ාපනයට ෙයොමුවන

පිරිස වැඩිෙව්වා

යන්න මෙග්

පාර්ථනාවයි Xi

Prof Chandana Jayaratne

Thuong Hien Tuan

J D P Deshapriya

M A Barucci

S Fornasier

P H Hasselmann

C Feller

H Sierks

A Lucchetti

M Pajola

N Oklay

S Mottola

N Masoumzadeh

C Tubiana

C Güttler

C Barbieri

P L Lamy

R Rodrigo

D Koschny

H Rickman

J.-L Bertaux

I Bertini

D Bodewits

S Boudreault

G Cremonese

V Da Deppo

B J R Davidsson

S Debei

M De Cecco

J Deller

M Fulle

O Groussin

P J Gutierrez

H V Hoang

S F Hviid

W Ip

L Jorda

H U Keller

J Knollenberg

R Kramm

E Kührt

M Küppers

L Lara

M Lazzarin

J J Lopez Moreno

F Marzari

G Naletto

F Preusker

X Shi

N Thomas

J.-B Vincent

M R El-Maarry

J Agarwal

M F A'hearn

A Gicquel

M Hofmann

J J Lopez Moreno

D Perna

A Pommerol

Exposed bright features on the comet 67P/Churyumov-Gerasimenko: distribution and evolution

Keywords: Comets, 67P/Churyumov-Gerasimenko, photometry, spectrophotometry, OSIRIS, Rosetta, H 2 O ice, astronomy, astrophysics, planetary science vi individual: 67P/Churyumov-Gerasimenko -techniques: photometric -methods: data analysis

Snowfall at the Paris Observatory in Meudon February 2018. aventure à la fois astronomique, spatiale et surtout humaine. Donc je pense fort au 'jeune' Dom et à tous les bénévoles que j'y ai rencontrés. සිංහල භාෂාෙවන් ලියෙවන ෙමම ෙඡ්දය, මම සිංහල පාඨකයන් ෙවනුෙවන් ෙවන්කරමි. තාරකාවිද ාව හැදෑරීමට, ශී ලංකාෙව්දී මාහට අත හිත දුන් පිරිස ෙබොෙහොමය. ආනන්ද විද ාලයීය තාරකාවිද ා සංගමය ෙමහිදී පාරම්භක විය. මක්නිසාදයත්, තාරකාවිද ාත්මක අධ යනයට මා අඩිතාලම ෙයදුෙව් ආනන්ද විද ාලයීය තාරකාවිද ා සංගමෙය්දී වූ නිසාය. ඉන් අනතුරුව, සුෙභෝධී අධ යන ෙක්න්දය හා ශී ලංකා තාරකාවිද ා සංගමය සමග සම්භන්ධ වී, තාරකාවිද ා අධ යන සහ පචලිත කිරීෙම් ව ාපෘථි රැසකට සහභාගී වීමට මට හැකිවිය. 2008 වසෙර්දී අ.ෙපො.ස. උසස් ෙපළ විභාගයට මුහුණ දී, 2009 වසෙර්දී, ෙකොළඹ විශ් ව විද ාලෙය් විද ා පීථයට ෙභෞතීය විද ා අංශෙයන් ඇතුලත්වූ විගසම, මම එහි ගණිත සහ තාරකාවිද ා සංගමයට ඇතුලත් වීමි. ෙකොළඹ විශ් ව විද ාලෙය් අධ යන කාලය තුලදී සහ විෙශේ ෂෙයන්ම 2009 ජාත න්තර තාරකාවිද ා වසෙර්දී, මට ෙනොෙයක් ෙදස් -විෙදස් තාරකාවිද ා ව ාපෘති වලට සහභාගීවීමට හැකිවිය. ෙමම ව ාපෘති මෙග් තාරකාවිද ා විෂයය දැනුම වර්ධනය කරගැනීමටත්, උගත් දැනුම ශී ලාංකීය පාසැල් සිසුන් අතර ෙබදා හැරීමටත්, විෙද්ශ සම්මන්තණයන් හීදී ශී ලංකාව නිෙයෝජනය කිරීමටත් මහත් රුකුලක් විය. ෙමෙලස ඇරඹි මාෙග් තාරකාවිද ා ගමන්මෙග්, මා හට උරදුන්, මා ෛධයමත්කල හා මා හට මග ෙපන්වූ පිරිස ෙබොෙහොමයකි. නම් වශෙයන් සඳහන් ෙනොකලත්, ඔවුනට මාෙග් ස් තූතිවන්ත ෙගෞරවය හා පණාමය පකාශකිරීමට ෙමය අවස් ථාවක් කරගනිමි.

දරුවන් ෛවද වරුන් හා නීතීඥයන් කරවීමට තිඹිරිෙගයි සිටම ෙපොළඹවනු ලබන, ස් ව ං-තීරණයට සහ නව තාවයට අභිෙයෝග කරන, ස් ථිතික ගතානුගතික සමාජ-සංස් කෘතික වපසරියකින් ෙපෝෂිත වත්මන් ශී ලංකාෙව්, තාරකාවිද ාෙවන් උසස් අධ ාපන කටයුතු කිරීමට ෙපළෙඹන්ෙන් ඉතා අල්ප පිරිසකි. ආචාර්ය උපාධීන් ෙවත ෙයොමුෙවන එම සීමිත පිරිස තුළද සියයට සියයක්ම පාෙහේ සාම්පදායිකව ඇතුලත්වන්ෙන් ඇමරිකානු, ඕස් ෙට්ලියානු ෙහෝ බිතාන විශ් ව විද ාලවලටය. ෙමයට පධාන ෙහේ තුවක් වන්ෙන් ජාත න්තර අධ ාපනය පිළිඹඳව ශී ලාංකික සාමාන ජනයා අතර පවතින ෙනොදැනුවත්කමය. ගෙව්ශන කටයුතුවලට සහ තාවකාලික අවදානම් ගැනීමට වත්මන් ශී ලාංකිකෙයෝ මැලිෙවති, එම නිසාම ඔවුන් විකල්ප විසඳුම් සඳහා අඩු උනන්දුවක් ෙයොමු කරති. විෙද්ශ රටකට ෙගොස් , ස් වාධීනව නැවුම් ජීවිත පරිච්ෙඡ්දයක් ඇරඹීමට සිහිනයකින් පවා එඩිතරවන්ෙන් අතෙලොස් සකි. එෙහයින්, ෙබොෙහෝ වටිනා විෙද්ශ අධ ාපන අවස් ථා ඔවුනට මගහැෙරති. වර්තමානෙය්, ෙකොළඹ විශ් ව විද ාලෙය්, තාරකාවිද ාෙවන් ආචාර්ය උපාධියක් හැදෑරීෙම් හැකියාවක් ඇතත්, එය ඉතා සුළු පිරිසකට සීමිතය. ෙමම ෙහේ තූන් නිසා, විෙද්ශයන් හී ඇති අධ ාපන අවස් ථාවලින් උපරිම පෙයෝජන ගතයුතුය යනු මෙග් හැඟීමයි. එම අරමුණ ලඟාකරගන්ෙන් ෙකස් දැයි සංකල්පනය කිරීමට සහ කියාත්මකවීමට, ෙමය කියවන ඔබට භාරය. නිදසුනක් ෙලස, වියට්නාමය සහ පංශය හරහා ෙගොඩනැගුණු මාෙග් උසස් අධ ාපන ගමන්මග ෙමහිදී ඉදිරිපත් කරමි. හිත ඇත්නම් පත කුඩාද! යන පස් ථාව පිරුළ පිළිබඳ සිංහාවෙලෝකනයක ෙයෙදන්නට පාඨක ඔබට ඇරයුම් කරමින්, ෙමම ෙකටි වදන්ෙපළ ෙමෙලස සමාප්තකරමි. සංක්ෂිප්තයක් ෙලස, ශී ලංකාෙවන් තාරකාවිද ා උසස් අධ

Abstract

The small bodies of the solar system are the residual material from which the planets got formed. The study of small body populations in the solar system allows to probe into the conditions that prevailed during the formation of the solar system. Unlike planets, these objects represent a reservoir of least modified material present in the solar nebula. The population of trans-neptunian objects (TNOs, located between ∼30 UA à ∼150 AU from the Sun), primarily consisting of volatile ices and dust, are a part of the small bodies of the solar system and embodies important clues about the different processes of their formation and the evolutionary history that they underwent. Given this context, Rosetta spacecraft was launched in 2004 by ESA, to explore the comet 67P/Churyumov-Gerasimenko, which originated in the TNO region. Rosetta reached the comet in 2014 and could dwell on an unprecedented cometary science mission lasting for more than two years. This thesis is based on the spectrophotometric properties of the comet 67P/Churyumov-Gerasimenko, using the OSIRIS instrument which is composed of two scientific cameras to observe the nucleus and the coma of the comet. Images acquired with multiple filters, that span the near-UV to near-IR wavelength range, allow to reconstruct the shape of the visible spectrum of the observations. I used the OSIRIS data to study the spectrophotometric curves of the exposed bright features that appeared on the surface of the cometary nucleus. Following on, I got involved with a comparative study of such bright features, that was carried out in collaboration with the VIRTIS spectro-imager aboard Rosetta, that demonstrated that these exposures are related to H 2 O ice, using its absorption band located at 2 microns. Next, I concentrated on a spectrophotometric study of the Khonsu region in the southern latitudes of the comet, known for its morphological diversity. I studied the exposed bright features, identified in the Khonsu region and the seasonal variation of the spectral slope of different types of terrains in this region. Among the terrains studied, all the terrains corroborated the global phase reddening behaviour of the comet, except for one terrain, which was an outlier due to the intense activity of the comet, which would have eroded its surface during the perihelion passage. These findings led to the publication Deshapriya et al., 2016. My existing work on bright spots was then extended to include the entire nucleus of the comet, using the OSIRIS data gathered during more than two years. Thus, I identified more than 50 exposed bright features [START_REF] Deshapriya | [END_REF] that are due to the presence of H 2 O ice and I produced a map to reflect their whereabouts in the nucleus and find potential correlations. Following this, I could come up with four different types of exposed bright features in terms of their morphological aspects and conclude that these features are preferably located in equatorial latitudes of the comet. I also made albedo calculations, which yielded a mean value of 0.4 for these features. Some of the features were found to be related to activity sources such as outbursts, and boulder displacements and correspond to much higher albedos than the mean. The features located on smooth terrains with a lot of dust, are probably formed when the diurnal cycle of water ceased to continue on the comet as it gets away from the Sun upon passing through the perihelion. As such, once the comet passes beyond the current snowline, the frost would not longer sublimate away and would get mixed with the dust that gets redeposited. Moreover, the albedos of this type of features are lower than the other types, in agreement with the above hypothesis. Indeed, as the comet gets close to the Sun again, during its following perihelion passage, the sublimation-driven activity would result surface erosion, leading to the appearance of this type of features, buried underneath the dust.

Résumé

Les petits corps du système solaire sont les vestiges du matériel primordial avec lequel se sont formées les planètes. En les étudiant on peut remonter aux conditions qui régnaient lors de la formation du système solaire. Les objets transneptuniens font partie des petits corps et contiennent des indices importants sur les processus de leur formation et les évolutions qu'ils ont subies. Dans ce contexte, la sonde Rosetta, qui fut lancée en 2004 par l'ESA, a atteint la comète 67P/Churyumov-Gerasimenko en 2014 et a pu recueillir une importante quantité de données grâce aux divers instruments scientifiques qu'elle a embarqués jusqu'en fin septembre 2016. La comète 67P/Churyumov-Gerasimenko provient de la région des objets transneptuniens répartis entre ∼30 UA à ∼150 UA du Soleil et l'étudier a dévoilé de détails importants sur ces objets principalement formés des glaces volatiles et de poussière.

Dans le cadre de la mission spatiale Rosetta, cette thèse porte sur les propriétés spectrophotométriques de la comète 67P/Churyumov-Gerasimenko à l'aide de l'instrument OSIRIS. Cet instrument est composé de deux caméras pour les observations du noyau et de la coma de la comète. Elles permettent d'acquérir des images avec des filtres qui opèrent dans la gamme du proche UV au proche IR. Dans un premier temps, j'ai utilisé les données OSIRIS pour analyser les courbes spectrophotométriques des taches claires qui sont apparues sur le noyau de la comète. Ensuite, j'ai participé activement à l'élaboration d'une étude comparative sur ces taches, qui a montré qu'elles sont liées à glace de H 2 O . La présence de H 2 O a été confirmée grâce à sa bande d'absorption à 2 microns dans les spectres infrarouges obtenus par l'instrument VIRTIS, le spectro-imageur embarqué dans Rosetta. Dans un second temps, j'ai entrepris une étude spectrophotométrique de la région Khonsu, très riche en variations morphologiques, se trouvant dans les latitudes du sud de la comète. J'ai analysé les taches claires qui y sont été repérées et les variations de la pente spectrale de différents types de terrains dans cette région. Bien qu'il y ait un type de terrain avec une anomalie expliquée par le surfaçage dû l'intense activité qui aurait érodé la surface pendant le passage au périhélie, trois types de terrains corroboreraient le rougissement de phase connu au noyau. Par la suite, j'ai élargi mon analyse des taches à tout le noyau de la comète et j'ai détecté plus de 50 taches claires dues à la présence de la glace de H 2 O et j'en ai produit une carte qui sert à repérer leurs emplacements dans le noyau, afin d'étudier plus en détail leur répartition et évolution au fil de temps. Ceci m'a permis d'identifier quatre types de taches regroupés en fonction de leur morphologie et de constater qu'elles sont plutôt présentes dans les latitudes équatoriales. J'ai également calculé une valeur moyenne d'albédo d'environ 0.4 pour ces taches. Certaines sont susceptibles d'être liées aux sources d'activité et ont un albédo plus élevé que la moyenne. Les taches qui se trouvent sur les terrains lisses (enrichi en poussière) sont probablement issues de la cessation du cycle diurnal d'H 2 O sur la comète. En effet, lorsque la comète s'éloigne du Soleil et passe la ligne des glaces, le givre ne se sublime plus de la surface et se mélange avec la poussière qui s'y dépose. De plus, l'albédo de ce type de taches est plus faible que l'albédo moyen des taches, ce qui soutient cette hypothèse.

Mots-clefs : Comètes, 67P/Churyumov-Gerasimenko, photométrie, spectrophotométrie, OSIRIS, Rosetta, glace de H 2 O , astronomie, astrophysique, planétologie vii

Acknowledgements

It is with great pleasure that I pen these lines, to reflect my gratitude and appreciation for all those who have helped me and contributed in numerous ways towards this thesis along the road during past three years or so.

First of all, my utmost thanks go to my thesis advisor Antonella Barucci, first for accepting my candidacy to pursue my doctoral studies with her, and then guiding me to do scientific research at such a prestigious astrophysics laboratory as LESIA. This gave me the opportunity to discover in detail the Rosetta space mission and eventually be a part of it as I continued to work with the OSIRIS team. I really enjoyed taking part in team meetings and teleconferences during the mission planning phases and felt rewarded as my suggestions were evaluated and ended up implemented in OSIRIS observational sequences. Furthermore, Antonella promptly shared with me her vast expertise and knowledge, inculcated in me the sense of critical thinking and offered me the possibility to work independently with autonomy. Antonella, I thank you again for your patience with me, listening to me always and specially, opening me the doors to the realm of space missions, which I've become quite fond of over the recent years. It is my firm conviction that our collaborations will continue.

Next I wish to thank the jury of my thesis defence for accepting to be a part of it. Thus, my cordial thanks are addressed to Daniel Hestroffer, Pierre Vernazza, Paolo Tanga, Cécile Engrand, Holger Sierks and Antonella Barucci. As reviewers, Pierre Vernazza and Paolo Tanga have been very helpful in improving the manuscript and pointing out ambiguities which led to its betterment. I thank you all for your contribution. My cordial thanks also go to the Ecole Doctorale Astronomie et Astrophysique d'Ile-de-France (ED 127) and Université Paris Diderot for the scholarship they offered me to conduct three years of research at LESIA-Observatoire de Paris.

Working at the Meudon site of the Observatoire de Paris has been a blessing despite the effort one has to make in order to get there (Fig. 1). The Meudon site offers quite a breathtaking view of Paris from its inherent vantage point. Things could be even better on a nice sunny day in late spring or summer with the tantalising panoramic views of Eiffel tower and Sacré-Coeur. Even, at night, whether you've just finished a long day at work or an observation session with the astronomy club at T60, it would make quite a sight with all the Parisian lights that might have hindered you from observing distant faint targets a while earlier. No doubt that they were savvy enough to appropriately name the Meudon neighbourhood 'Bellevue'. Other than that, one could practically invest some leisure time in diverse activities ranging from feeding the carps of the little lake in the site, going for a stroll in the adjacent woods or to even looking for mushrooms in the season. Then, earlier this year we had heaps of snow in Meudon (Fig. 2) and I was lucky to embrace it because, prior to this, I had only heard of such heavy snowfalls in Meudon ages ago. Thanks to the daily routine to and fro between my residence and my study in Meudon (including the daily walks to canteen), I managed to easily average more than 5 km a day, during past three years. It would be remiss to leave out my visits to my university 'Université Paris Diderot', although the number of visits would be orders of magnitude lower than the visits I made to Meudon. Although I went there mainly to administrative tasks and borrow books from the library, I enjoyed being a part of the university Chess team and playing at the national E&Y Trophy and emerging as finalists in each year to play the renowned simultaneous round viii with an international professional player (Alexander Morozevich , Laurent Fressinet and finally, Maxime Vachier-Lagrave this year). A big shout out to my team members Michael, Matthieu, Julien, Yasser and Jeaniero ! Being at LESIA has been full of interesting experiences since my arrival there as an M2 intern, six months prior to the beginning of my thesis. Over the time, I had the pleasure of meeting many generations of PhD students, with whom I've had fond memories and souvenirs. Some have already graduated and left LESIA and Observatoire de Paris for further adventures and have spread across the globe, while some are on the verge of graduating and some a bit later. As such, I wish to thank Batiste, Cateline, Mélody, Vincent, Jean-Loup, Gaël, Clément F, Clément P, Clément H, Diane, Charly, Oza, Lucas, Anelise, Evan, William, Isadora, Tabatha, J-B, Julien, Fernando, Paula, Fiorella, Doriann, Meriem, Corontin, Letizia, Léa, Charlotte, Lisa, Loïc, Christophe M, Nicholas, Victor, Christophe L, Tanya, Ioanna, Brisa, Rose, Laura, Adarsh, Trisha and all those who I managed to forget here for all these times we shared. Now I take this moment to expressively thank Davide, Sonia, Pedro, Marcello, Maarten, Dominique, Frédéric, Alain, Cedric, Vladimir, François, Benjamin, Nicholas, Emmanuel, Thierry, Sandrine, Mirel, Marcel, Bruno, Alice, Arianna, Thérèse, Jacques for all the interactions we had during the last three years within the planetology group of LESIA. Whether it was an interesting discussion during a leisurely stroll to the canteen in Meudon, playing a football tournament in Montevideo or basking in the Sun at a beach in Rhodes, I enjoyed their company a lot. Davide was my first professor during my Masters at USTH in Hanoi and thus was the first person from LESIA that I got to know. I'm grateful to his availability and attention that he paid to the questions I had during those early days. At the same time, I take this moment to thank all my professors at USTH during my Masters, with a special ix mention to Prof. Yannick Giraud-Héraud. As I moved to Paris to do my M2 internship with Antonella and Davide, I got to work closely with him and later we collaborated. Thank you Davide for everything. I could easily recall the time I met Cedric as a professor during my Masters as it was then that I had a bigger picture of the Rosetta space mission thanks to his lectures. At that point, little did I know that I would end up doing a thesis on the very space mission. So, thank you Cedric for exposing me to this glorious mission. Adventures have always been numerous with Pedro, leaving with many anecdotes. Be it in Montevideo, Göttingen, Berlin, Rhodes or Athens, not a single second has been boring with him. Thank you for the enlightening disucussions on photometry and above all, your ever-enriching Brazilian energy and spirit of life ! Muito obrigado por tudo cara, vamos fazer muitas coisas em breve e um dia no Sri Lanka ! Special thanks are in order to Clément for the constant availability and help with shapemodels and the analysis of space data at a time I was initially throwing myself towards the realm of small bodies and space missions. I would also like to personally thank Batiste for sharing the study for two years and showing solidarity of the Bureau 34 of the Bâtiment 17 at the CROSS de Meudon in February 2017. Thank you for the times shared at conferences (DPS in Pasadena and EPSC in Riga) and specially for the regular supply for French pâtisseries, which I have been missing since your departure from LESIA. I would also take this moment to address my thanks to the administration of LESIA and Observatoire de Paris for their assistance during administrative tasks and foreign missions. The colleagues at GIL are kindly reminded and acknowledged for their technical support and solutions in countless situations.

My thanks should also go to all the colleagues of the OSIRIS scientific team for their collaboration and times shared. It is with great gratitude that I recall Carsten, Cecilia, Holger, Marc, Jakob, Maurizio, Alice, Björn, Luca, Ramy, J-B, Stefano, Nilda, Julia, Katharina and all other members of the team spread around the world. I'm grateful to Holger the principal investigator of the instrument OSIRIS on-board Rosetta, for encouraging me and recognising my work in the team level since as early as my integration into the team. Carsten and Cecilia were always available to clarify doubts and get a better insight about a problem I would have and thank you very much for your support. Thank you J-B for the amazing shapeViewer: this research has made use of the scientific software shapeViewer (www.comet-toolbox.com), that he developed. Again, thank you dear OSIRIS team for all those colaborations and memories at Full Team Meetings and other international meetings.

Thanks are in order for the folks I met in Paris and suburbs during random encounters since my early days in Paris and those who were and are at the Fondation Marjolin. Marcellus, merci pour ton amitié qui date désormais de plus de trois ans. Merci Christophe pour tous tes soutiens pendent que je m'habituais à la vie parisienne. Les beaux Marjolinais, merci de vos belles amitiés pendent ces deux années. Je pense qu'on en a bien profité grâce aux formidables rencontres et sorties qu'on a faites au cours de ces temps. Mes chères pensées vont ainsi à Dani, Bérengère, Richard, Antonio, Marine (meilleure voisine que je connaisse), Elza, Alexine, Jess, Busra, André, Carlou, Viky, Sylvia, Silvia, Hamed, Raed, Omar, Juan, Adriana, Abdallaye, Florine, Mamadou, Joe, Tissas, Dilara, les deux Christina, M.C, Laura, Hipolyte, Rafa et forcément aux autres personnes qui sont malheureusement tombées aux oubliettes. Franchement ..... vous avez été géniax, les gars !! Par ailleurs, je pense également à Dimitri, Karel, Rémy, Wayan, Susie, Paul, Tam, Jodie, Mathilde, Anni, Kasia, Tareq et d'autres amis que j'ai cotoyés soit dans la région parisienne ou en Estonie ou bien au Viet Nam. Je n'oublierais également pas les bénévoles du projet ambitieux 'SpaceBus France 2018' avec qui j'ai passé un agréable et bel été dans le sud de la France cette année avec une xvii List of Figures The compositional distribution of solar system bodies is succinctly recapitulated in the above figure which is structured in several horizontal layers, with the exception of comets that span much larger heliocentric orbits than the range depicted therein. The colouring trend from brown to blue with the heliocentric distance represents the transition of composition dominated by silicates to volatile ices. The populations of small bodies are represented as a function of their semi-major axes versus their inclinations to the ecliptic plane with a colour code for the population density. The approximate distribution of taxonomic classes for the main belt asteroids are given as well as some sample spectra for different small body populations. The horizontal gray bar refers to the average albedo as a function of heliocentric distance and the bar above it corresponds to the effective black-body temperature as a function of the heliocentric distance, calculated for an albedo of 0.1. The figure succinctly depicts major aspects of the dynamical history of the small bodies in the solar system according to the Grand Tack model [START_REF] Walsh | A low mass for Mars from Jupiter's early gas-driven migration[END_REF] and the Nice model [START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF][START_REF] Gomes | Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets[END_REF][START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF]. Although these models may not simulate the exact evolution of the solar system, they appear to be viable hypotheses. They connect the epochs of radial mixing, mass removal and planetary migration, leading to the present image of the solar system with its planets and populations of small bodies. Credit: DeMeo and Carry, 2014. . . . . . . . . . . . . . . . . . . . 6 The above figure depicts the relationship between semi-major axis and eccentricity for some of the small bodies located in the outer solar system. Neptune's orbit is marked as a vertical dotted line. The blue populations are in mean motion resonance with Neptune, which results in their stability over the age of the solar system. The most significant blue population is located around 40 AU of semi-major axis with 3:2 mean motion resonance with Neptune where Pluto and many plutinos belong. Figure credit : David Jewitt. . . 12 1.7 The observational, dynamical and composition criteria on which the definition of a comet is based. Jewitt, Morbidelli, and Rauer, 2007 . . . . . . . . . . 15 1.8 The population of active asteroids / MBCs (black dots) that display various mass-losing mechanisms compared to main belt asteroids (orange dots) and short-period comets/ SPCs (blue dots). The dotted lines correspond to the semi-major axis of Mars, orbit related to the 2:1 mean motion resonance with Jupiter and the semi-major axis of Jupiter, when viewed from left to right. 1.16 The variation of sub-solar latitude with the heliocentric distance. The blue horizontal line marks the 0 o sub-solar latitude to denote the equinoxes on the comet. The red vertical line corresponds to the perihelion passage at 1.24 AU, that took place on 13 August 2015. . . . . . . . . . . . . . . . . . . . . . . . . . 1.17 Left column gives different perspectives of the nucleus of the comet 67P with different orientations. Rotational axis of the comet is also indicated for better interpretation of the images. Left panel shows the same images with an overlaid texture to identify the regional boundaries, which correspond mainly to those located in northern latitudes. Figure credit: El-Maarry et al., 2015. . . . 1.18 The regions located at the southern latitudes of the comet along with their nomenclature. Left column offers different views on the regions located in southern latitudes, while the right column offers views similar as those on horizontal panels along with an overlaid texture to identify the regional boundaries. [START_REF] Tubiana | 67P/Churyumov-Gerasimenko at large heliocentric distance[END_REF] and the NAC and WAC spectrophotometric data of the comet recorded on 28 July 2014 between 23:25-23:45 UT (Fornasier et al., 2015). It is noted that the comet was inactive during the acquisition of its spectrum in 2007, while the same is not true for the OSIRIS spectrophotometric data in question. . . . . . . . . . . . . 2.5 Phase curve of the nucleus of the comet 67P from NAC observations (F23).

The continuous line refers to the IAU HG model that best fits the data whereas the dashed-line corresponds to the linear fit to the data. 6 A schema illustrating the bi-directional reflectance from a surface element ∆A with surface angles i, e, α and ψ which refer respectively to the incident angle, emission angle, phase angle and the azimuthal angle which is the angle between the planes of incident and emission. . . . . . . . . . . . . . . . . . . . 2.7 First and last images of an 11-filter-sequence of observation. Although the changes on the two images are subtle to discern, the arrow points to a location which is emerging from the shadows, where the changes are relatively easier to spot. This reflects the need of co-registration for the observational sequences in order to assure that same pixel corresponds to same surface features in all the images in the sequence. . . . . . . . . . . . . . . . . . . . . . . . 2.8 Stucture of an OSIRIS data cube produced using images acquired with four filters. For each spatial location of the image, it is possible to extract the intensity / reflectance for each wavelength of the filters and perform a spectrophotometric analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Orthographic renderings of the shape model SPG SHAP7 (along the primary axes of the body-fixed 67P/CG_CK coordinate frame). Indicated axes point towards the viewer, north is up for the +/-XY views, +Y is up for the +/-Z views. 16 RGB images created using the same data cubes used to create the spectral slope maps in the previous section. OSIRIS NAC filters centred at 882.1 nm (F41), 649.2 nm (F22) and 480.7 nm (F24) as input channels respectively for red, green and blue colours have been used in their creation. . . . . . . . . . .

Individual optical designs of VIRTIS-M and VIRTIS-H along with an image

showing both of them being assembled together prior to the integration to the Rosetta spacecraft. Adapted from Coradini et al., 2007. . . . . . . . . . . . . . xxi 3.2 Exposed bright features on a map of the comet 67P created using two shape models for northern and southern hemispheres respectively from Preusker et al., 2015 andJorda et al., 2016. All the 13 candidate features initially detected using OSIRIS as well as two other features (F) referred to as bright albedo patches (BAP) and studied by Filacchione et al., 2016a are populated in red across the map. The positive VIRTIS-M detections where the presence of H 2 O was confirmed, bear their respective numbers (1)(2)(3)(4)(5)(6)(7)(8). The figure is adapted from Barucci et al., 2016 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Average spectrum of the surface of the comet 67P derived from VIRTIS data.

The missing data from 0.8 -1.0 µm is due to a calibration artefact. The first column refers to a contextual NAC full frame image (F22 filter centred at 649.2 nm) and the second columns gives a zoom on the feature in question, where an arrow points to the location of the feature. The reflectance (I/F) plots, photometrically corrected using the Lommel-Seeliger disk function are given in the third column and in the forth column are their relative reflectance plots, normalised at the F23 filter, centred at 535 nm. The red curves correspond to the bright feature, whereas the black curves represent the behaviour of the typical nucleus, as sampled close to the feature. All the samplings have been done 3x3 pixels centred at the feature. The detailed OSIRIS observational circumstances for each feature are given in the Table 3.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 The infrared spectra of VIRTIS-M for the eight positive features. The left column represents the features from 1-4 and the right column represents the features from 5-8. Given are also the false colour RGB images created using the reflectances at 1.32 µm (channel B), 2.0 µm (channel G) and 4.0 µm (R).

The reflectances for each feature are plotted in black, while the best fits For each spot, we report the VIRTIS-M observed reflectance (black curve), not corrected for phase angle, and their respective best fits are plotted in red. The missing data in the spectral ranges correspond to order-sorting filter-junction wavelengths that can produce unreliable features and, hence, have been discarded. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Modelled reflectance spectra are illustrated to point out their variation as functions of abundance and grain size of H 2 O ice. In both plots, the black curve corresponds to the spectrum of the spot 3 and red curve is the best fit for area mixing modality as seen in Fig. 3.5. The top panel shows the variation of spectra as a function of H 2 O ice abundance (0%, 1.3%, 2.6%), while the grain size of H 2 O is fixed at 750 µm. The bottom panel shows the variation of the spectrum as a function of grain size (75 µm, 750 µm, 7500 µm), while the 12 Variation of the spectral slopes for both pre and post-perihelion periods. The sampling has been carried out in such a way that always same surface area is covered. The individual images have been co-registered to prepare the data cubes and are corrected for the illumination conditions, using the Lommel-Seeliger disk function, prior to computing the spectral slopes. . . . . . . . . . 3.13 Evolution of the spectral slope with the heliocentric distance and the phase angle. Spectral slope values are evaluated in the 535-882 nm range, averaged over one rotation, for observations acquired on August 2014 (red squares), on April-August 2015 (black circles) and on January-February 2016 (magenta stars). Excerpt courtesy of Fornasier et al., 2016. . . . . . . . . . . . . . . . . . 3.14 Temporal evolution of the bright feature K1 in the Khonsu region. An OSIRIS NAC context image along and a zoomed sub-frame image are given for each NAC observation listed in Table 3.9 for this feature. . . . . . . . . . . . . . . . 3.15 Spectrophotometric plots for the bright feature K1, where its reflectance is represented in red, while that of the typical nucleus sampled from nearby terrain is represented in blue. For A, C and D sampling has been done by 2x2 pixels whereas for B, the sampling was 3x3 pixels. . . . . . . . . . . . . . . . . 3.16 Evolution of the bright feature K2 as adapted from Deshapriya et al., 2016. . . 3.17 Spectrophotometric plots for K2 (red data points) multi-filter observation on 23 January 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 2x2 pixels. . . . . . 3.18 Spectrophotometric plots for K2 (red data points) multi-filter observation on 27 January 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. Since several pixels towards the centre of the feature K2 were saturated, the sampling was done towards its edges. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 2x2 pixels. . . . . . . . . . . . . . . . . . . . . . . . . . 3.19 Spectrophotometric plots for K3 (red data points) multi-filter observation on 10 February 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 1x2 pixels. . . . . . 3.20 Spectrophotometric plots for K4 (red data points) multi-filter observation on 16 July 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 1x2 pixels. . . . . . . . . . xxiii 3.21 Top left -A NAC mono-chromatic full frame OSIRIS NAC image (F22 filter centred at 649.2 nm), revealing a part of the Khonsu region on the comet where volatile-rich (bluish material on RGB images) and exposed bright patches are present in the form of a cluster, right -an RGB image created using the NAC filters centred at wavelengths 882.1 nm, 649.2 nm and 480.7 nm for red, green and blue colours. This image also depicts 3 regions of interest chosen for the spectrophotometric analysis. Bottom left -spectrophotometric plots of the aforementioned regions of interest, right -a zoom onto the 3 regions of interest on the RGB image. The red box refers to an exposed bright patch (Deshapriya et al., 2016) containing H 2 O ice, whose reflectance has been sampled (1x2 pixels per a filter) with scrutiny in order to avoid saturated pixels and blue box corresponds to volatiles mixed with desiccated refractory material from the nucleus (2x2 pixels) , whereas black box (2x2 pixels) represents the typical nucleus. Note that the dimensions of the boxes are deliberately exaggerated in order to point out their respective locations on the terrain. The red and green features that sometimes appear adjacent to the shadows and near the frame edges in the RGB images are artefacts arising from the imperfect co-registration/receding shadows of the images. 3.10) respectively and the black dot indicates the unique detection of CO 2 ice (Filacchione et al., 2016b) where H 2 O ice was detected few weeks later (Fornasier et al., 2016). This map is the cylindrical projection of a merged shape model resulting from the shape models SHAP4S (Preusker et al., 2015 andSHAP5 (Jorda et al., 2016) sponds to about 3 weeks before the perihelion passage and there is a couple of bright patches that have already appeared on the smooth terrain probably due to the erosion that had been escalating at this time. The next panels from c through g illustrate the appearance of a family of transient outbursts and a long-lived outburst with a lifetime in the order of several hours, as the comet was reaching the perihelion of its orbit. The next panel h, where the area of interest is resolved about 4 months later, makes it clear that there have been significant changes on the surface morphology therein. The observable boulder field is therefore likely to be a direct result of the aforementioned series of outbursts. Finally, the panel i reveals the bright patch that has appeared on the boulder, some 2 months later. Figure credit : Deshapriya et al., 2018. . . . . 118 xxiv 3.26 The field of boulders in Bes region, where a displacement of a boulder was observed, leading to the appearing of an exposed bright patch on it between February and March 2016. Its sublimation-driven evolution can be characterised by means of the diminution of its effective size calculated from pixels resolutions. However, the reader is cautioned of any bias due to the geometry of the observation. et al., 2016a, Fo16: Fornasier et al., 2016, Fo17: Fornasier et al., 2017, O: Oklay et al., 2016, P: Pommerol et al., 2015. N and V in the column Obs. refer to NAVCAM and VIRTIS respectively. Source: Deshapriya et al., 2018. . . . . . . 3.12 The ice grain sizes derived from spectroscopic modelling for different cometary nuclei, hitherto observed by in-situ spacecraft. The best mixing modality refers to the parameters obtained with the least χ 2 value between intimate (I) and areal (A) mixing. The references, S, A, DS, F and B refer respectively to Sunshine et al., 2006;[START_REF] A'hearn | EPOXI at Comet Hartley 2[END_REF]De Sanctis et al., 2015;Filacchione et al., 2016a;Barucci et al., 2016. The feature Types are also displayed where applicable, in order to find any correlations. . . . . . . . . . . . . . . . . . . . .

Chapter 1

Introduction 1.1 Formation of the solar system and its evolution A stellar system forms following the gravitational collapse of a dense core of a cloud of gas and dust, generally referred to as a nebula. Such cores generally correspond to about 10 4 solar masses (M ). The core collapse gives rise to the formation of an accretion disk around a centralised mass. The conservation of the angular momentum of the system results in faster rotational rates for the material closer to the centre of the disk and this leads to increasing rotational rates for inner material of the disk as the collapse continues. As more and more disk material gets accreted towards the centre of the disk, their kinetic energy is released in the form of heat and consequently, the temperature and the pressure in the centralised mass keep on increasing. As soon as the temperature reaches about 1000 K, it begins to radiate in infrared wavelengths (∼2.9 µm). Although this radiation is initially able to escape the centralised mass, continued collapse results in greater densities in the centralised mass, thus making it opaque to infrared radiation. As this radiation gets trapped, the temperature and pressure continue to rise. The increased pressure reaches a threshold value and prevents more gas from falling into the centralised mass and at this point it is said to be a protostar. At this stage, the protostar contains a fraction of its final mass. However, the stellar envelopes continue to grow as more infalling material gets accreted from the disk. As the protostar grows, its core pressure becomes sufficient enough for thermonuclear fusion of hydrogen atoms to take place. What follows this phenomenon, is the release of a powerful stellar wind, preventing the flux of infalling mass, at which point the protostar becomes a young star. Its evolutionary path is thus set depending on the mass the young star has at this time. The remaining circumstellar disk eventually dissipates as the planetary systems get formed through hierarchical accretion processes of small refractory and volatile grains and evolve through dynamical interactions of planetary bodies in the system.

The above represents a general scenario of the formation of a planetary system. The Sun formed about 4.6 Gyr ago through such a process and evolved into the solar system observed today. The objective of the planetary science consists of understanding how the planetary systems form, evolve and lead to the situations observed today in the solar system and other extrasolar systems [START_REF] Morbidelli | Challenges in planet formation[END_REF]. As the protosun was in place and still growing, a temperature gradient would have existed across the protoplanetary disk. This temperature gradient would have determined the radial distances across the disk, where different material would condense and form small grains. As such, metals and dust would have condensed closer to the protosun, while volatiles would have condensed Chapter 1. Introduction furtherer from the protosun. The radial distance beyond which H 2 O molecule condenses is colloquially referred to as the snowline. Indeed, this is a variable distance depending on the evolution of the central star and is currently located at about 3 AU from the Sun in the solar system. Hence, it is safe to assume that the disk would be primarily populated by rock-forming metals and minerals up to the snowline, beyond which the disk would be dominated by volatile species. Consequently, the thermal gradient would have defined the compositional gradient of the protoplanetary disk. Nevertheless, strong evidence exists for radial and vertical mixing (Fig. 1.1) of material in the protoplanetary disk, triggered by the transport of angular momentum. The analysis of dust grains returned to Earth from the comet 81/Wild by the Stardust mission [START_REF] Zolensky | Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples[END_REF] indicated the presence of minerals such as olivine, pyroxene, sulphides generally attributed to primitive meteorites. The dust grains further contained forsterite 1 and CAIs 2 , thought to have been formed in the hottest inner regions of the solar nebula. This suggests that these minerals, originally formed in the inner parts of the solar system, somehow got dynamically implanted in the outer regions of the solar system, where cometary nuclei get formed. In addition to the discovery of forsterite in cometary dust grains, recent spectroscopic observations of star-forming regions indicated the presence of forsterite [START_REF] Poteet | A Spitzer Infrared Spectrograph Detection of Crystalline Silicates in a Protostellar Envelope[END_REF], which stands as a strong evidence of extensive radial mixing at the early stages of the formation of the solar system [START_REF] Brownlee | Comet 81P/Wild 2 Under a Microscope[END_REF][START_REF] Ciesla | The distributions and ages of refractory objects in the solar nebula[END_REF][START_REF] Ciesla | Residence Times of Particles in Diffusive Protoplanetary Disk Environments. II. Radial Motions and Applications to Dust Annealing[END_REF]. Although the protoplanetary disk gradually dissipated through the formation of planets following the early compositional gradient, in some areas of the solar system, there remain 1 Forsterite is a type of olivine with the formula Mg 2 SiO 4 . 2 CAIs are Calcium-Aluminium-rich Inclusions of sub-millimetre to centimetre sizes, usually found in carbonaceous chondrite meteorites. CAIs are considered to be the oldest substances in the Solar System.

1.1. Formation of the solar system and its evolution 3 several populations of remnant small bodies. These small bodies did not evolve into planets due to divers reasons, such as strong gravitational interactions with giant planets, inadequate availability of accreting material in their respective feeding zones, etc. These small bodies bear witness to phenomena that the young protoplanetary disk underwent in the process of forming planetary embryos and planets. These small body populations, principally knowns as comets and asteroids are hence the remainders of the building blocks of the planets. The key differences of these two small body families arise from their respective formation regions (heliocentric distances) and different evolutionary paths that they followed. Since they are several orders of magnitude smaller in size than planetary satellites and planets, most of them have not been able to undergo geological differentiation through internal heating, which means that they are not radially layered as planets do, with different stratification processes. As such, they contain relatively primitive material that was present as planetary building blocks during the formation of the solar system, 4.6 Gyr ago. Hence, this material found on small bodies are supposed to have undergone very little alteration since their formation. In the solar system, these small bodies comprise of asteroids in the main belt located between the orbits of Mars and Jupiter, the Trojan asteroids 3 of giant planets, the dynamically excited transient population of centaurs and transneptunian objects (TNOs) 4 . A comprehensive understanding of their formation, physical characteristics, distribution, evolution is key to answer the fundamental questions of planetary science concerning the formation of the solar system, its evolution and origin of life on Earth.

More than two centuries later their discovery, the understanding of the asteroids has drastically changed in recent decades. The original idea was that the asteroids had formed where they are observed presently, in the main asteroid belt located between the orbits of Mars and Jupiter (Fig. 1.2). Hence asteroids were thought to preserve information about their original formation environments, which directly translate to the temperature and compositional gradient across the main belt at the time of the nascent planetesimals in the protoplanetary disk. Equipped with the growing know-how of the composition of meteorites, emanating from the asteroid belt, it was established that the inner main belt contained highly thermally processed igneous objects, whereas the outer main belt contained objects that had experienced little thermal processing. As a result, the major taxonomical classes of asteroids were discovered in the 1980s as a function of their heliocentric distance and were interpreted as the remnant thermal gradient present in the main belt of the evolving protoplanetary disk [START_REF] Gradie | Compositional Structure of the Asteroid Belt[END_REF]. The efforts that followed, aimed to explain the steep thermal gradient across the main belt, bridging thermally processed stony objects and relatively primitive objects across a relatively short distance of about 1 AU [START_REF] Grimm | Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations[END_REF]).

Yet, over the next decades that followed, this classical static view of the evolution of 3 Trojans are the asteroids that are co-orbital with a large body, and are located in two specific points of the orbit of a large planet, known as the L4 and L5 Lagrangian points. The stability of their orbits arises from the three-body system, as objects at L4 and L5 are dynamically stable due to the equilibrium of forces acting on them by the Sun and the large planet. Numerical simulations indicate that Trojan asteroids are dynamically stable to survive the age of the solar system. Jupiter Trojans (The asteroids that are in front of Jupiter in its orbit at L4, are referred to as the Greek camp, while the asteroids that trail behind the Jupiter at L5, are referred to as the Trojan camp.) account for the most numerous asteroids in the Trojan population. Earth [START_REF] Connors | Earth's Trojan asteroid[END_REF], Mars and Neptune have also been credited with Trojans of their own, although with significantly fewer numbers than those of Jupiter.

4 Each population will be detailed later on in this chapter. FIGURE 1.2: The compositional distribution of solar system bodies is succinctly recapitulated in the above figure which is structured in several horizontal layers, with the exception of comets that span much larger heliocentric orbits than the range depicted therein. The colouring trend from brown to blue with the heliocentric distance represents the transition of composition dominated by silicates to volatile ices. The populations of small bodies are represented as a function of their semi-major axes versus their inclinations to the ecliptic plane with a colour code for the population density. The approximate distribution of taxonomic classes for the main belt asteroids are given as well as some sample spectra for different small body populations. The horizontal gray bar refers to the average albedo as a function of heliocentric distance and the bar above it corresponds to the effective black-body temperature as a function of the heliocentric distance, calculated for an albedo of 0.1. the solar system has undergone colossal modifications (DeMeo and Carry, 2014), principally triggered by the continued discoveries of new asteroids and better characterisation of asteroids owing to space missions that questioned the static formation scenario of the preexisting models of the formation of the solar system. The samples of the S-type asteroid (25143) Itokawa, returned to Earth by the Hayabusa spacecraft, indicated that indeed, it had undergone some thermal processing, but not as intense as it had been previously thought and that it was relatively primitive [START_REF] Nakamura | Itokawa Dust Particles: A Direct Link Between S-Type Asteroids and Ordinary Chondrites[END_REF]. This subsequently helped resolve a long-standing debate about the spectral mismatch between ordinary chrondrite (OC) meterorites 5 and the S-type asteroids, by invoking the effects of space weathering [START_REF] Vernazza | Planetesimals: Early Differentiation and Consequences for Planets[END_REF]. In the meantime, the pool of available compositional data for asteroids began to grow and several 'rogue' asteroids (iron asteroids being discovered in the outer belt and primitive asteroids in the inner belt) appeared to be contaminating the asteroid taxonomical classes at their corresponding heliocentric distances. Although, such cases were treated with caution initially and interpreted as exceptions, the copious amounts of data [START_REF] Ivezić | Solar System Objects Observed in the Sloan Digital Sky Survey Commissioning Data[END_REF][START_REF] Mainzer | Preliminary Results from NEOWISE: An Enhancement to the Wide-field Infrared Survey Explorer for Solar System Science[END_REF] that was becoming available, suggested that a radial mixing of small bodies is in fact the general trend of the asteroids and that the asteroid taxonomic classes indeed are heliocentrically inter-mixed to a considerable extent (Fig. 1.3). Despite the fact that the distribution of asteroid taxonomic classes had been canonically studied as a function of their corresponding heliocentric distances, recent work [START_REF] Demeo | The taxonomic distribution of asteroids from multi-filter all-sky photometric surveys[END_REF][START_REF] Demeo | Solar System evolution from compositional mapping of the asteroid belt[END_REF] has demonstrated that asteroid taxonomic classes could be more accurately accounted for, by studying them as distributions of mass (Fig. 1.3A) and size (Fig. 1.3B). As such, it is clear that irregularities of the distribution of taxonomic classes are omnipresent in the main belt.

Today, the number of known asteroids surpasses 700,000 and it is the modern dynamical models of the planetary migration [START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF][START_REF] Gomes | Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets[END_REF][START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF][START_REF] Walsh | A low mass for Mars from Jupiter's early gas-driven migration[END_REF], that try to convincingly explain the aforementioned current picture of the asteroid belt, invoking the migration of giant planets that would have caused the small body populations in the early solar system to dynamically interact and change their original orbits. As such, the current main asteroid belt not only contains objects that formed in-situ between ∼2 AU and ∼3.3 AU, but also, objects that formed in the telluric planet region as well as beyond the snowline and the orbit of Jupiter (including those formed in the region of TNOs and preluding a volatile-rich cometary nature), thus corresponding to various heliocentric distances (Fig. 1.4). The first dynamical model to effectively explain the vivid physical structures of the present solar system was the Nice model [START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF][START_REF] Gomes | Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets[END_REF][START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF]. It was capable of explaining the current whereabouts of the giant planets, their respective orbital eccentricities [START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF], presence of irregular satellites of Saturn [START_REF] Nesvorný | Capture of Irregular Satellites during Planetary Encounters[END_REF] and the orbital properties of the Jupiter Trojan asteroids [START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF]. According to the original version of the Nice model, Jupiter would migrate inward while the other giant planets would migrate outwards, causing the 1:2 mean motion resonance between Jupiter and Saturn to occur, at which point the system would be cataclasmically shaken and destabilised, leading to various aftermath such as the rise of eccentricities of the orbits of Uranus and Neptune eventually leading to their switch, removal of objects present in the region of TNOs, etc. Nevertheless, the latest version of the Nice model states that this cataclasmic disruption would have taken place following the interaction of giant planets and the distant objects in the region of TNOs [START_REF] Levison | Late Orbital Instabilities in the Outer Planets Induced by Interaction with a Self-gravitating Planetesimal Disk[END_REF]. Naturally, the consequences are multifold. One of the prominent ones is the removal of pre-existing Jupiter Trojans and their re-population from the bodies that would have been scattered inward from the region of TNOs. This carries important implications to accurately reproduce the orbital distributions of masses of Jupiter Trojans as observed in the present solar system, evident from the Fig. 1.3B. As such, the Nice model successfully accounts for the compositional distinction of Jupiter Trojans from the rest of the main belt asteroids, caused by the implantation of what used to be TNOs, naturally different in composition compared to the original main belt asteroids. The present Jupiter Trojans are compositionally characterised by the dominant D-type objects, followed by the P-type objects with a minute component of C-type objects. Neither they are contaminated by asteroids formed in-situ in the main belt, nor do they appear to contain any S-type or M-type asteroids formed in the inner parts of the main belt and around telluric planets. This links them directly with comets emanating from the region of TNOs and Jupiter Trojans and other D-type and P-type asteroids present in the main belt, The figure succinctly depicts major aspects of the dynamical history of the small bodies in the solar system according to the Grand Tack model [START_REF] Walsh | A low mass for Mars from Jupiter's early gas-driven migration[END_REF] and the Nice model [START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF][START_REF] Gomes | Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets[END_REF][START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF]. Although these models may not simulate the exact evolution of the solar system, they appear to be viable hypotheses. They connect the epochs of radial mixing, mass removal and planetary migration, leading to the present image of the solar system with its planets and populations of small bodies. Credit: DeMeo and Carry, 2014.

could simultaneously be cometary nuclei, embodying volatile components. Recent discoveries have even led to the emergence of a new population of small bodies, presently referred to as main belt comets or active asteroids6 [START_REF] Hsieh | A Population of Comets in the Main Asteroid Belt[END_REF] attributed to mass-losing processes. These main belt comets/ active asteroids can directly be correlated with the small 8 Chapter 1. Introduction bodies that originally emanated from the region of TNOs during the migration of giant planets as per the Nice Model, in an attempt to explain their volatile-rich nature and mass-losing phenomenon once they are close enough to their perihelia. Given this interesting context of D-type and P-type asteroids or former TNOs, upcoming space missions like Lucy [START_REF] Levison | Lucy: Surveying the Diversity of the Trojan Asteroids: The Fossils of Planet Formation[END_REF] to explore the population of Jupiter Trojans seem quite appealing for their interesting science returns [START_REF] Barucci | Small D-type asteroids in the NEO population: new targets for space missions[END_REF]. In addition, the Nice model could relate to the timescales of the late heavy bombardment (LHB) 7 , supposed to have taken place ∼700 Myr (Fig. 1.4) since the formation of the solar system, as it postulates the influx of small bodies on the telluric planets from the region of TNOs and the asteroid belt, following the aforementioned cataclysmic disruption. It is noteworthy, however, that the Nice model does not accommodate for the internal 7 The LHB is also referred to as the lunar cataclysm and stems from the radioactive isotopic dating of the lunar rock samples returned to Earth by Apollo astronauts. mixing of the reddish and bluish objects in the asteroid belt. This is where the dynamical model referred to as the Grand Tack model [START_REF] Walsh | A low mass for Mars from Jupiter's early gas-driven migration[END_REF] offers a potential solution while responding to the low Mars problem [START_REF] Raymond | Building the terrestrial planets: Constrained accretion in the inner Solar System[END_REF]. The scenario explained by the latter corresponds to an epoch of the formation of the terrestrial planets, much earlier than that of the Nice model. The Grand Tack model postulates that Jupiter would have migrated inward up to about 1.5 AU, approximately where the current orbit of Mars is (Fig. 1.5). In doing so, Jupiter would have traversed through the primordial asteroid belt, emptying it in the process and then repopulating it [START_REF] Raymond | The empty primordial asteroid belt[END_REF] with the material from both the inner and outer parts of the then solar system, as it reversed its course and migrated back. The inward migration of Jupiter would have resulted from the torques exerted by the gases still present in the protoplanetary disk (net inward torque due to the larger gas masses present beyond the orbit of the Jupiter), allowing the inward migration of the Jupiter that would have truncated the telluric planet forming region to an annulus extending to about 1 AU. As soon as Saturn grew up it also would have migrated inwards due to the same reasons, but faster than Jupiter, as Jupiter would have already cleared the material behind in the process of migrating inward. Once Saturn is close enough to Jupiter, they would have got locked in mean motion resonance and Jupiter would have migrated outward, due to the reversed net torque. This leads to a scenario where Jupiter, Saturn and other giant planets would have migrated back, setting the ground for the Nice model to work later on, once the accretion disk is depleted of gas and the formation of the telluric planets is complete. Therefore, this scenario is able to explain the radial mixing of material in the early solar system, thus implanting asteroid belt not only with S-type and M-type asteroids, but also with small bodies containing organics and volatiles formed beyond the snowline. This agrument is further supported by the analysis of the collected anhydrous interplanetary dust particles (IDPs) [START_REF] Vernazza | Interplanetary Dust Particles as Samples of Icy Asteroids[END_REF]. Complimentary to the study of meteorites which mostly contain thermally processed objects formed in the inner parts of the solar system, IDPs offer insights into the P-type and D-type asteroids and thereby to the primordial material from which the objects in the outer solar system such as giant planets satellites, TNOs / comets formed.

It would be remiss not to mention that there exist observations that the evoked dynamical models do not seem to answer. The asteroids of the Hilda family (Fig. 1.3) located at around 4 AU from the Sun between the main belt and the orbit of the Jupiter appear similar in composition with the Jupiter Trojans. Hildas belong in majority to P-type, followed by Dtype and C-type taxonomical classes. This shared homogeneity still remains to be explained in a resolute manner. Although the Nice model confidently explains the compositional nature of Jupiter Trojans, the composition of Hilda asteroids and their similarity with Jupiter Trojans appear puzzling. The dynamic models fail to explain the dichotomy between the orderly trend among the largest asteroids and the increased mixing of asteroid types at smaller sizes, evident from the Fig. 1.3 (DeMeo and Carry, 2014). Now it is clear that the solar system has been subject to a unique evolutionary path, sculpted by the myriad of physical, chemical and dynamical processes over the course of past 4.6 Gyr. Today, the solar system still remains dynamic, more in the areas where the small body populations are present. The asteroids in the main belt regularly undergo collisions and experience divers outcomes. The collisional fragments end up dynamically evolving and eventually impacting telluric planets. Similar events may occur if they get dynamically excited and reach the heliocentric distances corresponding to mean motion resonances with Jupiter. Following interactions with giant planets, TNOs and centaurs could also reach Chapter 1. Introduction the inner parts of the solar system, where they would become comets. As such, alike asteroids, the study of comets is fundamental to understanding how the solar system works and retrieve the compositional gradients of material and their distribution in the protoplanetary disk and decipher how they gradually evolved to the current solar system. Although the asteroid belt is relatively explored more, the existing knowledge of TNOs is minuscule. Extended surveys of cometary nuclei in the region of TNOs are required to better characterise them and retrieve their original birth places in the solar system. But, this task remains challenging due to their small sizes and distant orbits. The detection of comets by ground-based observations is favourable only when a TNO gets dynamically excited and reaches the inner part of the solar system and develop a coma, which leads to their detection. Numerous space missions and the advent of better instruments equipped with advanced techniques could attempt to give a response to remaining questions in planetary science. The remainder of this chapter is dedicated to explore the realm of small bodies in the current solar system and introduce important definitions and establish the framework, in order to set the base for studying a comet that bears the name 67P/Churyumov-Gerasimenko and that has been investigated for more than two years by the Rosetta space mission of the European Space Agency.

Small bodies of the outer solar system

Apart from the Sun, the eight planets with most of their satellites and dwarf planets, all other objects that orbit the Sun in the solar system fall into the category of small bodies of the solar system. The small bodies in the outer solar system are the centaurs and TNOs, which contain cometary nuclei. The above definition of small bodies was established according to the resolutions B5 and B6 at the 26 th General Assembly of the International Astronomical Union (IAU) 8 , held in Prague in 2006 in an attempt to avoid any potential confusion in classifying an object in the solar system and to establish a coherent distinction among planets, planetary satellites, dwarf planets and small bodies. The planets are located in the range of 0.4 to 30 AU from the Sun and the small bodies are found all the way to the edge of the solar system at some 200,000 AU and easily outnumber the planets and planetary satellites by many orders of magnitude. This wealth of small bodies can be characterised by their physical properties and orbital dynamics. In contrast to the asteroids that are principally populated in the main asteroid belt between the orbits of Mars and Jupiter, generally with a composition of non-volatile materials, comets got formed at heliocentric distances greater than the frost line of the early protoplanetary disk of the solar system and are rich in volatile ices. Each of these populations is discussed in detail in the following sections, with the exception of asteroids that have been already discussed in the previous section.

Centaurs and Transneptunian Objects (TNOs)

Centaurs are icy small bodies located between the orbits of Jupiter and Neptune while TNOs are icy small bodies located beyond the orbit of the Neptune. Their presence compared with that of the small bodies in the inner solar system implies that there had been a compositional gradient in the early solar nebula and that different bodies underwent different level of thermal processing depending on their heliocentric distances (Fig. 1.2) as discussed in the previous section.

Numerical modelling suggests that the populations of centaurs, TNOs and short-period comets (SPCs) are related. The orbits of centaurs are dynamically unstable for periods more than 1-10 Myr and end up impacting giant planets, being ejected from the solar system or evolving into either an SPC or an NEO [START_REF] Horner | Simulations of the population of Centaurs -II. Individual objects[END_REF]. The fact that the population of centaurs keeps being replenished, is attributed to the presence of the TNO population. This TNO reservoir is supposed to be the remnants of volatile-rich planetesimals (cometary nuclei), from which the giant planets formed in the outer solar system. Regular collisions between TNOs and gravitational instabilities due to the giant planets result in some TNOs to evolve their orbits, which lead them to end up between the orbits of Jupiter and Neptune, where they become a part of the transient centaur population. During their dynamically stable life time of 1-10 Myr, these centaurs or the small bodies that once were TNOs, face the aforementioned aftermaths depending on how the dynamics of the solar system plays out at the time their orbits become unstable. Following this, unless they get ejected out of the solar system, these icy small bodies start to reach the inner solar system, where, for the first time H 2 O ice on them receives enough solar flux for sublimation to occur, giving rise to a temporary thin atmosphere, called coma, leading them to be classified as a comet. In the presence of super-volatiles (CO, CO 2 ) on the body, sublimation activity could be detected earlier, at longer heliocentric distances, thus claiming for the cometary status much earlier. This goes on to show that an icy small body originally formed in the TNO region in the solar nebula could evolve to be a centaur and then finally into an SPC, provided that the right dynamic conditions are met. As such, one object, initially classified as an asteroid (absence of a coma), could later be reclassified as a comet upon the manifestation of a coma as the object undergoes dynamic orbital evolution. One such example is (2060) Chiron, the first discovered centaur, that was reclassified as 95P/Chiron upon the detection of its cometary activity.

The first TNO to be discovered was Pluto by Clyde Tombaugh in 1930, despite that fact that it was attributed the status of a planet immediately after its discovery. It took more than 60 years for the next TNO to be discovered, as David Jewitt and Jane Luu co-discovered (15760) Albion (provisional designation 1992 QB 1 ) in 1992 [START_REF] Jewitt | Discovery of the candidate Kuiper belt object 1992 QB1[END_REF]. Soon afterwards, followed the discovery of more TNOs and the abundance of such bodies with dimensions comparable to those of Pluto, which challenged its planetary status, thus eventually demoting it to the status of a dwarf planet in 2006, according to the new resolutions of the IAU, mentioned earlier in this section.

A classification of TNOs can be made as follows, depending on their heliocentric distances and orbital parameters.

Kuiper belt objects (KBOs)

KBOs are small bodies that are distributed at orbits generally located between 30-50 AU. They have less eccentricities and low inclinations to the ecliptic plane.

(a) Resonant objects : These are the small bodies mainly in 3:2 (plutions) and 2:1 (twotinos) mean-motion resonances with Neptune, resulting in their stability over the age of the solar system. Although they have perihelia inside the orbit of the Neptune, their mean-motion resonances help them preferably stay away from Neptune's gravitational perturbations. The unique characteristic about resonance is that it allows the cumulation of small gravitational forces, resulting in a larger effect over large time scales [START_REF] Malhotra | The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune[END_REF].

(b) Classical Kuiper belt objects : Synonymous with cubewanos, classical Kuiper belt objects do not undergo orbital resonance with Neptune and preserve their closeto-circular orbits whose semi-major axes are strictly bound (40 AU a 47 AU) as seen in the Fig. 1.6.

Scattered disk objects (SDOs)

1.2. Small bodies of the outer solar system 13 SDOs have dynamically unstable orbits with high eccentricities and high inclinations with the ecliptic plane. Their perihelia (30 AU q 38 AU) are located near the orbit of Neptune and consequently they gravitationally interact with Neptune, which results in their eccentricities to grow. As such, they subsequently end up with larger aphelia. This is why they appear to have semi-major axes spread in a wide range in Fig. 1.6. Thus, they become unstable over ∼Gyr time scales and get ejected from the scattered disk, as they are launched towards the Oort cloud, or into the inter-stellar medium or towards the inner solar system, where they could potentially become SPCs or Near Earth Objects (NEOs) 9 .

Detached objects

Unlike SDOs, detached objects have perihelia in such distances that they are generally out of the influence of the Neptune's gravity. Hence they are named 'detached' objects. It is common to find them referred to as extended scattered disc objects (E-SDO) as well. One noticeable property of this population is that they appear to have high values for arguments of the perihelion compared to other TNOs.

Oort cloud objects (OCOs)

Oort cloud is a sphere-like structure that is hypothesised to be located at the outer edge of the solar system, at the limits of the Sun's Hill sphere with respect to the closest stars. It is supposed to extend from 50,000 AU -200,000 AU forming what resembles a shell or a cloud and contain a reservoir of icy small bodies, or in other words, cometary nuclei that orbit the sun. Upon gravitational perturbations from passing-by stars or due to galactic tidal torques, some such cometary nuclei become unstable in their orbits in the Oort cloud and they either get completely ejected from the solar system or they reach the inner solar-system in very elliptic orbits where they are observed isotropically (coming from varying directions) at varying inclinations to the ecliptic plane. Once inside the inner solar system, they begin to develop a coma and display cometary characteristics due to sublimation of volatiles present on their nuclei. Thus, the comets coming from the Oort cloud fall into the category of long period comets (LPCs) usually with periods larger than 200 years, which will be dealt in detail in the next sub-section. About half of these LPCs are in retrograde orbits, which is consistent with a random distribution of cometary nuclei.

Although Oort cloud, itself or the individual cometary nuclei therein cannot be observed due to their large heliocentric distances, the presence of the Oort cloud can be inferred from the observation of LPCs. Analysing the available distribution of gravitational binding energies of the comets at the time, Jan Oort in 1950 [START_REF] Oort | The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin[END_REF] noticed that there were very few comets in hyperbolic orbits (negative binding energy with eccentricities > 1) while the vast majority of the comets corresponded to elliptic orbits around the sun (positive binding energy with eccentricities < 1). Among the family of comets on hyperbolic orbits, he noticed that they all were only barely hyperbolic, which he interpreted to be due to uncertainties arisen from inadequate observational data coming from a minute observational arc of the comet's orbit in question or to the fact that the comet might have had a fortuitous encounter with a giant planet which Chapter 1. Introduction would have resulted in a change of its orbit and hence eccentricity. Oort further observed a narrow and high peak among the bound comets, which he attributed to newly arriving comets in the inner solar system, because otherwise if they were to maintain regularly returning orbits with steady orbital parameters, it would not have been possible due to the gravitational scattering that would take place during encounters with giant planets, triggering changes in their binding energies and hence, distributing them randomly in the distribution of the binding energies without any noticeable peak. Thus, using this model and using the orbital elements of the observed comets, Oort reasoned the presence of a reservoir of small icy bodies that would spherically surround the solar system at the aforementioned heliocentric distances. Therefore, the observed peak in the binding energies was ascribed to the replenishing newly arriving comets following a gravitational perturbation in the Oort cloud. The spherical nature of this reservoir was derived from the isotropy of observations of the Oort cloud comets and their random inclinations to the ecliptic plane. The way in which these cometary nuclei became available in the Oort cloud is via the migration of giant planets, as they exchanged angular momentum with icy small bodies present in the Kuiper belt in the process, during the early stages of the solar system. This resulted in depletion of the reservoir of icy small bodies in the Kuiper belt as many of them got ejected into the inter-stellar space while some of them ended up in far-flung orbits, thus forming the shell-like structure, now referred to as the Oort cloud. Moreover, as a result of this planetary migration, some small icy bodies ended up as comets reaching inner solar system, thus impacting terrestrial planets, giving rise to the LHB, while some others can be seen as captured planetary satellites of giant planets (i.e. Hyperion of Saturn, Triton of Neptune, etc.) identifiable using their irregular shapes, retrograde orbits around the planet and high inclinations to the equatorial plane of the planet.

Comets

Comets are composed of rock, volatile ices, dust and different minerals which can collectively be called the cometary nucleus. They were initially referred to as dirty snowballs [START_REF] Whipple | A comet model. I. The acceleration of Comet Encke[END_REF]Whipple, 1951;Whipple, 1955) and as of early 2018 the model of comets has evolved a lot thanks to many findings from space missions that visited comets. Comets have very elliptical and parabolic orbits and they are difficult to be detected when they are away from the Sun and thus from the Earth, due to the typical dimensions of the nuclei (a few kilometers to 10 km).

Comets could originate from the TNO region, which is the region beyond (> 30 AU) the orbit of the Neptune. The Kuiper belt and the Oort cloud (beyond 50,000 AU) are sub-categories of the TNO region and are principally the main sources of the comets. Periodic comets (periods less than 200 years usually) with low inclinations to the ecliptic plane are known to originate from the Kuiper belt, whereas the long period comets with varying inclinations to the ecliptic plane are thought to be originating from the hypothesised Oort cloud [START_REF] Oort | The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin[END_REF] supposed to exist from 50,000-200,000 AU. The English polymath Edmond Hally was the first to recognise the periodic nature of the comets [START_REF] Halley | A synopsis of the astronomy of comets[END_REF] and subsequently, the comet whose observations he used to come up with its periodicity, got posthumously named after him as the Halley's comet. Halley's comet has a period of about 75 years and made its latest apparition in 1986 and was the first comet to be visited by human spacecraft (including Giotto which was an ESA mission and several others). From then onwards, several other comets have been visited by spacecraft as fly-by missions and in August 2014 the ESA spacecraft, Rosetta arrived at the comet 67P/Churyumov-Gerasimenko and performed an unprecedented and a detailed study during a period of more than 2 years, up to September 2016.

Definitions based on observations, composition and dynamics

Comets are solar system bodies that can fall under several definitions due to their unique characteristics. Basically, observations, composition and dynamics can each be used as a proxy to identify a body with cometary behaviour. Observationally, a comet is any celestial body that has a tenuous atmosphere, called 'coma' and additionally having a single tail or more, depending on the cometary activity level when it is in close proximity to the Sun, or in other words, when the comet is adequately close to the perihelion of its orbit. As the comets approach the perihelion, they begin to develop their comae as a result of the sublimation of the volatile ices in their nuclei, giving rise to their ephemeral atmospheres. Consequently, the on-going sublimation leads to a lot of surface erosion (as evidenced by Groussin et al., 2015a on the comet 67P/Churyumov-Gerasimenko) as more and more dust particles on the surface are ejected out by the drag force exerted by sublimating molecules [START_REF] Biver | Distribution of water around the nucleus of comet 67P/Churyumov-Gerasimenko at 3.4 AU from the Sun as seen by the MIRO instrument on Rosetta[END_REF][START_REF] Hansen | Evolution of water production of 67P/Churyumov-Gerasimenko: an empirical model and a multi-instrument study[END_REF]. This process liberates a large amount of dust, which is in the order of few microns of size and these dust particles temporarily end up in the coma, where they contribute to add to the brightness of the coma, which, in the first place, allows the observation and identification of the cometary nature of the body in question. The coma owes its brightness to the sublimated Chapter 1. Introduction molecules, radicals and dust particles that scatter away the incoming sunlight. Once the liberated dust particles arrive at the coma, they are pushed back by incoming solar photons, exerting on them a radiation pressure, which leads them to trail behind the comet forming what is known as the dust tail of the comet. In some cases, especially when the comet is close to the perihelion, large chunks of rock could get ejected and end up in heliocentric orbits, potentially leading to meteor showers if their orbits intersect that of the Earth. Since the dust particles keep scattering the sunlight, this phenomenon results in the observation of a long tail that trails behind the comet extending up to several tens of millions of km. In addition to this dust tail, comets could form another type of tail, called the plasma tail or the ion tail, which is the aftermath of the ionisation of previously sublimated gas molecules in the coma by ultraviolet solar radiation. The ions get de-excited by fluorescence, giving away visible photons in the process and this is how the ion tail gets its characteristic colours. The ion tail is directed radially away from the sun and can be of different colours depending on the ions present in the ion tail (i.e. blue colour due to CO + ions and green colour in the coma due to the Cyanogen (CN) 2 gas and diatomic Carbon (C) 2 ). The direction of the dust tail on the other hand, mimics the orbit of the comet as dust particles could conserve more of their original momentum when pushed back by the radiation pressure, compared to the ions present in the coma.

As evoked above, the identification of a comet by means of observations is very subjective to its visibility which depends on the sensitivity and angular resolution of the observing instrument. It is not rare that an object first classified as an asteroid gets reclassified as a comet following the detection of its coma by better instruments. Since the sublimation of ices in the cometary nuclei, which gives rise to the manifestation of their comae, depends on the heliocentric distances of the comets, their detection becomes difficult beyond about 5 AU in most of the cases. Besides, what may display a coma in present observations, could appear non-cometary to future observers as the comets could eventually run out of their volatile species in the nucleus and appear inactive, leading to a more asteroidal nature.

In terms of composition, a comet can be described as a small body containing a lot of volatile ices and complex organic species (Capaccioni et al., 2015). Generally, the main volatile component of the composition is H 2 O ice, followed by CO ice. Therefore, one could assume that all the objects beyond the snowline may contain volatile ices. Nevertheless, unless an object is probed by a spacecraft, ground based observations are not favourable in determining the nucleus composition of an object suspected to be a comet. In the absence of spacecraft based in-situ instruments, it is the manifestation of the coma that allows the characterisation of the composition of the comet by studying the abundance ratios of different molecular species present in the coma. However, this requires that the object in question be sufficiently close to the sun in order for a coma to be developed. Depending on the heliocentric distances of the comet, ∼ 3 AU, ∼ 13 AU and ∼ 120 AU, can be given as approximate distances inside which, H 2 O, CO 2 and CO volatile ices will respectively start to sublimate (Kramer, 2014 Ph.D. thesis).

Dynamically, comets represent a very active population among small bodies. Due to their very elliptical orbits compared to other small bodies, comets are observable in high angular resolution when they enter the inner solar system and pass through the perihelia of their orbits. Their elliptic orbits make them far-fetched and allow us to study (be it groundbased techniques or in-situ spacecraft missions) these objects that carry remnants of the constituents available at the formation of the solar system, which would otherwise remain far and be difficult to reach, as is the case with other TNOs. Sometimes, comets come across giant planets on their way around the solar system and gravitationally interact with them once they are inside the Hill sphere of such a giant planet, which leads to various outcomes. For instance, the encounter of the comet Shoemaker-Levy 9 with Jupiter in 1992 resulted in its disintegration and eventual collision with Jupiter in 1994 after becoming a family of temporary satellites of Jupiter. The comet 67P/Churyumov-Gerasimenko, which was originally a TNO, after having several close encounters with Jupiter, has presently ended up as a Jupiter-family comet (JFC), with a considerable modification of its aphelion from more than 30 AU to less than 6 AU.

Traditionally, comets have been principally placed under two categories : short period comets (SPCs), having an orbital period (P) of < 200 years and long period comets (LPCs), having P > 200 years. The SPCs then fall into two sub-categories : Jupiter-family comets (JFCs) with P < 20 years and Halley-type comets (HTCs) with 20 < P < 200 years. However, recent numerical simulations suggest that the above classification could sometimes be ambiguous, for a JFC comet could dynamically fall into different definitions during its dynamical lifetime [START_REF] Duncan | Dynamical Evolution of Ecliptic Comets, Comets II[END_REF]. In such cases, in order to identify whether the object in question has been the same, prior to a potential encounter with a giant planet, the Tisserand's parameter (T) can be computed for the object for the two interested epochs. Following is the definition of this dynamical quantity, T j , with respect to Jupiter in the restricted circular three-body problem, where a j is the semi-major axis of Jupiter; a, the semi-major axis of the object (comet); e, the eccentricity of the orbit of the object; i, the inclination of the object with respect to the orbital plane of Jupiter.

T j = a j a + 2 cos(i) a a j (1 -e 2 ) (1.1)
Although T j is expressed with respect to Jupiter since it has the greatest angular momentum in the solar system and hence being able to exert the greatest perturbation on small bodies, T j can also be calculated with respect to other giant planets as desired. If T j is the same or approximately the same for both epochs, it is assumed that the Tisserand's parameter is conserved and it can be concluded that it is most likely to be the same object. Thus, Tisserand's parameter offers a way to link between pre-encounter properties and post-encounter properties of an object. For example, JFCs generally have a Tisserand's parameter that ranges from 2 to 3, whereas asteroids typically have a T j value greater than 3. On the other hand, Tisserand's parameter for HFCs and LPCs varies between 0 and 2 (Fig. 1.7).

Active asteroids / Main belt comets (MBCs)

The population of active asteroids or MBCs has been recognised as a new domain [START_REF] Hsieh | A Population of Comets in the Main Asteroid Belt[END_REF] of small bodies in the solar system, with the progress of planetary science in recent decades. Their unique characteristics is that they have orbital parameters similar to those of asteroids in the main belt, while displaying physical properties that are similar to those observed in comets. This leads to an ambiguous interpretation of these small bodies. They have orbits located in the main asteroid belt (a < a j ), which means that they have approximately circular orbits like other asteroids, while they display comae and tails as comets

1.
3. What can be learnt from comets/ small bodies ?

do. In terms of Tisserand's parameter, they have T j > 3, indicative of an asteroidal nature with respect to orbital dynamics.

As far as the observations are concerned, some of these bodies have been reported to develop dust tails mainly due to the sublimation of volatile ices and impacts (cases of P/2010 A2 and (596) Scheila as reported by [START_REF] Jewitt | A recent disruption of the main-belt asteroid P/2010 A2[END_REF]Jewitt et al., 2011 respectively) that would expose sub-surface volatile ices, although less favourable, because their frequencies are low. 133P/Elst-Pizarro [START_REF] Hsieh | The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids[END_REF], the first small body to have been assigned to this new population, has displayed cometary activity in three consecutive perihelion passages, which suggests the existance of a sublimation-driven activity leading to mass loss. Although the possibility of impacts for the recurrent activity of 133P/Elst-Pizarro can be discarded, it is possible that the initial precursor was an impact that exhumed the subsurface volatile ices to be exposed and sublimated when its perihelion passages brought it at adequate heliocentric distances for the sublimation to take place. The dwarf planet (1) Ceres also falls into this category of peculiar small bodies, for H 2 O vapour has been spectroscopically detected around it [START_REF] Butterfield | Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes[END_REF][START_REF] Küppers | Localized sources of water vapour on the dwarf planet (1) Ceres[END_REF], which could be due to an outgassing activity of cometary nature. Then, with the data from NASA's Dawn space mission, evidence for H 2 O ice (De Sanctis et al., 2016;[START_REF] Combe | Detection of local H2O exposed at the surface of Ceres[END_REF][START_REF] Nathues | Oxo Crater on (1) Ceres: Geological History and the Role of Waterice[END_REF] was found on the surface of (1) Ceres, thus corroborating the mass-losing, ascribed to it earlier.

It is noteworthy that the empirical definition of active asteroids / MBCs takes into consideration the observations of activities which are mass-losing processes of small bodies and hence should not be confused with the nature of comets in the classical sense as described in the previous section. More observations will make more data available which will help further the understanding of these bodies as their known population will continue to grow. However, the volatile-rich nature of these objects could potentially be traced back to the region of TNOs by evoking the Nice model and planetary migration, whereby the TNOs are supposed to have been implated in the main belt and at the locations hosting Jupiter Trojans, currently classified under the taxonomical classes of D-type and P-type. Furthermore, as evident from the Fig. 1.8, (3200) Phaethon, (2201) Oljato and 107P/Wilson-Harrington are not part of the main belt, although they are part of this interesting population of active small bodies, which suggests the better applicability of the name 'active asteroids' rather than main belt comets.

What can be learnt from comets/ small bodies ?

For example, abundance ratios of volatile ices present in a comet could give clues to the origin and formation of comets, dating back to the same epoch about 4.6 Gyr ago. These abundance ratios reflect the how they ended up condensed and agglomerated into cometesimals in the cold parts of the early solar nebula and the kind of alterations that they underwent later on in the solar system. Because of this reason, the study of the relative abundance of volatile ices on comets is crucial to understanding the primordial outset of cometesimals. Therefore, by studying the small bodies which are the remains of the reservoir of material available for the formation of planets and planetary satellites, we could gain insight into the material that had been key to the formation of the solar system. Consequently, asteroids and comets have been at the forefronts of the proposals for space missions in recent decades Chapter 1. Introduction [START_REF] Barucci | Space missions to small bodies: asteroids and cometary nuclei[END_REF].

It is noteworthy that, the impacts of comets have played an important role in the evolution of life on Earth and forging the Earth with a diversity of species. Comets are thought to be the sources that brought organic ingredients that are vital to the life on Earth. One of the goals of cometary science is to understand to what extent, the materials in comets (e.g., water ice, organics) are similar to those found in Earth. For instance, the D/H ratio (relative abundance of deuterium atom, which is an isotope of hydrogen, to that of hydrogen atom) is a measure of the 'flavour' of the water and can be used as a tracer to compare the water present in the terrestrial oceans and water found elsewhere, being present on planetary atmospheres, comets, asteroids or other small bodies. This follows from the theoretical studies of the interstellar chemistry and corroborative astronomical observations of deuterium-rich water on envelopes surrounding protostars that indicate the abundance of deuterium in H 2 O molecules corresponding to D/H ratios ranging from 0.001 to 0.08 [START_REF] Hallis | D/H ratios of the inner Solar System[END_REF][START_REF] Ceccarelli | Deuterium Fractionation: The Ariadne's Thread from the Precollapse Phase to Meteorites and Comets Today[END_REF] at temperatures below 50 K. This yields that the water molecules present in the solar nebula would have probably had a D/H ratio in the above range and that later, when this deuterated water was introduced to the hot inner regions of the protosolar nebula (PSN), the presence of other hydrogen-bearing species would lead to thermophilic isotopic exchanges to occur, eventually resulting in lower D/H ratios in the order of 2 x 10 -5 [START_REF] Yang | The D/H ratio of water in the solar nebula during its formation and evolution[END_REF] in the inner parts of the protosolar nebula. However, in the outer parts of the protosolar nebula, where the temperatures are lower, aforementioned isotopic exchanges are less prominent. Therefore, the inherited nebular D/H ratios are likely to have received little to no variation and hence the D/H ratio of water in the protosolar nebula would have depended from the temperature of the surrounding, following a radial trend. This is why an in-depth understanding of D/H ratios of comets is essential to the interpretation of their birth places in the early solar system, complimentary to the D/H ratios measured on planets [START_REF] Bergh | The D/H ratio and the evolution of water in the terrestrial planets[END_REF]. Fig. 1.9 excerpted from Altwegg et al., 2015, com-pares the measurements of the mass spectrometer ROSINA aboard the ESA's Rosetta spacecraft that studied the JFC, 67P/Churyumov-Gerasimenko for more than two years. These results reflect the latest understanding of the flavour of water found in terrestrial oceans and other solar system bodies, precluding the idea that the water on Earth was delivered by comets like 67P. Ironically, apart from bringing the constituents to the organic-based life on Earth, cometary/asteroidal impacts are also known to bring catastrophe to the life on Earth, for mass destruction events like the Chicxulub impact resulted in the annihilation of many biological species and consequently leading to the thriving of mammals, some 66 Myr ago. Thereupon, stemming from this very dual nature of comets, studying comets has direct implications in modern astrobiological research.

The study of small bodies is also important to understand the processes and forces that are pronounced only on small bodies such as Yarkovsky and YORP effects. The former is due to the thermal radiation of a body upon being heated by the Sun and it results in a small thrust. Although the resulting recoil force is quite weak compared to gravitational forces that govern the orbital dynamics of a body, in small bodies, this effect could be observed as it gets accumulated over large time scales (several Myr to few Gyr) and can consequently modify the orbital configurations of small bodies. The Yarkovsky-O'Keefe-Radzievskii-Paddack effect, often abbreviated as the YORP effect is a combination of the aforementioned Yarkovsky effect and scattering of solar photons off the surface of a small body which leads to a torque on the rotating small body. As a result, the small body's spin state is affected as this effect gets integrated over time, leading to changes in the spin period and the obliquity of the small body. The disintegration of the asteroid P/2013 R3 [START_REF] Jewitt | Disintegrating Asteroid P/2013 R3[END_REF] observed by Hubble Space Telescope is thought to be an aftermath of the accumulated YORP torque. Therefore, comprehensive studies of these effects are essential to study the orbital dynamics of small bodies and especially for learning more about Potentially Hazardous Asteroids (PHA) such as (101955) Bennu which was once a Main Belt asteroid, that is now in an orbit that crosses the orbit of the Earth. Indeed, one of the objectives of the NASA mission OSIRIS-Rex that will arrive at the asteroid Bennu in 2018 is to study in depth how the Yarkovsky and YORP effects [START_REF] Bottke | In search of the source of asteroid (101955) Bennu: Applications of the stochastic YORP model[END_REF] work, in order to mitigate potential collision hazards in the future. In the light of cometary studies, activity sources such as outbursts could significantly alter the spin state of a small body, by excerting non-cancelling torques on themselves. A couple of examples can be given here. [START_REF] Samarasinha | Rotation of Comet 103P/Hartley 2 from Structures in the Coma[END_REF] report the non-principal axis rotation (triggering a tumbling motion) of the nucleus of the very active comet 103P/Hartley as observed by the NASA EPOXI mission. [START_REF] Bodewits | A rapid decrease in the rotation rate of comet 41P/Tuttle-Giacobini-Kresák[END_REF] investigate an interesting case of a rapid slowing-down of rotational rate of the comet 41P/Tuttle-Giacobini-Kresák by more than an order of magnitude, attributed to intense outgassing activity.

Space missions to comets

1985 marked an important year for cometary science as it was when the spacecraft International Cometary Explorer (ICE) passed through the ion tail of the comet Giacobini-Zinner at about 7860 km away from its cometary nucleus, becoming the first spacecraft to explore a comet as it did so on CO and OCS (Fig. 1.10). It further revealed an emission band located around 3.2 µm, which was interpreted to be due to complex hydrocarbon molecules. In addition, Halley's spectrum in the range of 6-12 µm hinted the presence of silicates with a signature akin to olivine. Among the five spacecraft, Giotto flew-by closest to the nucleus of the Halley's comet at a distance of 596 km, yielding unprecedented images and data of the nucleus, which, combined with the data from the other four missions, eventually led to broaden the understanding of comets (Fig.

1 .11). The gathered data helped determine the physical parameters of 1P/Halley, such as albedo, size and surface temperature and analysing the dust particles in the coma, the presence of elements such as C, H, O, N which formed organic compounds such as polymerised formaldehyde, was established [START_REF] Kissel | Composition of comet Halley dust particles from VEGA observations[END_REF]. The Giotto scientific camera [START_REF] Keller | First Halley multicolour camera imaging results from Giotto[END_REF] determined the dimensions of the nucleus of 1P/Halley to be about 14.8 x 7.4 x 7.4 km 3 and several studies that followed in the direction of abundance ratios of elements (C, H, O) confirmed that 1P/Halley was indeed a primitive small body of the solar system as these ratios matched well with those observed in the sun. Following the fly-by of Halley's comet, Giotto was re-routed to visit the comet 26P/Grigg-Skjellerup in July 1992. Although Giotto was able to get as close as 200 km from the cometary nucleus, no images could be acquired due to the failure of its imaging instrument during the previous fly-by of the Halley's comet.

Then, on 22 September 2001, NASA spacecraft Deep Space 1 made a fly-by of the comet 19P/Borelly and returned valuable scientific data to Earth. Deep Space 1 had earlier flownby the asteroid (9969) Braille, which was the primary target of the mission on 29 July 1999 and was an extended mission to validate new advanced technologies and encounter the comet 19P/Borelly, which consequently became the second comet to be closely encountered by a spacecraft. The images of the comet 19P/Borelly sent to Earth by Deep Space 1, were of higher resolution than those acquired by Giotto, which were the only images of a cometary nucleus obtained by a spacecraft at that time.

Shedding more light on cometary science, NASA spacecraft Stardust was the next space mission to encounter a comet. Stardust was launched on 7 February 1999 with the objective of collecting dust samples from the coma of the comet 81P/Wild (Wild 2) and cosmic dust and returning them to Earth for analysis, to become the first sample return mission. After flying past the asteroid (5535) Annefrank on 02 November 2002 on its journey to Wild 2, the spacecraft encountered the comet on 2 January 2004 and was able to get as close as 237 km from the nucleus during the fly-by. Apart from collecting the envisaged dust grains from the coma, Stardust managed to obtain detailed images of the nucleus of the comet Wild 2 [START_REF] Brownlee | Surface of Young Jupiter Family Comet 81 P/Wild 2: View from the Stardust Spacecraft[END_REF] at varying phase angles. Subsequently, the spacecraft delivered the samples to Earth on 15 January 2006 as the sample return capsule returned to Earth, successfully completing its mission. As the sample analysis followed, the unprecedented discovery of glycine (NH 2 -CH 2 -COOH), which is a simple form of an amino acid, fundamental to the process of protein synthesis, became one of the major highlights of the mission due to its obvious astrobiological implications. Shortly afterwards, given that the spacecraft still had 10 IKS being the initials for the IR spectrometer in Russian. Launched in July 2002, COmet Nucleus TOUR (CONTOUR) was a NASA mission that was planned to visit the comets 2P/Encke, 73P/Schwassmann-Wachmann (Schwassmann-Wachmann 3) with a potential third target of 6P/d'Arrest. Nevertheless, following an orbital manoeuvre that would have placed the spacecraft in a heliocentric orbit out of the geocentric orbit around the Earth, the contact with the spacecraft was lost in August 2002 and hence, the CONTOUR mission failed.

The NASA mission Deep Impact, launched in January 2005 had the objective of studying the interior of the comet 9P/Tempel, by dropping an impactor to its surface [START_REF] A'hearn | Deep Impact: Excavating Comet Tempel 1[END_REF]. In comparison to all the previous missions to comets, Deep Impact had the unique goal of ejecting the material off the surface of a comet and creating a crater, which would be studied in detail afterwards. The collision took place successfully on 4 July 2005, releasing more dust than ice, while increasing the brightness of the comet by a factor of six. As such, the impact site got ephemerally obscured by the released dust and resultant crater could not be observed in detail as expected. Although the crater could not be resolved with Deep Impact mission, soon afterwards, NASA approved an extension to the Stardust mission that got renamed Stardust-NExT and got re-routed to visit the comet 9P/Tempel, as mentioned earlier. Although several modifications on the terrains could be established by Stardust-NExT which arrived at the comet in 2011, the much sought-after crater could not be clearly identified, leading to the conclusion that either the resultant debris would have fallen onto the crater, thus camouflaging its features or the possibility of a smaller nested crater, surrounded by a shallow excavation crater [START_REF] Schultz | The Deep Impact crater on 9P/Tempel-1 from Stardust-NExT[END_REF], which would have been challenging to accurately identify.

Much like the initial Stardust spacecraft, upon finishing its primary mission, Deep Impact spacecraft too, underwent a renaming and got a new mission assigned to it as it had more fuel left at the end of the encounter with the comet 9P/Tempel. Thus, Deep Impact was renamed EPOXI and was tasked with studying the comet 103P/Hartley (DIXI -Deep Impact Extended Investigation) and exoplanets (EPOCh -Extrasolar Planet Observation and Characterization). Having returned to Earth at the end of 2007, after its encounter with the comet 9P/Tempel, freshly renamed EPOXI spacecraft made several fly-bys of Earth and made the closest encounter with the comet 103P/Hartley on 4 November 2014 at a distance of 694 km from the cometary nucleus. The acquired data allowed mission scientists to discern dust and gas jets on the nucleus and observe their surface features. Data analyses further revealed that CO 2 volatiles were a species that was more abundant than H 2 O on the comet 103P/Hartley [START_REF] A'hearn | EPOXI at Comet Hartley 2[END_REF].

The most recent cometary spacecraft mission was the ESA Rosetta, which, unlike all its predecessor fly-by missions, studied the comet 67P/Churyumov-Gerasimenko (67P) for an extended period of more than two years and delivered the Philae lander to its surface. Rosetta was launched in March 2004 and after a long journey of more than ten years, during which it made two fly-bys of asteroids [START_REF] Kossacki | Comet 67P/Churyumov-Gerasimenko: Hardening of the sub-surface layer[END_REF] Lutetia and (2867) Šteins, Rosetta arrived at the comet 67P in August 2014. Rosetta was a scientific laboratory equipped with 11 instruments to carry out an unprecedented scientific study of the comet 67P and further the scientific know-how on fundamental questions about comets. Since following sections of this chapter are dedicated to the comet 67P and the Rosetta mission, they shall not be elaborated here.

As a concluding remark for this section, it is noteworthy that a sample-return mission, named CAESAR (Comet Astrobiology Exploration Sample Return) to comet 67P [START_REF] Squyres | The CAESAR New Frontiers Mission: 1. Overview[END_REF] was proposed in 2017 to NASA's New Frontiers program mission 4 and it was selected as one of the two finalists to receive funding for further development. The final selection is scheduled at the end of 2019, where NASA will green-light one of the missions. If CAESAR will be selected, its launch could take place in 2024-2025 with the delivery of samples to Earth pending in 2038.

Finally, the Table 1.1 summarises the cometary space missions that have taken place to date. -3) and got renamed ICE upon the assignment of the cometary mission. 1 ISAS is now a part of the Japanese Space Agency (JAXA). 2 Shortly following its launch, contact to the spacecraft was lost during an orbital injection manoeuvre on 15.08.2002 leading to the mission failure. Hence, the three comets referred to in the text were not encountered. † Indicates the spacecraft when the original mission name got changed upon being assigned to a secondary target/mission.

The comet 67P/Churyumov-Gerasimenko and the Rosetta mission

At the time when the Rosetta definition study report, also known as the 'red book' was presented to ESA for approval, only three small bodies of the solar system had been explored by space missions (Barucci and Fulchignoni, 2017). The first was the comet 21P/Giacobini-Zinner explored by the NASA-led International Cometary Explorer flying-by its nucleus at a distance of about 7800 km in September 1985 and the second was the comet 1P/Halley that was explored by multiple spacecraft called 'Halley armada' in March 1986, where ESA's Giotto spacecraft had the closest encounter with the comet at 596 km away from the nucleus. The third was the main belt asteroid (951) Gaspra, which became the first asteroid to be visited by a spacecraft as the Galileo spacecraft flew-by it on 29 October 1991, on its way to study the Jovian system. Rosetta was thus to be a milestone project to shred more light to population of small bodies, especially the comets, in order to further the understanding of the formation of the early phases of the solar system. In 1991, ESA chose the Rosetta mission as the Planetary Cornerstone mission of its Horizon 2020 program and the planned mission included a rendezvous mission with a comet, one or two fly-bys of asteroids plus a delivery of a lander to the surface of the comet. The entire mission was pivoted under three main scientific rationales.

• The close link between the small bodies and the early planetesimals through in-situ analysis of primitive material.

Chapter 1. Introduction

• Investigation of physical and chemical processes that are specific to comets and asteroids.

• Furthering of the understanding of small bodies that preserve a wealth of knowledge about the origin and the evolution of the solar system.

On 02 March 2004, Rosetta was launched from Kourou, in French Guyana to rendezvous with the comet 67P/Churyumov-Gerasimenko, which would take about 10 years of space travel. During this journey, Rosetta made several gravity-assist maneuvers using Earth and Mars and flew-by the asteroids (2867) Šteins and ( 21) Lutetia, respectively in 2008 and in 2011. The very first data of the target comet 67P/Churyumov-Gerasimenko came from the OSIRIS imaging instrument, revealing the presence of a bi-lobed nucleus. As Rosetta got closer to the comet, more details about the nucleus could be resolved and other scientific instruments were also systematically initiated for data acquisition. Thus, in early August 2014, after more than a decade of interplanetary travelling, Rosetta started to execute its primary scientific mission, in order to respond to many scientific questions stemming from the aforementioned three main scientific rationales.

At the time of Rosetta's arrival to the comet 67P, it was on the in-bound phase of its orbit around sun, reaching its perihelion which would take place in August 2015. As a matter of fact, the decreasing heliocentric distances meant that the cometary activity was rising, which might pose problems to the spacecraft, should the spacecraft were to stay close to the nucleus. Therefore, the very first data of the comet involved deriving its physical properties and characterising the geomorphology of the nucleus as Rosetta was to deliver its Philae lander to the surface, before intense cometary activity would hinder its deployment. This involved a careful mapping of the nucleus and selecting a potential landing site before the cometary activity rose to impede scientific plans. Closer orbits to the surface permitted OSIRIS camera to look out for the ideal landing site and at the same time to acquire a lot of images which would then help reconstruct a shape model (Preusker et al., 2015;Jorda et al., 2016) of the comet, being very important in orbital dynamics of both Rosetta and Philae around this irregular bi-lobed object as the gravity field was very far from being uniform.

On 12 November 2014, Philae lander was released from Rosetta, to follow a descent trajectory of about 7 hours. Following this ballistic descent, Philae landed in the region named Agilkia, as planned, but could not manage to deploy the harpoons that were supposed to fix it to the cometary surface. As a result, Philae bounced back and after several more jumps, came to a stand still in an arbitrary landing site, known as 'Abydos'. On 15 November 2014, Philae entered into its hibernation mode, thus ceasing to nominally operate as the illumination conditions were not sufficient for it to use the solar panels to generate energy. Although Philae's eventual location was narrowed down to a certain area, it was not until September 2016, that its location was retrieved by OSIRIS images (Fig. 1.12). Nevertheless, important scientific discoveries were made from the data that Philae did return before it went into hibernation. These include the fact that the material on the surface forming a stiff crustlike structure while the inside material is fluffy [START_REF] Knapmeyer | Structure and elastic parameters of the near surface of Abydos site on comet 67P/Churyumov-Gerasimenko, as obtained by SESAME/CASSE listening to the MUPUS insertion phase[END_REF], the non-magnetic nature of the cometary surface [START_REF] Auster | The nonmagnetic nucleus of comet 67P/Churyumov-Gerasimenko[END_REF], derivation of thermal and mechanical properties [START_REF] Spohn | Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko[END_REF] of the near-surface layers of the nucleus, local porosities in the scales of about 75%-85% [START_REF] Kofman | Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar[END_REF], etc. The early observations of OSIRIS instrument allowed the characterisation of a cometary nucleus in an unprecedented manner. The bi-lobed nucleus of the comet 67P was rich in geological features with vivid morphologies (Sierks et al., 2015;Thomas et al., 2015;El-Maarry et al., 2015). The imaging data further revealed that some of the regions11 like Hapi and Seth (Fig. 1.13) were active as early as August 2014, as they demonstrated outgassing from dust-rich areas and pits. This corresponded to about 3.6 AU heliocentric distance and indicated that the sublimation of volatiles had already been taking place, which would later contribute to the formation of a cometary atmosphere (coma), as the comet would be reaching the perihelion. OSIRIS further identified exposed bright features (Pommerol et al., 2015;Oklay et al., 2016;Deshapriya et al., 2016;[START_REF] Deshapriya | [END_REF] and attributed them to the presence of H 2 O on the comet, depending on their spectrophotometric nature and long-term stability, despite on-going sublimation.

The initial data from the VIRTIS spectro-imager led to the identification of a diurnal cycle of H 2 O , (De Sanctis et al., 2015), where the volatile H 2 O ice found on the sub-surface is sublimated from the locally day-lit side of the comet and some of the resulting H 2 O vapour re-condenses as it gets deposited on the surface which has low thermal inertia, during the local night. This phenomenon results in the manifestation of frost at the dawn at the local sunrise, followed by the quick sublimation of H 2 O frost. Hence, this cyclic process would continue as long as the comet receives adequate solar irradiation to sublimate away the H 2 O ice/ frost. Apart from this, one of the core findings of VIRTIS was the presence of omnipresent refractory organic material (Capaccioni et al., 2015) on the surface of the comet, with a strong absorption band at 3.2 µm. The individual species of this organic component, however is still under the process of interpretation, with the support of laboratory measurements (Quirico et al., 2016;[START_REF] Rousseau | Laboratory simulations of the Vis-NIR spectra of comet 67P using sub-micro meter sized cosmochemical analogues[END_REF]. VIRTIS further studied the exposed bright features on the surface on the comet and spectroscopically identified H 2 O (Filacchione et al., 2016a;Barucci et al., 2016) and CO 2 (Filacchione et al., 2016b) ices on the surface of the comet, whose presence is highly subject to both diurnal and seasonal effects on the comet.

The oblique rotational axis of the comet 67P, results in long summers in the northern latitudes with short intense summers in the southern latitudes in its present orbit around the Sun. The Fig. 1.16 shows the variation of the sub-solar latitude of the comet with the varying heliocentric distance from the beginning of August 2014 up to the end of September 2016. As such, the comet 67P experienced the spring equinox in the southern hemisphere in early 2015, which meant the illumination on the southern latitudes of the comet, leading to identification of new regions. Therefore, following Rosetta lore, which is based on the names of Egyptian deities to name geological regions on the comet, the existing nomenclature was extended to 26 regions. Fig. 1.17 and Fig. 1.18 present the regions located respectively at the northern and the southern latitudes of the comet.

The continued observations further revealed the two lobes to be originally two different objects [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF] that would have incorporated themselves together, resulting in the bi-lobed comet as seen by Rosetta. Davidsson et al., 2016 demonstrate that the formation of these two lobes took place at an epoch when the solar system was about 3 Myr old and that they formed hierarchically from the material that remained after the TNOs were formed. As the comet approached the perihelion, a lot of cometary activity (Vincent

Instruments aboard Rosetta

In this subsection, 11 scientific instruments aboard the Rosetta spacecraft as well as Philae lander are briefly presented. As seen in the Fig. 1.19, the bus of the spacecraft is a rectangular parallelepiped, with dimensions of 2.0 m x 2.1 m x 2.8 m, which weighs 3064 kg, including the scientific payload, the Philae labder (100 kg) plus the fuel mass (1719 kg) 12before the launch. Rosetta is powered by two solar arrays fixed on either side of the spacecraft, with each composed of 5 solar panels (2.3 m x 2.7 m), yielding a total surface of 64 m 2 . The spacecraft with the solar panels deployed, measures 32 m end-to-end and the solar arrays are capable of producing between 850 Watts at 3.4 AU and 1500 Watts at the comet's perihelion at 1.24 AU, which correspond respectively to the Rosetta's approach to the comet 67P during its in-bound orbit and to the perihelion passage of the comet.

The 11 scientific instruments on-board Rosetta, with a brief description are listed below in the alphabetic order, along with their respective principal investigators.

• Alice : An ultraviolet imaging spectrometer, which studied gaseous species in the coma and the tail of the comet and measures the production rates of H 2 O , CO 2 and CO.

Principal Investigator: Joel Parker, Southwest Research Institute, Boulder, Colorado, USA.

• Principal Investigator: Alain Herique, Institut de Planétologie et d'Astrophysique de Grenoble, Grenoble, France.

• COSIMA : Cometary Secondary Ion Mass Analyser was a mass spectrometer, aimed to collect and study the characteristics and composition of dust grains emitted by the comet, using an optical microscope and an ion gun as the irradiation source.

Principal Investigator: Martin Hilchenbach, Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany

• GIADA : Grain Impact Analyser and Dust Accumulator collected in an aerogel, the dust grains coming directly from the nucleus as well as the dust grains coming from other directions due to the dust-photon interaction and measured the parameters like their mass, momentum and velocity distribution.

Principal Investigator: Alessandra Rotundi, Università degli Studi di Napoli "Parthenope", Naples, Italy.

• MIDAS : Micro-Imaging Dust Analysis System was an atomic force microscope to study the particles in dust environments, shredding light on the size, shape and volume of dust particles.

Principal Investigator: Mark Bentley, Institut für Weltraumforschung, Graz, Austria.

• MIRO : Microwave Instrument for the Rosetta Orbiter was used to study the abundances of major gases in the coma and the nucleus sub-surface temperatures.

Principal Investigator: Mark Hofstadter, Jet Propulsion Laboratory, Pasadena, California, USA.

• OSIRIS : Optical, Spectroscopic and Infrared Remote Imaging System, composed of two imaging units to study the nucleus and coma respectively with broad band and narrow band filters ranging from near infrared to the near ultraviolet wavelengths. The heart of this doctoral work is based on the data coming from OSIRIS and the instrument is presented with a dedicated section in the next chapter.

Principal Investigator: Holger Sierks, Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany.

• ROSINA : Rosetta Orbiter Spectrometer for Ion and Neutral Analysis was a mass spectrometer to study ions and neutral species in the coma with one of the main objectives to determine the D/H isotope ratio of H 2 O of the comet to constrain the 'flavour' of the water of the comet.

Principal Investigator: Kathrin Altwegg, Universität Bern, Switzerland.

• RPC : Rosetta Plasma Consortium, equipped with several magnetometers and ion sensors, studied the physical properties of the nucleus, examined the structure of the inner coma, monitor cometary activity, and investigated the comet-solar wind interaction.

Investigators:

Hans Nilsson, Institutet för rymdfysik, Kiruna, Sweden James Burch, Southwest Research Institute, San Antonio, Texas, USA Anders Eriksson, Institutet för rymdfysik, Uppsala, Sweden Karl-Heinz Glassmeier, Technische Universität, Braunschweig, Germany Pierre Henri, Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, Orléans, France Christopher Carr, Imperial College of Science, Technology and Medicine, London, United Kingdom

• RSI : Radio Science Investigation was an experiment to measure the mass and gravity of the cometary nucleus and deduce its density and internal structuring, using radio waves.

Principal Investigator: Martin Pätzold, Rheinisches Institut für Umweltforschung an der Universität zu Köln (RIU-PF), Cologne, Germany.

• VIRTIS : Visible and Infrared Thermal Imaging Spectrometer was composed of two optical heads: VIRTIS-M imaging spectrometer and VIRTIS-H spectrometer, respectively to determine the surface composition and physical conditions of the nucleus and determine the species present in the coma. Since some major results derived from this instrument will be discussed in this work and the instrument is briefly presented in Chapter 3.

Principal Investigator: Fabrizio Capaccioni, Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy.

Instruments aboard Philae

The 10 instruments on the Philae lander (1.20) are briefly presented below in the alphabetic order, along with their respective principal investigators. The reader is referred to [START_REF] Bibring | The Rosetta Lander (Philae) Investigations[END_REF] for a detailed report of Philae instruments.

• APXS : Alpha X-ray Spectrometer was an experiment to detect α particles and X-rays from the surface of the comet, to yield insight about the elemental composition of the cometary surface.

Principal Investigator: Göstar Klingelhöfer, Johannes Gutenberg-Universität, Mainz, Germany

• ÇIVA : This was a setup of six identical micro-cameras to take panoramic pictures of the cometary surface. ÇIVA was equipped with a spectrometer to study the composition of samples collected from the surface.

Principal Investigator: Jean-Pierre Bibring, Institut d'Astrophysique Spatiale, Université Paris Sud, Orsay, France. • CONSERT : Comet Nucleus Sounding Experiment by Radiowave Transmission, the counterpart of the experiment, equipped with a transponder and installed on the Philae lander, to investigate the internal structure of the nucleus by the propagation of radio waves between the Rosetta spacecraft and the Philae lander.

Principal Investigator: Alain Herique, Institut de Planétologie et d'Astrophysique de Grenoble, Grenoble, France.

• COSAC : Cometary Sampling and Composition experiment was set up to detect and characterise complex carbon-bearing molecules of the cometary surface by means of acquiring in-situ samples and combusting them and analysing the resulting gases. As such, COSAC consisted of a gas chromatograph and a time-of-flight mass spectrometer (TOF-MS) to implement its analyses.

Principal Investigator: Fred Goesmann, Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany.

• PTOLEMY : With a functionality similar to that of COSAC, PTOLEMY was targetted to analyse gases derived from samples collected in-situ. Additionally, it was capable of studying the cometary atmosphere by sampling volatile gases and deriving accurate measurements of isotopic ratios of light elements (i.e. C, N, O).

Principal Investigator: Ian Wright, Open University, Milton Keynes, UK.

• MUPUS : Multi-Purpose Sensors for Surface and Subsurface Science was an experimental setup that consisted of several temperature sensors attached to 35 cm long penetrator, that was designed to be deployed away from the landing module and to record the sub-surface temperature as the penetrator would be hammered into the ground. Using these data it was supposed to investigate the density, thermal and mechanical properties of the surface, such as porosity, thermal inertia, etc.

Principal Investigator: Tilman Spohn, Institut für Planetenforschung, Deutsches Zentrum für Luft-und Raumfahrt, Berlin, Germany.

• ROLIS : Rosetta Lander Imaging System was a CCD imaging system to acquire increasingly high-resolution images of the landing site during Philae's descent. Once landing has taken place, ROLIS was supposed to image beneath the lander by means of using light emitting diodes functioning at several wavelengths. It was also tasked to support other instruments such as APXS and MUPUS by imaging their sampling sites to obtain complimentary contextual data.

Principal Investigator: Stefano Mottola, Deutsches Zentrum für Luft-und Raumfahrt, Berlin, Germany.

• ROMAP : Rosetta Lander Magnetometer and Plasma Monitor was an experiment composed of a magnetometer and a plasma monitor to study any potentially residual local magnetic field and the plasma environment through comet/solar-wind interaction.

Principal Investigators: Hans-Ulrich Auster, Technische Universität, Braunschweig, Germany István Apáthy, KFKI , Budapest, Hungary.

• SD2 : Sample and Distribution Device was an amalgamation of a drilling mechanism and a suite of ovens to heat the samples. SD2 was capable of penetrating to about 230 mm below the surface and securing samples whose resulting gases, once heated, would be distributed to ÇIVA, COSAC and Ptolemy for eventual gas analyses. Hence, it could heat samples to ∼180 o C and ∼800 o C in its ovens, depending on the requirements of the corresponding end-instrument.

Principal Investigator: Amalia Ercoli-Finzi, Politecnico di Milano, Milan, Italy.

• SESAME : Surface Electric Sounding and Acoustic Monitoring Experiment combines three instruments that were created to function mutually to address the question of the formation of the comets. The three instruments are:

1. CASSE : Cometary Acoustic Surface Sounding Experiment was aimed to study the surface and subsurface of the comet 67P by means of sensors that were integrated to the feet of the Philae lander. CASSE had a bi-fold functionality as it would record the noise produced within the nucleus of the comet, thus being analogous to a seismometer and would act as a sonar by generating sound waves that would pass through the nucleus in order to get insights about the layering and voids inside the comet.

Principal Investigator : Martin Knapmeyer, German Aerospace Center, Institute of Planetary Research, Asteroids and Comets, Berlin, Germany.

2. PP : Permittivity Probe consisted of 5 electrodes attached to Philae: 3 to each of its feet, one to APXS and the other to the penetrator of MUPUS. These transmitter electrodes were supposed to send an electric current through the surface of the comet to understand the electrical conductivity of the surface, which would then be important to determine the presence and abundance of sub-surface H 2 O .

Principal Investigator : Walter Schmidt, Finnish Meteorological Institute, Helsinki, Finland 3. DIM : Dust Impact Monitor would record the impact of cometary particles. These particles were supposed to be originally volatilised and ejected from the surface of the comet and eventually return to the surface in the event of inability to surpass the escape velocity of the comet. The measurements could be used to estimate the velocity at which the particles were originally ejected from the surface of the comet.

Principal Investigator : Harald Krueger, Max-Planck-Institute for Solar System Research, Göttingen, Germany Principal Investigator of the SESAME consortium : Martin Knapmeyer, German Aerospace Center, Institute of Planetary Research, Asteroids and Comets, Berlin, Germany.

Chapter 2

Method of Data Analysis T he acronym OSIRIS refers to Optical, Spectroscopic and Infrared Remote Imaging Sys- tem, embarked on the Rosetta spacecraft. A complete documentation on its technical specifications and details including OSIRIS lore can be found in Keller et al., 2007 as only selected technical details are discussed here. It is noted that the spectroscopic and infrared functionalities of the initial design of OSIRIS were not eventually realised due to budgetary constraints, despite the original acronym remained unchanged. It is also mentioned that some of the content on this chapter is based on internal documents prepared by the OSIRIS scientific team.

OSIRIS imaging instrument

Comprising of two scientific cameras: the Narrow Angle Camera (NAC) and the Wide Angle Camera (WAC), the OSIRIS imaging instrument was one of the main scientific instruments aboard the Rosetta spacecraft, that was crucial throughout the mission. OSIRIS cameras were designed to complement each other and to shed light on a wide set of questions about comets. These include, but are not limited to:

• Characterising physical (size and shape, albedo, density, porosity, rotational state, etc.) and compositional properties of the cometary nucleus, in order to link them to those of the protoplanetary disk, where they formed initially.

• Derive photometric properties (albedo, phase curve, opposition effect, reflectance properties of the surface) of the cometary nucleus.

• Verify the applicability of existing models of cometary nuclei and refine them with the new results from Rosetta.

• Identifying surface inhomogeneities of the nucleus at varying scales and their comparison to already studied/known cometary nuclei.

• Investigating the evolution of the cometary surface and activity along its orbit around the Sun, specially at the perihelion.

• Evolution of comets, especially when they are close to the perihelion and how such changes affect the observed properties of the comets.

• Further the understanding of sublimation and processes through which the dust gets released.

• Understand the changes of dust-to-gas ratios.

• Understand the processes at work in the inner coma, closer than few cometary radii.

• Monitor the activity and characterise active regions. The NAC, optimised with a better spatial resolution than the WAC, was intended for the observation of the cometary nucleus and studies on the composition, colours and geological morphology, while the WAC was aimed at studies on the coma, dust particles and to give a contextual perspective for NAC observations with its wide field-of-view. In contrast to wide bandpasses of NAC, WAC had narrow bandpasses to be sensitive to study the different gaseous species present in the coma (Fig. 2.2). Basic characteristics of both NAC and WAC are summarised in Table . 2.1 and their perspectives are given in Fig. 2.1 along with their optical designs. NAC contains 11 wide-band filters assembled in two filter wheels, while WAC contains 14 filters, majority of which are narrow-band filters. Alternatively, as a redundancy to NAC, WAC further included a filter (filter R, centred at 629.8 nm) of wide bandpass with the purpose of studying the cometary nucleus and asteroid targets during the fly-bys.

OSIRIS image calibration

Calibrations are essential measures for all the photometric systems embarked on spacecraft, in order to be able to meaningfully interpret the observations they are making. This is an action-item that must be performed at regular intervals during the space mission to guarantee the consistency of the instrument and to perform scientific evaluations. The entire calibration process could be a sophisticated process which involves both ground-segment and space-segment operations. Thus, calibration of OSIRIS data requires several corrections.

• Bias and dark current removal

These corrections are related to the CCD (Charge-Coupled Device) that is used during the image acquisition. Bias corresponds to the individual pixel-to-pixel variations of offset in the CCD and hence needs to be removed. As this bias varies over the CCD, the correction is achieved by creating a bias frame which will be removed from all the other images. A bias frame is created by recording an image with a zero-second exposure time. Thus, the CCD is not exposed to the light by keeping the optical system closed with the shutter and the measurement is the bias. Usually several bias frames are recorded and their average is used as the 'master bias frame' to de-bias the other images. This averaging ensures that the signal-to-noise (S/N) ratio of the master bias frame is good enough that it does not introduce undesired noises to the corrected image. In the case of OSIRIS cameras, the bias is very stable over the CCD, with almost no pixel-to-pixel variations for a given operational mode and a temperature. Hence, the average of all the pixels is taken as the bias value, which is then scaled to the CCD temperature (a correlation between the bias and the CCD temperature has been found) of the image needing correction and then subtracted from the image.

The dark current is a phenomenon that arises due to the stochastic generation of electrons and holes in the depletion region of the semi-conductor material used for the CCD. Their random motion results in a thermal component and consequently extra electrons get added to the pixels in the CCD, thus making it a nuisance for the interpretation of the image. Therefore, the CCD is required to be maintained in cold temperatures so as to minimise such motion and reduce the residual electrons being added to the pixels. In order to remove the dark current from an image, it is necessary to create a dark current frame. This is achieved by recording an image with an exposure time at a temperature similar to the images that need to be processed while the optical system is closed with the shutter. Since dark current depends on time, it is of great importance to have the exposure times of dark current frames matching the exposure times of images that need to be corrected. The typical procedure is to acquire several dark current frames and average them to obtain a master dark frame per a given exposure time. It is noted that the dark current removal takes place after the bias removal. In long exposure images, some pixels can accumulate increasing amounts of residual electrons due to the dark current and lead to 'hot pixels'. Such hot pixels could often get mixed with background stars in astronomical observation and thus, increase the chances of misinterpretation and sometimes could result in image saturation. They are to be closely monitored and removed with dark current. The dark currents of OSIRIS images are very negligible even at long exposure times and are much smaller than the read-out noise at operational temperature, that they are not subtracted from the images. Nevertheless, they are periodically recorded in order to monitor their behaviour.

In practice, during the acquisition of bias and dark images for OSIRIS cameras, the door is closed and optical path is blocked by two filters whose bandpasses do not overlap. Since the door is not sealed completely, but only closed, this extra cautious step of using two filters to block, is implemented to minimise the external photons from entering the CCD. A good combination of two filters for example, would be F15 and F71 which have bandpasses in near-UV and near-IR wavelengths. Finally, the acquired bias and dark current images are subsequently inserted in the OSIRIS standard data reduction pipeline, which contains the programs OsiTrap and OsiCalliope1 .

• Flat-field correction

The final correction related to the CCD is the flat-field correction. This arises from the fact that the sensitivity of each pixel varies over the CCD due to two main concerns.

The arrangement of optics of the instrument could result in a non-uniform transmission of photons across the entire CCD due to a process known as vignetting (a reduced throughput close to the edges of the CCD) or due to the presence of dust on the CCD or the mirrors (an off-axis optical system is hence preferred in many cases as is the case for both OSIRIS NAC and WAC). The other concern is the variable quantum efficiency of individual pixels across the CCD even though the arrangements of the optics are in a favourable condition. Therefore, a correction becomes necessary to mitigate this effect and flat-field frames are required for that. This is achieved by using a homogeneous source of illumination with a sufficient exposure time to get a high signal-to-noise ratio. A master flat-field could be obtained by averaging several flat-field frames and it will be used for the correction. Finally, the correction is performed by dividing the image that is already corrected for bias and dark current, by the master flat-field. Since flat-field is related to the optics of the system, an individual master flat-field should be created for each filter of the instrument.

As for the case of both NAC and WAC, flat-field frames cannot be obtained in-flight due to the inability to have a lighting source that would homogeneously illuminate all the pixels. As such, this has been done in ground prior to Rosetta's launch, using an integrating sphere. The inside of the integrating sphere is coated by a homogeneous high reflective material in order to obtain a uniform illumination condition. The illumination sources were the calibrated halogen lamps for the flat-fields of filters corresponding to near-IR/ visible wavelengths and the calibrated Xenon lamps for the flat-fields of filters corresponding to UV wavelengths. Once the OSIRIS cameras are spaceborne, reference lamps acting as illumination sources, are used to monitor the transmission of the optical system and the results are constantly compared with the flat-field frames that have been obtained at the ground-based laboratory beforehand. They are further compared with the flat-fields that are being regularly obtained in the ground-based identical model of the instrument, in order to check for any potential degradation in space. This is done by using the rear side of the lid of the front door of the optical system which behaves as a diffusing screen. It is illuminated by two redundant sets of two Tungsten lamps acting as the light sources, thus enabling the acquisition of reference images that would be compared with flat-field frames recorded in the ground laboratory (Keller et al., 2007). Hence, the used flat-field frames come directly from the ground-based laboratory.

• Correction for the geometric distortion Both NAC and WAC inherit a significant amount of geometric distortion on the images due to their off-axis optical designs which lead to non-rectilinear projections of rectilinear components on the images. However, it is possible to effectively counter this problem by adopting a calculated or measured two dimensional distortion function. Thus, camera distortions are measured or calculated over the full field-of-view and these distortion values are fitted to a two dimensional, third order polynomial.

Next, the correction is achieved by re-sampling the original pixels according to this polynomial. For OSIRIS cameras, the corresponding distortion correction parameters were derived during the ground calibration and in-flight calibration sequences.

• Radiometric calibration

In order to be able to calibrate the images radiometrically and convert DNs (digital numbers) value of a pixel to radiometric units (W m -2 nm -1 sr -1 ), absolute calibration factors are needed. As conventional, OSIRIS cameras achieve this by observing standard photometric stars such as Vega (Alpha Lyrae), 16 Cyg B (a solar-type star with low flux in UV wavelengths) and Zeta Cassiopeiae (for the calibration of near-UV filters as this star has good flux in UV wavelengths) to perform aperture photometry. Images need to undergo all the aforementioned corrections beforehand followed by a normalisation by the division of their respective exposure times. Then, aperture photometry is carried out on the selected stars to extract corresponding stellar fluxes. It is noteworthy that the fluxes (F star,measured ) are recorded in DN per second at this stage with respect to the given filter.

Afterwards, known tabulated spectra (F star,c ) of the respective stars are used to extract their fluxes (in W m -2 nm -1 ) at the centre wavelength of each filter and integrated through each filter bandpass. For Vega, the spectrum is obtained from the Hubble Space Telescope (HST) catalogue, while the solar spectrum scaled to the magnitude of 16 Cyg B is used for 16 Cyg B , since it belongs to the same spectral class as the Sun.

The absolute calibration factors for a solar-type star are defined as follows, where k is the pixel size in steradians. It is noted that k N AC = 3.54744 × 10 -10 sr and k WAC = 9.91055 × 10 -9 sr.

C solar = k × F star,measured F star,c [sr] × [DN/s] [Wm -2 nm -1 ] (2.1)
In case of aperture photometry carried out with non-solar-type stars such as Vega, the tabulated spectrum of the star should be scaled to the solar spectrum in order to obtain the absolute calibration factors, as follows.

C non-solar = C solar × F star,c F Sun,c × F Sun (λ)T(λ)dλ F star (λ)T(λ)dλ (2.2) C non-solar = k × F star,measured F star,c × F star,c F Sun,c × F Sun (λ)T(λ)dλ F star (λ)T(λ)dλ (2.3) C non-solar = k × F star,measured F Sun,c × F Sun (λ)T(λ)dλ F star (λ)T(λ)dλ (2.4)
F star,c -Flux of the non-solar-type star at the centre wavelength of the filter. F Sun,c -Flux of the Sun at the centre wavelength of the filter. F star (λ) -Flux in the stellar spectrum available from the HST catalogue. F Sun (λ) -Flux in the solar spectrum available from the HST catalogue. T(λ) -Throughput of the system, derived from the optics, filters and ARPs transmission curves, mirror reflectivity and the CCD quantum efficiency.

Thus, using the above formulae, absolute calibration factors can be derived for all the filters of both NAC and WAC. Finally, the DN values are divided by the absolute calibration factor corresponding to the respective filter, in order to obtain the radiance values (W m -2 sr -1 nm -1 ). Following the hibernation phase of the mission, in May 2014, Rosetta implemented an observation campaign known as the delta calibration, where Vega observations were used to calibrate most of the filters of both cameras. However, for several filters, the absolute calibration factors derived prior to the hibernation phase were used as they did not perform observations of Vega in the post-hibernation phase.

It is trivial to note that if a non-solar-type star is used for the derivation of absolute calibration factors, the resulting errors will be larger than those resulting following a derivation using a solar-type star, simply because of the more operations involved.

• Linearity of the camera responses

It is important to make sure that an instrument behaves uniformly throughout a space mission, in order to accurately interpret its observations and to be able to compare them with other observations done previously. Therefore, the analysis of the linearity of the instrument becomes an important tool to validate the performance of an instrument. This analysis is done periodically by observing standard stars same as those for the absolute calibration at varying exposure times, ranging from low exposure times up to complete saturation.

The CCDs used in OSIRIS cameras have full well capacity of about 120,000 electrons. Given the gaining modes (HIGH gain mode and LOW gain mode) and parameters of the electronics used, the saturation level is fixed below the limit of 65,536 DNs. Based on Vega observations, saturation levels of NAC and WAC are fixed respectively at 54,000 DNs and 48,000 DNs and they both demonstrate satisfactory linearity (with a deviation of less than 2 % from the fitted line) over the allowed range of DNs. The intensity conversion from electronics to DNs is implemented as per the Eq. 2.5, where Gain equals to 3.1 e -/DN and 15.5 e -/DN, respectively in HIGH gain mode and in LOW gain mode for both NAC and WAC, whereas I e -and I DN refer, respectively to intensities in electrons and DNs.

Gain = I e - I DN (2.5)

The OSIRIS calibration pipeline

Once Rosetta data are received to the Earth via the Deep Space Network telescope facilities, they are transferred to ESA and then to the servers of different institutes. In the case of OSIRIS instrument, the data get transferred to the Max Planck Institute for Solar System Research in Göttingen, Germany, where they are calibrated and processed for scientific usage. Depending on the calibration steps and corrections mentioned in the previous section, OSIRIS image archive is structured in several levels. Over the years, this structure has 2.3. The OSIRIS calibration pipeline 51 evolved in order to address and accommodate different mission aspects and this section reflects the most up-to-date2 version of this structure. OsiTrap and OsiCalliope are the data processing programs that are used for the calibration of OSIRIS images and generation of data products in different levels and the description of OsiCalliope given here, corresponds to the functionalities of its version v1.42.0 or later. Following are some of the tasks performed by OsiTrap and OsiCalliope.

• Downloading data from the Data Delivery System (DDS) -OsiTrap.

• Generating raw images with instrument hardware read-outs (PDS3 Level 0) -OsiTrap.

• Generating raw images with calibrated hardware parameters and spacecraft position and pointing (PDS Level 1) -OsiTrap.

• Generating calibrated scientific images (OSIRIS Level 2 and higher) -OsiCalliope.

• Generation of additional image formats (JPEG, FITS) -OsiCalliope.

• Generating reports and notifications -OsiTrap / OsiCalliope.

The hierarchical levels, reflecting different processing levels of the OSIRIS images are listed below.

• Level 0: PDS format, raw header (proprietary, non-standard labels containing actual hardware parameters), raw binary with 16-bit unsigned integer image data.

• Level 1: PDS format, calibrated header (standard labels containing calibrated hardware parameters plus spacecraft pointing information), raw binary with 16-bit unsigned integer image data in DNs per second.

• Level 2: Level 2: PDS format, calibrated header, radiometrically calibrated 32-bit float image data in radiance units (W m -2 sr -1 nm -1 ) achieved through division by the absolute calibration factors.

• Level 3: PDS format, calibrated header, radiometrically calibrated 32-bit float image data, corrected for the camera optical distortion in radiance units (W m -2 sr -1 nm -1 ).

-Level 3B: PDS format, calibrated header, radiometrically calibrated 32-bit float image data, corrected for the camera optical distortion, in dimensionless radiance factor (reflectance or I/F) units. The conversion from radiance to I/F is achieved as follows;

I/F(i, e, α, λ) = I(i, e, α, λ) F(λ) (2.6)
where I is the observed scattered irradiance from the comet and F(λ) is the solar irradiance divided by π at the corresponding heliocentric distance of the comet.

i, e and α correspond respectively to the incident, emission and phase angles of the observation and F(λ) is calculated at the centre wavelength of the filter in question, in order to be consistent with the method used to derive the absolute calibration factors.

Chapter 2. Method of Data Analysis -Level 3C: PDS format, calibrated header, radiometrically calibrated 32-bit float image data, corrected for the camera optical distortion in radiance units (W m -2 sr -1 nm -1 ), solar stray light removal applied.

-Level 3D: PDS format, calibrated header, radiometrically calibrated 32-bit float image data, corrected for the camera optical distortion in dimensionless radiance factor (reflectance or I/F) units, solar stray light removal applied.

• Level 4: PDS format, calibrated header, radiometric calibrated 32-bit float image data, corrected for the camera optical distortion in radiance units (W m -2 sr -1 nm -1 ) with comet 67P geo-referencing data layers.

In addition, OsiCalliope also archives various calibration data files necessary for data reduction steps in the pipeline.

Basic photometric concepts 2.4.1 Flux and magnitudes

Photometric measurements are at the heart of this thesis and this section is used to introduce useful photometric concepts used throughout this work. In a nutshell, OSIRIS imaging instrument records photon fluxes that are reflected from the comet 67P as it is being irradiated by the Sun. The degree of brightness of an individual pixel of an OSIRIS camera depends on various factors which include, but are not limited to: illumination and observational angles, both physical and chemical properties of the surface, observing wavelength, exposure time, optical parameters of the camera itself, etc. Irrespective of the nature of the instrument (whether it be NAC or WAC), the photon flux (F 0 ) received at a CCD pixel is an accumulation of photons reflected from the comet (F λ ) with wavelengths that fit within the bandpass (∆λ) of the filter used for the observation. For instance, if NAC F22 filter centred at λ c -649.2 nm with the bandpass of 84.5 nm, the accumulated photons would be having wavelengths between 606.95 nm and 691.45 nm. Since these photons travel through the optical system of the instrument as they arrive to the CCD, F 0 is also a function of the transmission function (R λ ) of the filter used, reflectivity of the mirrors (M λ ) n (n refers to the number of mirrors in the system) 4 of the system and the quantum efficiency (QE λ ) of the CCD. Fig. 2.3 represents the transmission curves of the mirrors and focusing plates, as well as the variation of the QE with the wavelength. Thus, the integrated photon flux (F 0 ) received at a pixel can be written as follows.

F 0 = λ c + ∆λ 2 λ c -∆λ 2 F λ R λ M n λ QE λ dλ (2.7)
The combined effect of all the aforementioned instrument-inherent parameters is referred to as the system throughput (T λ ) and therefore Eq. 2.7 can be rewritten as follows.

F 0 = λ c + ∆λ 2 λ c -∆λ 2 F λ T λ dλ (2.8)
In order to make sense of flux measurements and be able to compare among the brightnesses of celestial objects, the absolute magnitude scale is used. Generally, there are two different systems used for stars and objects in the solar system. For stars, the absolute magnitude would be their apparent magnitude if they were observed at a distance of 10 parsecs5 away. For the objects in the solar system, the absolute magnitude would be their apparent magnitude if they were observed at a zero phase angle 6 (α) at a distance of 1 AU both from the Sun and the Earth, despite the fact that this is an imaginary situation. Since the illumination conditions of the bodies in the solar system are governed by the Sun and the apparent magnitude depends on the geometry of the observation related by the phase angle, the convention is to compute the absolute magnitude at zero phase angle. Hence, the observed flux is related to the phase angle through the phase curve that describes its variation as a function of the phase angle.

In the measurement of fluxes, the general convention is to measure the brightness of the object with respect to the bright star Vega (Alpha Lyrae), which, by the latest definition has 0.03 absolute magnitude in all the Bessel filters [START_REF] Bessell | Standard Photometric Systems[END_REF]. The standard Bessel filters and their bandpasses are given in the Table 2.3. Following this convention, using the Bessell V filter centred at 554 nm, the apparent magnitude (m) of the comet 67P could be expressed as follows. m = -2.512 × log 10 (A

λ c + ∆λ 2 λ c -∆λ 2 F λ T V(λ) dλ) + C (2.9) 54
Chapter 2. Method of Data Analysis

A = F 0 × λ c + ∆λ 2 λ c -∆λ 2 F (λ) T λ dλ λ c + ∆λ 2 λ c -∆λ 2 F comet(λ) T λ dλ (2.10)
In the Eq. 2.9, the factor A modulates the apparent magnitude (m) by taking into consideration the spectral differences of the Sun and the comet whereas the factor C is the difference between the apparent solar magnitude -26.75 [START_REF] Cox | Allen's astrophysical quantities[END_REF] and the convolution of solar spectrum with the Bessell V filter. F (λ) and F comet(λ) correspond to the solar spectrum and the spectrum of the comet, respectively and T V(λ) is the transmission of the Bessell V filter. Here, F comet(λ) for the comet 67P comes from ground-based observations carried out by [START_REF] Tubiana | 67P/Churyumov-Gerasimenko at large heliocentric distance[END_REF] and in the event a spectrum is unavailable, a spectrum could be approximated using the OSIRIS spectrophotometry. A demonstration of both is given in Fig. 2.4 (Fornasier et al., 2015).

Once the apparent magnitude is computed, a reduced magnitude V(1,1,α) could be computed, which assumes that the observed object is located 1 AU both from the Sun and the Earth at the time of observation. V(1,1,α) only has the dependence of the phase angle, α and could be computed as follows, where r is the heliocentric distance of the object and ∆ is the distance between the object and the observer at the time of the observation, in AU.

V(1, 1, α) = m -5 log(r∆) (2.11)
As for the comet 67P, the reduced magnitude V(1,1,α), which only depends on the phase angle (α) has been calculated (Fig. 2.5) using the transmission of standard Bessell V filter for the OSIRIS NAC observations done with the filter F23 centred at 535.7 nm by Fornasier et al., 2015. They adopted the IAU HG7 system [START_REF] Bowell | Application of photometric models to asteroids[END_REF] to model the phase function of the nucleus of the comet 67P and obtain the absolute magnitude, H V (1,1,0) of 15.74 ± 0.02 mag, using a chi-squared fitting.

Reflectance (Radiance Factor/ RADF) and albedo

Reflectance (I/F) or the radiance factor (RADF) , which is already defined earlier in Eq. 2.6 is an important radiometric quantity that is used throughout this dissertation. This essential photometric quantity describes how much light is reflected from a given surface element. As developed earlier in this section, reflectance depends on illumination angles i, e and α which correspond respectively to the incident, emission and phase angles of the observation as well as λ, which is the wavelength of the observation (centre wavelength of the filter in question for OSIRIS cameras). Nevertheless, it is possible to come across the term 'bidirectional reflectance' (BDR) which is closely related to the radiance factor in the literature (Fornasier et al., 2015). It is noted that the comet was inactive during the acquisition of its spectrum in 2007, while the same is not true for the OSIRIS spectrophotometric data in question.

and they are related according to the Eq. 2.12. BDR is defined as the ratio of the radiance scattered from the surface into a given direction to the collimated power per surface unit perpendicular to the direction of incidence.

RADF = πBDR (2.12)
As depicted in Fig. 2.6, the incidence angle (i) and emission angle (e) are defined by the surface normal vector and the respective vectors of the incident ray (J) and scattered ray (I) at the surface element ∆A. The angle subtended at the surface element by the J and I is the phase angle (α). The incidence and emission planes are defined by the surface normal vector and the respective vector of the incident ray J or the scattered ray I and hence are normal to the surface element. The angle subtended by these two incidence plane and the emission plane at the surface element is the azimuthal angle ϕ. The plane containing the vectors J and I is referred to as the scattering plane. If it happens to contain the surface normal vector as well, it is then called as the principal plane. The relation between these illumination angles is expressed is the Eq. 2.13. cos(α) = cos(i) cos(e) + sin(i) sin(e) cos(ϕ) (2.13) Once the notion of reflectance is established, the albedo of a planetary body can be defined. Naturally, the albedo refers to the portion of diffuse radiation compared to the total radiation received at a given surface. In the context of remote-sensing, the notion of geometric albedo (A p ) is often used for planetary bodies and it can be defined according to the Eq. 2.14, as the ratio of the reflectance (I/F) of the entire surface observed at zero phase angle (α=0) to the reflectance of a Lambertian disk (I/F L ) with the same cross-section. A Lambertian disk refers to an ideal diffusely reflecting surface, corresponding to an isotropic brightness.

A p = I/F(i = 0 o , e = 0 o , α = 0 o ) I/F L (i = 0 o , e = 0 o , α = 0 o ) (2.14)
When referring to a selected surface area of a planetary body or a resolved surface, the notion of the geometric albedo is generalised to the notion of normal albedo (A 0 ). In practice, the observations are hardly acquired at zero phase angle and hence it is needed to convert them to the normal or geometric albedo in order to compare among different observations and carry out meaningful scientific studies. As such, this process is referred to as the photometric correction. Usually this follows a bi-fold process, involving a correction for different illumination conditions due to local topography of a given image and a correction for the difference between phase angles among images. The former is modelled by means of a disk function and the latter is accounted by the phase function of the planetary body.

A photometric model simulates the photometric behaviour of the surface of an atmosphereless planetary body for a given phase angle. In the context of small bodies, several photometric models are being used to simulate the observed reflectance. The Lommel-Seeliger law (Fairbairn, 2005), the Akimov model [START_REF] Shkuratov | Optical measurements of the Moon as a tool to study its surface[END_REF]) and the Minnaert model [START_REF] Minnaert | The reciprocity principle in lunar photometry[END_REF] are some of the notable photometric models, presently used in the study of small bodies. Each model has its restricted applicability and its choice should be made with care, depending on the planetary body in question. As for the comet 67P, the Lommel-Seeliger disk law has been used extensively by many studies, due to the better applicability given its low albedo. The Lommel-Seeliger disk function is expressed in Eq. 2.15.

D(i, e) = 2cos(i) cos(i) + cos(e)
(2.15) Some photometric models (i.e. Lommel-Seeliger disk law) can be decoupled as a phase function and a disk function as per the Eq. 2.16, where A eq (α) is the equigonal albedo and D(i,e,α) is the disk function.

I/F(i, e, α) = A eq (α)D(i, e) (2.16)
The equigonal albedo and normal albedo are related by the phase function, f(α), according to the Eq. 2.17. The phase function describes the variation of brightness with the phase angle.

A eq (α) = A 0 (i = 0 o , e = 0 o , α = 0 o ) f (α) (2.

17)

Therefore, A 0 can be calculated from the above relation, making it possible to compare observations, irrespective of the illumination conditions of the observation, as it is corrected for the varying i and e, depending on the local topography and α, which results in varying brightnesses.

Image Co-registration

FIGURE 2.7: First and last images of an 11-filter-sequence of observation. Although the changes on the two images are subtle to discern, the arrow points to a location which is emerging from the shadows, where the changes are relatively easier to spot. This reflects the need of co-registration for the observational sequences in order to assure that same pixel corresponds to same surface features in all the images in the sequence.

During an OSIRIS data acquisition, images of the cometary nucleus are usually acquired either consecutively using a single filter or using sequences with a combination of filters. During the latter, given the individual filter exposure time and the time to switch between a pair of filters in the filter wheel, a delay of about several seconds is introduced between the acquisition of a pair of consecutive images. Since both the comet and the Rosetta spacecraft are in motion during this time, the fields-of-view of a given pair of consecutive images are somewhat shifted. The amount of shift depends on how close Rosetta is to the comet, as when it is closer, the relative speeds are greater, thus introducing a larger shift of the fieldsof-views. Hence, it becomes necessary to align all the images acquired during a sequence (up to 11 images for NAC if all the 11 NAC filters were used for the observations.) in order to perform a spectrophotometric analysis on the observed cometary surface. This alignment of images and creating a three-dimensional data cube is generally referred to as the image co-registering and it is achieved using several image analysing techniques.

The ideal case would have been to have a data cube on which spatial features on a pixels of a given image correspond to the same spatial features on all the images of the cube. However, since it is impossible in practice due to the way the instrument functions, I had to resort to a solution which would achieve a satisfactory co-registration of the images, which would in turn allow a reliable spectrophotometry on the surface features. In order to counter this situation, I used an algorithm that had been developed in the planetology group of my host laboratory, LESIA and I made use of this algorithm for the co-registration of the images. Using the python machine-learning library scikit-learn, this algorithm is capable of automatically identifying identical features in consecutive images and estimating the projective transformation matrix between each pair of consecutive images (ORB and RANSAC tools implemented by [START_REF] Walt | scikit-image: Image processing in Python[END_REF], leading to an adequate alignment of the images.

Thus, throughout the active ROSETTA mission phase 8 during my thesis, where new data was constantly flowing in, in an on-going manner, I performed co-registration of images and their validation, which was very important prior to scientific analysis. Following a successful validation, the members of the planetology team at LESIA, would use the co-registered data cubes that I produced. Depending on the types of observations and number of filters used, the resulting cubes contained between 3 to 11 images for OSIRIS NAC. The validation phase consisted of verifying whether a satisfactory alignment of the images has been made, necessary for the succeeding spectrophotometric analyses. The check was done either using the SAO DS99 [START_REF] Joye | New Features of SAOImage DS9[END_REF] software or the IDL ATV10 procedure, which allowed blinking between the images of the spectrophotometric data cube. In the data cubes where the co-registration was not satisfactory, I implemented segmented co-registering to obtain better results. This involved cropping the original images so that they only contain the regions of interest. In the image sequences, where both of the lobes of the comet were in visibility, the co-registration seemed challenging, due to the differential movement of the lobes, as the algorithm was trying to find a transformation that would apply to the both of the lobes, thus compromising an optimal co-registration. In such cases, segmented coregistration proved successful, as the differential movement of the lobes could be treated separately depending on the regions of interest. Fig. 2.7 shows first and last images of an 11-filter-sequence of observation for the NAC. As indicated therein, the imaging sequence starts with an image taken with F23 (λ -535.7 nm) filter at 2015-05-02T04.53.48.579 UTC and terminates with an image taken with F15 (λ -269.3 nm) filter at 2015-05-02T04.55. [START_REF] Vincent | Summer fireworks on comet 67P[END_REF].779 UTC, spanning almost two minutes. Although it may not be trivial to identify the subtle changes in the fields-of-view due to the motions of both comet and the spacecraft and varying illumination geometry, it is possible identify some surface features as they emerge from shadows in the location pointed by an arrow in each of the images. At the time of this observation, Rosetta spacecraft was at a distance of about 123.86 km from the surface and the large lobe of the comet was fully in the field-ofview of NAC. The pixel values are in reflectance and both of the images are scaled to the same range of reflectances and hence the image taken with F15 filter is less bright due to its lower transmission.

As such, all the images (up to eleven for OSIRIS NAC) of an observational sequence are co-registered following the work of [START_REF] Walt | scikit-image: Image processing in Python[END_REF], which results in a three-dimensional data cube. Fig. 2.8 represents a three-dimensional data cube produced with four OSIRIS NAC filters, each of which is a monochromatic image. As evident from the figure, this data cube could be used to perform a spectrophotometric analysis of spatial data points of interest. The more there are evenly distributed filters from near-UV to near-IR wavelengths in the observational sequence , the better the data cube will be able to simulate the real spectrum of a given spatial data point in the corresponding range of wavelengths. Nevertheless, contrary to what has been pointed out in the figure, observational sequences could be made at shorter distances from the surface, which leads to images that occupy the entire field-ofview of NAC. In the event of co-registration of such a sequence of images, it is inevitable that some spectrophotometric information is lost as the areas near the borders of the image do not necessarily appear on all the images of the sequence.

Working with shape models

A shape model is a three-dimensional reconstruction of an object, such that it resembles the original object to a very high accuracy. Such a 3D shape model is a fundamental requirement for various detailed analyses of a planetary body. This requirement becomes more demanding and of paramount importance when the planetary body in question is as irregular and complex as the bi-lobed nucleus of the comet 67P, for it is essential to be able to reproduce local topographical features and derive local illumination conditions to implement meaningful scientific studies such as photometric or thermal analyses. The illumination conditions for a given surface element are greatly influenced by the topography of the surrounding terrain as they could potentially cast shadows onto them. Hence, computing the illumination angles for all the pixels in the OSIRIS images, which is key to surface photometric analysis, highly depends on the accurate rendering of local topographical features.

Low resolution shape models were initially derived from ground-based telescopic observations [START_REF] Lowry | The nucleus of Comet 67P/Churyumov-Gerasimenko[END_REF] and using OSIRIS data from Rosetta's approach phase [START_REF] Mottola | The rotation state of 67P/Churyumov-Gerasimenko from approach observations with the OSIRIS cameras on Rosetta[END_REF] to the comet 67P. Then, Preusker et al., 2015 published the SPC SHAP4S shape model, created using OSIRIS data from August-September 2014, by means of the technique of stereo-photogrammetry (SPG). Afterwards, the new SPC SHAP5 shape model was introduced by Jorda et al., 2016 using the stereo-photo-clinometry (SPC) techniques, while using the OSIRIS data acquired up to June 2015. Although, the latter had a better coverage in the southern hemisphere of the comet due to the fact that it became fully illuminated starting from early 2015, the former was of better local accuracy, representing minute topographical details. As the mission evolved and the database of OSIRIS images constantly grew covering the entire nucleus, this allowed the SPC SHAP4S to be extended and upgraded over time. This led to the succession of SPG SHAP7, [START_REF] Preusker | The global meter-level shape model of comet 67P/Churyumov-Gerasimenko[END_REF] produced using the SPG technique, which is more accurate than its precursor and spans the entire nucleus of the comet 67P. The OSIRIS data acquired between August 2014 and February 2016, have been injected to produce this latest shape model, which corresponds to 1-1.5 m horizontal sampling, a vertical accuracy at the decimetre scale and more than 44 million facets (Fig. 2.9). It is underlined that the choice of a shape model should be made with care, giving attention to the envisaged requirements. In case of calculations requiring less precision, it would be more effective to use a decimated version of a shape model as it could enable obtaining the results faster.

Analysis of remote-sensing data from space missions

When working with remote-sensing data, it becomes essential to know the geometric parameters of the observations. For example, in order to apply photometric corrections and obtain accurate results, the aforementioned angular parameters such as incident angle (i), emission angle (e), phase angle (α), etc. per each surface element on a pixel should be known. Therefore, to determine these parameters, it becomes necessary to have ancillary data such as the ephemeris of the Sun, comet and the spacecraft with respect to a given reference frame, location of the instrument (OSIRIS NAC/ WAC) with respect to the coordinate system of the spacecraft, instrument boresight 11 , etc. Usually, in the case of remote sensing of planets, an ellipsoid can be assumed for their shape. But, in the case of a very irregular and complex body like the comet 67P, it is fundamental to resort to a 3D shape model which could reproduce the shape and the topological features of the cometary nucleus to a very high accuracy.

This is where the SPICE toolkit and SPICE ancillary data (Acton, 1996;[START_REF] Acton | A look towards the future in the handling of space science mission geometry[END_REF], usually referred to as kernels, become useful. The SPICE architecture was conceived by the Navigation and Ancillary Information Facility (NAIF), acting under the directions of NASA's Planetary Science Division to assist the both phases of the execution of spacecraft missions and scientific analysis of returned data. SPICE kernels contain navigation and other ancillary data, processed to facilitate an efficient data retrieval by the end-user and are sourced directly from the operations centres of a given space mission. In the early phases of this thesis, I used the IDL-based SPICE tooklit offered by NAIF for the data analysis and later on, migrated to its pythonic version offered by Andrew Annex, under the name 'Spiceypy' 12 .

As such, I have been using and processing SPICE data throughout my thesis to calculate different parameters: spatial resolution, phase angle, spacecraft altitude, surface illumination angles, latitude and longitude of a given surface element, etc. and generate plots to monitor their variations over time. Producing synthetic images to create maps of illumination angles involves a ray-tracing algorithm which should be capable of accurately rendering the shadows according to local topographies. Therefore, I adopted the Optimized Astrophysical Simulator for Imaging Systems (OASIS) simulator (Jorda et al., 2010) that was available within the planetology group I was working in. OASIS simulator uses data files generated from SPICE kernels and by using a shape model, creates synthetic images of illumination angles that mimic the real OSIRIS observations. In practice, it was discovered that the field-of-view of some of the reproduced synthetic images had been shifted by several pixels. Hence, the synthetic images were always cross-checked with real images in order to guarantee that the fields-of-view are the same, prior to utilising them for the photometric corrections. In the event of mass-generation of synthetic images, the work involved with OASIS was somewhat tedious due to the systematic manual steps that it involved and the cross-checking to verify the accuracy of the field-of-view and its correction in the case of mismatches. Nevertheless, this task was relaxed by the introduction of the SUMFILES 13within the OSIRIS scientific team that allowed faster and automatic generation of synthetic images containing illumination angles and other observational parameters. I would also like to recollect the software shapeViewer 14 of Vincent, 2018, that I was using, in order to visualise different observation scenarios and perform useful calculations. Because of its 3D functionality, it was particularly useful when it was required to co-register an OSIRIS image directly to the shape model in order to get a better insight about the observation geometries.

Data analysis techniques 2.8.1 Spectrophotometric analysis

Spectrophotometry is the photometry at different wavelengths of the spectrum, or in other words, it is the measure of photon flux at different wavelengths of the spectrum. This is a classical photometric technique that has proved very useful in the planetary science, especially in the realm of the small bodies of the solar system to study the physical properties of atmosphereless bodies [START_REF] Mccord | Asteroid Vesta: Spectral Reflectivity and Compositional Implications[END_REF][START_REF] Mccord | Asteroids -Spectral reflectance and color characteristics[END_REF]. Depending on the wavelengths of data acquisitions and integrated bandpasses, it is possible to yield a spectrophotometric curve that mimics the real spectrum of the observed object. Most of the spectrophotometric studies are based on ground-based telescopic observations in a limited wavelength range (from ∼0.4 µm to ∼2.4 µm) due to the atmospheric absorption of the Earth, which restricts the transmission of radiation. This 'window' of observation has therefore been crucial to the characterisation of small bodies, notably asteroids [START_REF] Demeo | An extension of the Bus asteroid taxonomy into the nearinfrared[END_REF] and classify them in different taxonomical groups. The spectrophotometric curves of observed bodies yield the possibility to study their composition. Such studies are important to derive the chemical constituents present on the body and with better accuracy to constrain their forming regions in the early solar nebula depending on the quantity of volatiles present and their types. Such spectrophotometric measurements make it possible to compare the small bodies with meteorite analogues and to explore potential links among them. However, since these are small bodies, they almost appear to be point-like sources in most of the ground-based telescopes, which impedes the detection of principal chemical species present in localised regions of their surfaces. For example, the dwarf planet (1) Ceres, which is the largest body in the main asteroid belt with a diameter of about 945 km, has an angular diameter in the range of 0."8 to 0."3 when observed from the best ground-based telescopes given an ideal atmospheric seeing 15 . While this being the case and only a limited window of the spectrum of small bodies has been exploited by ground-based telescopes for many years, success of space observatories and recent space missions have spearheaded the Chapter 2. Method of Data Analysis characterisation of small bodies. Equipped with their space-borne instrumentation, space missions have returned a wealth of spectroscopic data for many small bodies, which have enormously boosted the furthering of their prevailing understanding. The possibility of obtaining in-situ data with both high spatial and spectral resolution has resulted in refining existing knowledge, replying to long-standing questions and raising more questions. Additionally, space missions could provide data at different observation conditions, such as carrying out observations at varying phase angles, which would otherwise be impossible from the orbit of the Earth.

In the context of this dissertation, I have substantially been using spectrophotometric analysis to study the nucleus of the comet 67P with OSIRIS observations at different spatial resolutions and phase angles. Since a comet is a dynamical object which receives varying solar irradiation throughout its orbit, it is interesting to globally monitor the spectrophotometric behaviour of the comet 67P as it approaches the perihelion and passes beyond it, as well as to study the spectrophotometric variations on local scale. Furthermore, I used spectrophotometric measurements to compute spectral slopes 16 for different regions of the nucleus of the comet 67P. I note that the spectrophotometric plots presented here are in reflectance (I/F) units and are corrected for the illumination conditions using the Lommel-Seeliger disk function, described earlier. The reflectance values give an idea about the albedo of the regions of interest while one should be cautious in comparing different observations corresponding to varying phase angles, since the reflectance depends upon the phase angle and it could vary a lot in the lower phase angles (usually < 7 o ) as the comet 67P displays an important opposition effect (Fornasier et al., 2015;Hasselmann et al., 2017). The Fig. 2.10 shows the spectrophotometric behaviour of the three regions of interest of a data cube produced from an 11-filter image sequence recorded at 2015-05-02 15:09 UTC. The full frame NAC F22 image shows 2 lobes of the comet and the regions of interest are located in the large lobe which is to the left of the image. The red, blue and black colours represent smooth 17 terrain of Imhotep, Khonsu and Anubis regions of the comet. There are zoomed-in cropped images of the regions of interest giving close-up views up to the 5x5 pixels of sampling as indicated by squares. Then, spectrophotometric plots are presented for the regions in question. The reflectance plot to the left of the bottom panel reveals their albedo characteristics, which bring out the higher reflectance of the studied smooth terrain in the Khonsu region for all the wavelengths in the sequence. Nevertheless, all of them show increasing reflectance with the wavelength, which is generally referred to as a red spectral behaviour.

The relative reflectance plot to the right on the bottom panel shows the relative variation of each of the regions of interest. This plot is achieved by normalising all the reflectances of a given region of interest, to a selected filter, which in this case is the F23 filter, centred at 535.7 nm. The relative reflectance plots are useful to identify the compositional differences of the studied regions. However, in this case, all the curves coincide with each other upon the normalisation, which indicates a similar composition for 3 of them. Thus, using spectrophotometric analysis one could already get an idea about the surface variegation of the comet 67P. But, in order to specifically characterise the surface composition, one has to explore the characteristic spectral signatures present in the infra-red part of the spectrum, which is beyond the capability of the OSIRIS instrument.

Detection of H 2 O ice using spectrophotometry

Unambiguous detection of H 2 O ice can be done with spectroscopy using typical absorption bands of H 2 O ice. These characteristic absorptions bands are however present in the wavelengths larger than the near-IR wavelengths (> 1000 nm) or wavelengths shorter than the near-UV wavelengths (< 200 nm), which results in a featureless visible spectrum for H 2 O ice, which is flat. As such, the detection of characteristic absorption bands becomes impossible with OSIRIS spectrophotometry, which is restricted to the wavelength range from 269 nm to 989 nm. Yet, the only means to detect the presence of H 2 O ice on the comet using OSIRIS spectrophotometry, is to reconstruct the spectrum of the potential candidate for H 2 O ice and look for its flat nature. If an observational sequence contains more filters that span the visible spectrum, one could be more confident to claim the presence of H 2 O ice.

The characteristic spectrum of H 2 O frost, obtained from laboratory measurements is given in the Fig. 2.11. As evidenced from this spectrum of H 2 O frost, an observation of a flat spectrophotometric curve could be used as a proxy to look for the presence of H 2 O ice on the nucleus of the comet 67P. However, one must be cautious in explicitly concluding the presence of such a flat spectrophotometric curve to be an indication of the presence of H 2 O ice, as there are other volatile species which possess a characteristic flat spectrum in the visible wavelengths. For example, volatiles ice of CO 2 , NH 3 and SO 2 also result in a flat visible spectrum as demonstrated by Wagner, Hapke, and Wells, 1987 (Fig. 2.12) using laboratory experiments and respective frosts of these volatile species. Nevertheless, depending on the heliocentric distances of the observations and the observed life times of the volatiles, it is possible to rule out the presence of certain volatile species as they might already have sublimated away. At the same time, as evident from Fig. 2.12, SO 2 ice remains a species that can be spectrophotometrically distinguished from the other other volatile ices by OSIRIS NAC due to its characteristic absorption feature centred at ∼290 nm. Indeed, Fornasier et al., 2015 potentially

Spectral slope analysis

In the study of atmosphereless bodies in planetary science, spectral slope relates the dependence of the reflectance with the wavelength. Following Delsanti et al., 2001, spectral slope (S) is defined in Eq. 2.18 as the percentage of normalised reddening (red spectral behaviour) per 100 nm of a given spectrum.

S(%/(100nm)) = 10000 × F (λ2) -F (λ1) F (λ1) (λ2 -λ1) (2.18)
Spectral slope depends on the phase angle of the observation and its variation can be characterised by the gradient of the spectral slope with the phase angle. If the gradient tends to increase with the phase angle, it is referred to as a phase-reddening effect, while decreasing spectral slope with the phase angle is referred to as a phase-blueing effect. Given the context of the comet 67P, spectral slope analysis can be an important tool to monitor the variation of local spectral slopes on different morphological regions of the comet, in order to constrain the changes in the surface properties of the comet, as its insolation varies throughout the orbit. Especially, when comet approaches the perihelion of its orbit, this spectrophotometric technique can be used to compare the spectral properties of the surface at different epochs.

To be consistent with the literature and other studies of the comet 67P by the OSIRIS instrument, spectral slopes presented in this thesis are evaluated between F23 (λ c -535.7 nm) and F82 (λ c -882.1 nm) filters, unless otherwise specified. Fig. 2.13, 2.14 and 2.15 show the spectral slope maps produced using the above definition and they give a glimpse of the variation of the spectral slopes of the comet in general as well as locally over the time.

Using false colour RGB images

This is an image processing technique that can be used to make potential local heterogeneities in terms of composition stand out, in an observational sequence implemented with at least three filters. I've used this technique to identify exposed bright features, terrains containing dust and volatiles, frost, and transient activity sources, as each of these has characteristic intensities in different wavelengths. For this task, I've used the STIFF program presented by Bertin, 2012. This programs takes as inputs three channels of data and produces a false colour image depending on the values of its internal parameters that can be adjusted. During the making of these false colour RGB images, I stuck with the OSIRIS NAC filters centred at 882.1 nm, 649.2 nm and 480.7 nm as input channels respectively for red, green and blue colours. This is a technique that has been implemented in numerous studies (Fornasier et al., 2015;Feller et al., 2016;Oklay et al., 2016) to investigate the surface features and discover subtle compositional differences on the surface of the comet 67P. Thus, it allows to visually inspect the images and detect any characteristic spectral features that could otherwise go unnoticed in the broad band NAC images. Given in the Fig. 2.16 are some RGB images that have been created using the same data cubes that have been used for the examples given in the previous sub-section to demonstrate the spectral slopes.

In comparison to the spectral slope maps presented in the previous section, it is possible to note in that more local diversity can be observed in the RGB images. Evidently, this is due to the fact that three colour channels are used in the creation of RGB images, whereas the spectral slopes maps are derived only from two colour channels. However, in terms of scientific relevance, spectral slopes are of more importance as they are used to express the spectral behaviour of a body and to study the photometric effects, such as the phasereddening effect.

There are also several artefacts observable near the shadows and edges of the images as the imperfect co-registration is easily discerned in such locations.

(A) The OSIRIS NAC full-frame context images acquired with the F22 filter centred at 649.2 nm in June (∼ 1.37 AU) and July (∼ 1.26 AU) 2015, when the comet was approaching perihelion of its orbit at 1.24 AU from the Sun.

(B) The spectral slope maps evaluated between 882 nm and 535 nm, corresponding to the above observations of the nucleus. Coma contributions and an outburst also appear in the spectral slope maps as the comet has become active. T he early observations of the comet 67P by OSIRIS that resolved well the surface features of the comet started to reveal a world with very complex geological features (Sierks et al., 2015). This was around early August 2014 and the lower orbits that followed in order to map and characterise the complete nucleus prior to the Philae landing, revealed the surface features at even a better spatial resolution. The preliminary data indicated a very dark nucleus of a geometric albedo of 6.5±0.2% at 649 nm (Fornasier et al., 2015). This was a quite unexpected result, for cometary nuclei had been traditionally thought be made of dust and volatile ices, which would not correspond to such a low geometric albedo, given the icy component. This raised the question as to why the volatile ices are not seen on the surface of the comet. This study is, hence, inspired by this question and we attempt to respond to it by using the capabilities of the OSIRIS instrument. As such, observed regions of the entire nucleus of the comet, are scrutinised to find local inhomogeneities, characterised by a contrasting high albedo compared to the nucleus. We seek to locate such spatially resolved high albedo features in as large sizes as possible, to be candidates of volatile ices, which will be systematically verified by a spectroscopic analysis to unambiguously identify the volatile species locally present on the nucleus of the comet.

Search for localised albedo variations

Followed by the preliminary studies of the comet, further surface characterisation continued with the high resolution imaging data of the OSIRIS instrument. Careful photometric analysis of these images started to reveal some localised peculiar bright features on the nucleus and they were generally referred to as bright spots by Pommerol et al., 2015. Analysing the data from August to November 2014 and using complimentary laboratory experiments, these authors concluded the bright spots to be meter-sized exposures of H 2 O ice at the surface of the comet. Their conclusion was based on following observations and interpretations.

• Bright spots having reflectance of up to ten times the reflectance of the typical cometary nucleus.

• The spectrophotometric curves of bright spots are spectrally bluer than the typical nucleus, which has a red spectral behaviour.

• Bright spots having an apparent stability of their morphology and photometric properties over a few weeks.

• Bright spots are preferably located in areas subject to low insolation conditions.

As such, the question began to be partially answered, owing to the local heterogeneity of nucleus, which revealed localised bright spots in the order of several meters of average size. Following up on these peculiar bright features, Filacchione et al., 2016a used infrared spectral data from VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument (Coradini et al., 2007), the spectro-imager on-board Rosetta to study two debris falls in the Imhotep region of the comet, where they identified the presence of H 2 O ice. Their study points out that the ice has been exposed on and at the base of two of the elevated structures that surround the basin covered with smooth material in the Imhotep region. This finding could suggest a cliff collapse event possibly triggered by some thermal event as the comet was receiving progressively increasing amounts of solar flux during this time. It is also recalled that Pajola et al., 2017 reported the exposure of bright material following a cliff collapse in the Aswan (Pajola et al., 2016) area, located in the Seth region (El-Maarry et al., 2015) of the comet, with an attempt to give a notion of the frequency at which these events are taking place on the comet. The latter, which took place during the perihelion passage of the comet, about a year later from the detection of the former, strongly confirms the direct link associated to the incoming solar flux on the cometary surface. Filacchione et al., 2016a suppose a size distribution for icy grains that corresponds to milli-metric dimensions, which implies the presence of grains that are much larger than the ones corresponding to other cometary nuclei such as 9P/Tempel 1 (Sunshine et al., 2006) and 103P/Hartley 2 (A' Hearn et al., 2011), usually in the order of micrometers. Moreover, De Sanctis et al., 2015 reported the presence of the H 2 O frost, coupled to the diurnal cycle of H 2 O on the comet 67P, where they estimated grain sizes in the order of micrometers. Although, usually, a distribution of micro-metric sized grains is explained as a result of re-condensation of H 2 O vapour, the observed case of larger icy grains on the debris falls in the Imhotep region best fits to processes such as vapour diffusion in ice-rich layers or sintering.

Despite these limited detections of the exposures of H 2 O , it remained the most observed species [START_REF] Hässig | Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko[END_REF][START_REF] Biver | Distribution of water around the nucleus of comet 67P/Churyumov-Gerasimenko at 3.4 AU from the Sun as seen by the MIRO instrument on Rosetta[END_REF] in the coma of 67P. The difficulty of spectroscopic detection of exposed H 2 O on the nucleus, lies with the relatively low spatial resolution of the VIRTIS instrument, despite the fact it has a much better spectral resolution compared to OSIRIS cameras. This requires much laborious task of looking for absorption bands of H 2 O ice in the hyper-spectral data cubes of VIRTIS instrument, in the absence of any spatial information, as to where the exposed H 2 O might be located. This is where the inter-instrumental coordination becomes important as OSIRIS has much better spatial resolution and could relatively easily detect exposed bright features on the nucleus, as they are better resolved. Therefore, it was decided to use both OSIRIS and VIRTIS instruments complimentary to each other, in order to unambiguously detect the presence of H 2 O on the nucleus (Barucci et al., 2016).

Using OSIRIS NAC data, we carefully selected candidate exposed bright features on the nucleus. The candidate features were chosen following a spectrophotometric analysis, which relied on the characteristic flat behaviour of the spectrum of H 2 O ice in the visible wavelengths, as explained earlier in the sub-section 2.8.2. We further focussed on selecting features that would be as spatially large as possible in OSIRIS NAC images such that the corresponding VIRTIS signal-to-noise ratio would be satisfactory for their detection1 . Additional attention was given to select features that would have as long observed life time as possible before their complete sublimation, as this would increase the probability of VIRTIS detection for a given candidate feature. Analysing OSIRIS data in this way, we came up with 13 candidate bright features to be cross-checked with VIRTIS (Fig. 3.1) as exposures of H 2 O ice.

The VIRTIS data

VIRTIS instrument is made up of two optical heads: VIRTIS-M and VIRTIS-H. VIRTIS-M is an imaging spectrometer used for mapping of the nucleus of the comet. It has 438 spectral channels that are shared between two segments, with one ranging from 0.22 µm to 1.05 µm while the other functions in the infrared range from 0.95 µm to 5.06 µm. On the other hand, VIRTIS-H is a high resolution spectrometer generally used to observe the the coma and operates in the infrared wavelength range from 1.88 µm to 5.03 µm (Reader is referred to the PhD theses of [START_REF] Raponi | Spectrophotometric analysis of cometary nuclei from in situ observations[END_REF][START_REF] Debout | Spectroscopie infrarouge moléculaire avec VIRTIS/Rosetta[END_REF][START_REF] Rousseau | Étude de la composition et des propriétés physiques de surface de la comète 67P/Churyumov-Gerasimenko. Interprétation des données VIRTIS/Rosetta et mesure en réflectance d'analogues cométaires[END_REF] which describe in detail the functionalities of the VIRTIS instrument). In this study, we made use of the infrared segment of the VIRTIS-M in order to look for the absorption bands of H 2 O ice. Although I was only involved with the analysis of OSIRIS data in this work, following content is presented to briefly explain how the analysis of VIRTIS data went on, in order to give a complete picture of the detection of H 2 O on the comet 67P.

Search for H 2 O ice in VIRTIS data

The search for the VIRTIS-M spectra of exposed bright features was implemented using georeferenced data (based on cometary latitudes and longitudes), as the nucleus had been imaged with high redundancy from varying observational geometries and spatial resolutions. Making use of the ancillary data using the SPICE tool-kit (Acton, 1996), several pixel-based and facet based (combining a shape model) parameters such as latitude, longitude, incident angle, emission angle, phase angle are usually calculated and stored in a database for efficient retrieval purposes later on. As for the shape model, SHAP5 (Jorda et al., 2016), created using the method of stereophotoclinometry was used in this context, whilst adopting the coordinates based on Cheops frame (Preusker et al., 2015). Thus, based on the latitudes and longitudes of the exposed bright features, calculated using OSIRIS data, a search was performed on VIRTIS-M spectral cubes to look for the presence of the diagnostic absorption feature of H 2 O ice at 2 µm. This took into consideration, the pixels located within a circle with a radius of 2 o in both latitude and longitude, centred at the location determined using OSIRIS data. The sampled spectra were then grouped according to MTPs2 and band depth at 2 µm was calculated as the proxy to distinguish the presence of exposed H 2 O ice. The criterion was to consider the spectra with more than 5% of band depth as positive detections of H 2 O ice. H 2 O ice has notable characteristic absorption bands at 1.5 µm, 2 µm and 3 µm in the infrared regime and the decision to choose the absorption band at 2 µm was driven by two independent reasons. An accurate calculation of the band depth at 1.5 µm cannot be guaranteed due to the presence of an instrumental sorting filter, which could result in erroneous computations. On the other hand, the band at 3 µm inherits a complex shape due to the overlapping of H 2 O ice absorption and omnipresent organic material (Capaccioni et al., 2015) absorption on the nucleus. This could lead to unpredictable changes in the shape of the band, shifting of the centre of the band and consequently introducing modifications in the band depth, depending on the abundances of the organic material and H 2 O ice. Hence, the band at 2 µm becomes the available choice as it remains unaffected by these causes that disturb the use of other two bands. Additionally, the band at 2µm is well-established for a variety of grain sizes, thus making it a better proxy to identify the presence of H 2 O ice.

The detection of spectral signature of H 2 O ice

The VIRTIS analysis led to detecting 8 positive identifications of the presence of the H 2 O ice among the 13 candidate bright features that were used for this study. Their locations on a cylindrical projection of the comet are displayed in Fig. 3.22 The non-detections could be due to several reasons such as inadequate single-to-noise, low spatial resolutions and oblique observational geometries of the corresponding spectra. Hence, these non-detections cannot, nevertheless be ruled out as potential sources of exposed H 2 O ice. Following tables are intended to succinctly demonstrate how the search proceeded. Table 3.1 recapitulates the coverage of VIRTIS-M for the 13 candidate features that appear in the first column of the table with their respective IDs. Thus, the table makes it easier to identify the time intervals when the location in question has been observed and whether there have been detections of H 2 O ice in that location. Then, in Table 3.2, the OSIRIS observational circumstances are reported for each of the 8 exposed bright features, positively detected with VIRTIS-M to contain H 2 O ice. The Fig. 3.4 puts in perspective a NAC full frame image for the contextual purposes as well as a zoom on the exposed bright feature detected to contain H 2 O ice. It further presents the OSIRIS spectrophotometric curves for the given feature (in red) and typical terrain (in black), sampled from the vicinity of the feature. The Table 3.3 lists the VIRTIS-M observational parameters for the best detections (highest signal-to-noise ratio) for each of the 8 H 2 O ice patches along with their derived temperatures.

The surface temperatures of the exposed H 2 O features given in the Table 3.3 have been derived by modelling the spectral radiance between 4.5-5.1 µm of the spectrum at the pixel corresponding to the highest band depth of the absorption band of H 2 O ice located at 2 µm. The local temperature of a surface element on the nucleus is a function of local thermophysical properties such as albedo, composition, grain size, rugosity 3 , thermal inertia and instantaneous illumination conditions. As such, the spectral radiance, I(λ) emerging from the surface of an atmosphereless body can be written as:

I(λ) = r(λ)F(λ) + (λ)B(λ, T sur f ) (3.1)
where r(λ) is the surface spectral reflectance, F(λ) is the solar irradiance, scaled to the corresponding heliocentric distance, (λ) is the surface spectral emissivity, B(λ,T sur f ) is the Planck function and T sur f is the temperature of the surface element. The Planck function, expressing the spectral radiance per unit wavelength, λ can be written as:

B(λ, T sur f ) = 2hc 2 λ 5 1 e hc λk B T -1 (3.2)
where, h, c, k BT refer to the Planck constant, speed of the light in the medium, and the Boltzmann constant, respectively. The Eq. 3.1 holds as long as the variation of the temperature with depth in the surface material can be discarded within the vertical range where opacity becomes 1. It is assumed that this be the case of the H 2 O ice features for the depths in the order of the wavelengths (up to ∼5 µm) used for the analysis.

If we were to consider an ideal case of noiseless infrared spectrum sampled in N spectral channels, the retrieval of the surface temperature using the Eq. 3.1 violates the Hadamard criteria of a well-posed problem and manifests an ill-posed problem [START_REF] Hadamard | Sur une formule déduite de la théorie des cubiques[END_REF] as the number of unknowns (N+1 due to N emissivities corresponding to the spectral channels and the surface temperature) is greater than the number of measurements (N radiances corresponding to the N spectral channels). In addition, the temperature retrieval process could easily get more complicated due to rugosity of the surface. Given this context, it was decided to adopt a Bayesian approach to proceed, following the method described by Tosi et al., 2014 in the retrieval of the surface temperatures of bright and dark terrains of the asteroid (4) Vesta. Hence, initial guesses for the unknown parameters of T sur f and (λ) were injected with their respective maximum errors to the Bayesian algorithm to iteratively converge to the measured spectral radiance. The retrieved T sur f are given in the Table 3.3. As evident from the local solar time (LST), all the detections have been made during late morning to early afternoon, with VIRTIS-M pixel resolutions varying from about 5 m/pixel to about 23.5 m/pixel and the retrieved T sur f values range from 158 K to 218 K. It is also noted that the minimal detectable temperature correspond to about 150 K, as it is noise-equivalent temperature of the instrument.

Spectral Modelling

In order to further study and gain access to parameters like grain sizes of the H 2 O ice present in the bright features, a spectral modelling was performed using Hapke's radiative transfer model. This radiative transfer model was introduced and gradually developed by Bruce Hapke, (Hapke, 1981;[START_REF] Hapke | Bidirectional reflectance spectroscopy. 2. Experiments and observations[END_REF][START_REF] Hapke | Bidirectional reflectance spectroscopy 4. The extinction coefficient and the opposition effect[END_REF][START_REF] Hapke | Bidirectional reflectance spectroscopy 4. The extinction coefficient and the opposition effect[END_REF]Hapke, 2002;[START_REF] Hapke | Bidirectional Reflectance Spectroscopy. 5. The Coherent Backscatter Opposition Effect and Anisotropic Scattering[END_REF]Hapke, 2012a) with each latest version (Hapke, 2012b) of the model taking into account different different physical phenomena such as multiple scattering, opposition effect contributions from shadow hiding opposition effect (SHOE) and coherent backscattering opposition effect (CBOE), macroscopic rugosity of the planetary surface and porosity of the material to derive the observed reflectance of a planetary surface of an atmosphereless body. Therefore, Hapke spectral modelling was performed in a similar way as explained in Ciarniello et al., 2011. The observed spectra were modelled by two spectral end-members as follows. • Crystalline H 2 O ice:

The optical constants measured at 160 K between 1-4 µm were used to simulate the crystalline ice (Warren, 1984;Mastrapa et al., 2008;Mastrapa et al., 2009).

• Dark terrain unit: This corresponds to the photometrically corrected (Ciarniello et al., 2015) average spectrum of the nucleus attributed to desiccated refractory organic material, omnipresent on the nucleus (Fig. 3.3).

Further details of the spectral modelling are discussed in Filacchione et al., 2016a;Barucci et al., 2016 and will not be detailed here. The mixing modalities adopted during the modelling are as follows.

• Areal mixing:

The surface is modelled as patches of both end-members (pure crystalline H 2 O ice and dark terrain).

• Intimate mixing:

The surface is modelled from particles of both end-members that are in contact with each other.

The free parameters to be fit, are set as the grain size of H 2 O ice and percentage of the end-members. Most of the H 2 O -rich regions can be modelled by both areal and intimate mixtures, whose choice can be alternative. In case of the modelling based on an intimate mixing, the model requires larger abundance and grain size than the areal mixing case, in order to compensate for the lack of multiple scattering contribution, which happens to be low due to the interaction of light with the dark terrain. The Table 3.4 lists the results of the Due to the peculiar bi-lobed shape of the nucleus of the comet 67P and orientation of its rotational axis, the typical surface terrain is highly subject to both diurnal and seasonal variations of insolation. At the time of Rosetta's rendezvous with the comet in August 2014, it was the summer at the comet's northern hemisphere and hence only a limited portion of the entire nucleus could be observed. Several months later, it was only towards early 2015, as comet was approaching the perihelion, that the southern hemisphere of the comet could be observed. Owing to its current orbit, the present seasonal variations result in a long northern summer when comet is away from the sun, and a short southern summer with intense illumination conditions as the comet passes through the perihelion. The continued OSIRIS observations helped the characterisation of the nucleus with respect to morphological and geological features on the nucleus and there were 26 distinct regions identified on the comet. Following the Rosetta lore, the regions were named after Egyptian deities and reader is referred to El-Maarry et al., 2015;El-Maarry et al., 2016 for their nomenclature and additional information on their characterisation. As the mission proceeded up to September 2016, the entire nucleus could be well characterised, sometimes redefining the boundaries of some regions. This section is dedicated to the spectrophotometric analysis of the Khonsu region, one of the regions in the southern hemisphere of the comet.

Spectrophotometry of the Khonsu region

Chapter 3. Results

Khonsu (Fig. 3.7) is a peculiar region in the southern hemisphere of the comet due to several reasons. In terms of geology, Khonsu embodies a variety of surface elements characterised by both smooth and rough terrain, which are quite unique to this region. For example, the so-called pancake feature puzzles many a geologist due to its present look, which raises questions on its formation. Then, the boulder distribution on Khonsu (Deshapriya et al., 2016), suggests a formation scenario following a variety of processes such as collapses, sublimation of volatiles and gravitational events. Spectrophotometrically, the bright features (Barucci et al., 2016;Deshapriya et al., 2016) that have been observed therein are of research interest, as are the smooth terrains present therein. The latter got exposed following the surface erosion driven by sublimation as the comet approached the perihelion, thus revealing sub-surface material which is otherwise inaccessible to study. Furthermore, there have been several outbursts that originated in the Khonsu region, which are important in the study of the activity of the comet and surface modifications.

Spectral slope analysis of the Khonsu region

The spectral slope values of the comet 67P have been reported by several studies, using independent measurements. In this sub-section, I compare those with the spectral slope values that I computed for selected terrains in the Khonsu region, in order to understand to what extent this region corresponds to the average nucleus of the comet 67. [START_REF] Tubiana | Comet 67P/Churyumov-Gerasimenko at a large heliocentric distance[END_REF] report spectral slopes obtained from ground-based spectroscopy of 67P in the visible wavelengths to be 11±2%/(100 nm). It is useful to note that their observations were carried out in the range of 0.5 o to 10.6 o of phase angle and when the comet was between 4.89-5.62 AU away from the sun. This information is important to rule out any coma contribution as the comet 67P was not active at these heliocentric distances and was relatively close to the aphelion of its orbit. Following on, Fornasier et al., 2015, using the early OSIRIS data of the comet 67P and averaging the data to compensate for the rotational phase of the comet, calculated spectral slope to be increasing from 11%/(100 nm) to 16%/(100 nm) in 1.3 o to 54 o phase angle range (Fig. 3.8). This increase clearly demonstrates what is known as the phase-reddening effect and is generated due the changes in the phase angle. This was first observed on lunar surface [START_REF] Gehrels | Wavelength dependance of polarization. III. The lunar surface[END_REF], followed by asteroids in their broad band colours [START_REF] Gehrels | Photometry of asteroids[END_REF]. Its artificial nature has since been well-established by both ground-based observations as well as in-situ observations done by space missions on asteroids. This photometric phenomenon can be observed in many solar system bodies ranging from asteroids [START_REF] Clark | NEAR Infrared Spectrometer Photometry of Asteroid 433 Eros[END_REF], icy planetary satellites [START_REF] Filacchione | Saturn's icy satellites and rings investigated by Cassini-VIMS: III -Radial compositional variability[END_REF] to planetary ring systems [START_REF] Filacchione | Cassini-VIMS observations of Saturn's main rings: I. Spectral properties and temperature radial profiles variability with phase angle and elevation[END_REF]. Recent laboratory experiments [START_REF] Schröder | Laboratory observations and simulations of phase reddening[END_REF] have also been successful in reproducing this effect in order to better characterise it. This phenomenon is attributed to the wavelength dependence of the single-scattering albedo [START_REF] Gradie | The effects of scattering geometry on the spectrophotometric properties of powdered material[END_REF]. Thus, the results of Fornasier et al., 2015 indicate a spectral slope of 11.3±0.2 %/(100 nm) [START_REF] Feller | CHARACTERIZATION OF THE PHYSICAL PROPERTIES OF COMET 67P/CHURYUMOV-GERASIMENKO'S NUCLEUS WITH THE OSIRIS INSTRUMENT OF THE ROSETTA MISSION[END_REF] Ph.D. thesis) normalised to the 0 o phase angle (corrected for the phase-reddening). Ciarniello et al., 2015 use Rosetta/VIRTIS spectro-imager data and compute two spectral slopes for visible and infrared wavelengths. Upon the phase angle dependent correction to discard the phase-reddening effect, they obtain spectral slope values of 20%/ (100 nm) evaluated between 550-800 nm and 3.3%/ (100 nm) evaluated between 1000-2000 nm, which indicates a pronounced phase-reddening effect on the nucleus. The deviation between the normalised visible spectral slopes computed by Fornasier et al., 2015 andCiarniello et al., 2015 is probably due to the different wavelength ranges used and the variability of the phase angle in the different data sets used. Based on the definition of the spectral slopes, established earlier in the previous chapter, results obtained on selected terrains in the Khonsu region are presented here. I decided to measure the spectral slope at two different periods and compare the final values, in order to better understand, how the surface terrain of the Khonsu region behaved as the comet was passing through the perihelion 4 . Therefore, I make the distinction between pre-perihelion and post-perihelion periods in this analysis. Before concentrating on the spectral slopes, themselves, I visualise in the Fig. 3.10 and Fig. 3.11 the spectrophotometric variation of four selected terrains in the Khonsu region. For this spectrophotometric comparison I chose two data cubes (Fig. 3.9) from 02 May 2015 (∼3 months prior to the perihelion passage) and 10 February 2016 (∼6 months later to the perihelion passage) both of which have 11 filters in common. This choice was based on taking into consideration, the availability of the same filters, the difference between phase angles, difference between spatial resolutions for the two observational sequences imaging the Khonsu region in pre and post-perihelion periods. Since OSIRIS NAC observations included 11-filter observation sequences for the both periods, it was possible to create spectrophotometric plots for the purpose of comparison. The pre-perihelion observational sequence, having been imaged at a phase angle of ∼60 o and that of post-perihelion period at a phase angle of ∼65 o , make it not too difficult for the comparison, given the linear nature of the phase curve of the comet 67P in the range of 60 o -65 o . However, since there were no observational sequences that were acquired at similar altitudes, the spatial resolution has a difference of about a factor of about 2.5, which was accounted by sampling different number of pixels, such that the sampled area remains the same for both observational sequences. The Table 3.5 summarises the observation circumstances between the two observational sequences.

As evident from the Fig. 3.10 and Fig. 3.11, the four selected terrains are as follows.

(a) 'pancake' feature (b) smooth terrain 1 (A) The OSIRIS NAC full-frame context images of the sub-frames that have been used for the spectral slope analysis. Both images correspond to the F22 filter centred at 649.2 nm.

(B) The spectral slope maps evaluated between 882 nm and 535 nm, corresponding to the above observations. The phase-reddening effect of the nucleus is well evident from the spectral slope maps. The choice of these terrains was to check the effect of erosion on their surfaces and how their reflectance changed from pre-perihelion to the post-perihelion period. Basically the 'pancake' feature and rugged terrain would serve as surface elements that are more resistant to erosion, whereas smooth terrains would serve as surface elements that are less resistant to erosion.

As for the case of the data cube of 02 May 2015 (Fig. 3.10), the four terrains seem to have different reflectance values. With the available spectrophotometry, it is clearly noticeable that the smooth terrain 2 has the highest reflectance while the rugged terrains have the lowest reflectance among the terrains considered. The pancake feature and the smooth terrain 1 fall between the other two terrains with their intermediate reflectance. Despite these little variations in reflectance plots, the relative reflectance plots indicate that all the four terrains are compositionally related, as their relative reflectance curves (normalised at the filter centred at 535 nm) almost overlap with each other. Given this is the case of these terrains in the Khonsu region, three months prior to the perihelion passage, the data cube on 10 February 2016 (Fig. 3.11) gives insight into how they have been affected during the time elapsed in between. The reflectance of the terrains seem to have changed slightly with a noticeable change, if the terrains were to be ordered by their respective reflectance. The smooth terrain 2, which had the highest reflectance prior to the perihelion passage is now ranked at the third in the descending order of the reflectance. The pancake feature and the smooth terrain 1 share the highest reflectance in this post-perihelion data cube while the rugged terrain remains with the lowest reflectance.

In comparison with the data cube prior to the perihelion, this change in reflectance suggests a change in albedo between the two periods. In addition, the relative reflectance plot for the post perihelion data cube suggests a change in composition for the smooth terrain 2, which is interpreted from the deviation of its relative reflectance curve from those of the other three terrains which almost coincide with each other.

In order to further investigate this change in the smooth terrain 2, I carried out a spectral slope analysis, which included the same four terrains in order for the purpose of comparison. Therefore, I computed their spectral slopes for both pre and post-perihelion periods to study their evolution. Presented below are the multi-filter observational sequences that contain the four terrains in question from 25 March 2015 to 27 June 2015 for the pre-perihelion period and from 09 January 2016 to 09 July 2016 for the post perihelion period. Since spectral slopes are calculated between the reflectance of filters F41 and F23, respectively centred at 882.1 nm and 535.7 nm, the data cubes referred to in the Table . 3.6 were the only observational sequences that contained these filters. For the both data sets, phase angle varied in the range of 60 o to 90 o and computed spectral slopes are illustrated in the Figure 3.12.

As evident from the spectral slopes plotted in Fig. 3.12, the smooth terrain 2, which is represented therein by 'c', displays a behaviour that is completely different from the other three terrains for these observations, in the phase angles range from 60 o to 90 o . The a, b and d terrains have spectral slope values that increase with the phase angle, a characteristic which is otherwise referred to as the phase-reddening effect, parametrised by the gradient of the slope. The unique behaviour of c terrain is that it does not seem to possess a phasereddening effect for both pre-perihelion and post-perihelion data sets, whereas all the other a,b and d terrains do. In order to characterise the observed phase-reddening effect, I computed the slopes of the regression lines for the both data sets, with exception of the terrain c (smooth terrain 2). Thus, I obtain 0.041±0.0082% /(100 nm) /degree as the slope for the pre-perihelion period and 0.108±0.016% /(100 nm) /degree as the slope for the post-perihelion period, for a, b and c terrains. Indeed, Fornasier et al., 2015 reported a phase-reddening value of 0.104±0.003% /(100 nm) /degree for the rotationally averaged nucleus of the comet, using OSIRIS data from late July 2014 to early August 2014, where the comet 67P was orbiting the Sun from about 3.68 AU to 3.59 AU heliocentric distance. Their result matches well with the phasereddening value, I calculated for the three terrains for the post-perihelion passage. It is recalled that the southern hemisphere of the comet was not visible during the period when the phase-reddening was analysed by the aforementioned authors and that some cometary activity was already taking place as detected, for example on the Hapi region as reported by the same authors. From the work of Fornasier et al., 2016, spectral slopes for the preperihelion period from April to August 2015 yield a value of 0.042% /(100 nm) /degree (determined from the Fig. 3.13) which is well consistent with the value of 0.041±0.0082% /(100 nm) /degree that I calculated for the same period for the three selected terrains in the Khonsu region and that the value of Fornasier et al., 2015 takes into consideration the rotationally average nucleus. It is recalled that Khonsu is a region in the southern hemisphere of the comet, which started to be fully illuminated by the Sun around March 2015. Thus my spectral slope results for this analysis corroborate the work of Fornasier et al., 2015 andFornasier et al., 2016. Now it is interesting to look for what the above numbers mean and what is really going on, on the cometary surface as the comet approaches the perihelion. It is clear that the phase-reddening on the comet has changed by a factor more than two, during its transition between pre-perihelion and post-perihelion periods. This change in spectral slopes suggests a change in the physical properties of the surface of the comet. The phase-reddening value of 0.104±0.003% /(100 nm) /degree observed for the averaged nucleus in July-August 2014, corresponds to a value which is the highest observed for a cometary nucleus. Ciarniello et al., 2015 also report a strong phase-reddening for the nucleus using visible and infrared spectroscopic data from VIRTIS spectro-imaging instrument aboard Rosetta. As mentioned earlier, this is also the time when the comet's activity was progressively increasing as it was speeding up and getting closer to the Sun. Thus, sublimation-driven erosion was abundant on the surface of the comet at an increasing rate. Therefore, most of the surface got eroded, leading to the exposure of the volatile-rich material that had been covered by the surface dust layer. It is in this period that I find a phase-reddening value of 0.041±0.0082% /(100 nm) /degree for the three selected regions and Fornasier et al., 2016 a value of 0.042% /(100 nm) /degree for the rotationally averaged nucleus, both of which reflect the on-going surface changes. This process continued up to the perihelion passage of the comet in August 2015 and the activity slowly decreased as the comet was slowing down and getting further away from the Sun. Since the activity was no longer eroding the surface in a continuous manner and re-deposition of dust particles that did not escape the nucleus, was also taking place, the nucleus was slowly returning to its previous phase-reddening values. This is corroborated by my spectral slope calculations of the three selected terrains in the Khonsu region for the post-perihelion period and results derived from the Fig. 3.13 excerpted from Fornasier et al., 2016.

Considering the smooth terrain 2 or the terrain c, it is now clear that it was greatly subject to erosion, which removed a lot of surface dust and completely removed its phase-reddening nature, as no such behaviour was recorded for this terrain, during the pre-perihelion period. As for the pre-perihelion period, this peculiar terrain, in fact displays a phase-bluing effect (Table 3.7), which is that its spectral slope values decrease with the wavelength, resulting in a negative gradient. The observed phase-bluing effect is characterised by a gradient of -0.0603±0.0268 % /(100 nm) /degree. As for the post-perihelion period, this terrain displays a mild phase-reddening effect characterised by a gradient of 0.0585±0.0220 % /(100 nm) /degree, which is about half the value recorded for the average nucleus in post-perihelion period. This suggests that either there has been a re-deposition of dust on the eroded surface or the eroded surface has less subsurface volatiles left. Table 3.7 succinctly shows the evolution of the phase-reddening effect calculated in this study for the studied four terrains both individually and collectively. What is further interesting to remark is that, although smooth terrain 1 and 2 are characterised by the local presence of dust grains, their sub-surfaces could be different in composition, which adds to the local compositional heterogeneity of the comet. The summarised data also suggests that the rugged terrain (probably formed of consolidated material) was the terrain that has been most resistant to the erosion, with the least change of the phase-reddening effect observed between the pre and post-perihelion period. and the comet itself in general, it is interesting to see how these values compare with the other cometary nuclei. Table 3.8 lists the measurements of some of the spectral slopes for cometary nuclei known to date. Indeed, the spectral slope values of the comets 9P/Tempel and 103P/Hartley respectively reported by [START_REF] Li | Deep Impact photometry of Comet 9P/Tempel 1[END_REF]Li et al., 2013 indicate no evidence for phase-reddening of the nucleus, contrary to the case of the comet 67P reported by Fornasier et al., 2015;Ciarniello et al., 2015. This could be due to the relative inadequacy of data for the nuclei of 9P/Tempel and 103P/Hartley resulting from the transient data acquisition during the flyby phase of their respective missions, whereas Rosetta had a copious amount of data in comparison, which permitted a better characterisation of the phase-reddening as well as the determination of local heterogeneities of the spectral slope. Additionally, as far as the nucleus of the comet 103P/Hartley is concerned, the large error bars of the results of Li et al., 2013 for the 93 o phase angle are highlighted, which makes the task of interpretation of any phase-reddening impossible. In the analysis of the data of the comet 67P, one has to be cautious as to not interfere with the surface changes took place near and at the perihelion and take into consideration the data that are not affected by cometary activity. Hence, data presented in the Table 3.8 for the comet 67P have been chosen such that they correspond to periods when the comet was not active. I further included data which correspond to the post-perihelion data for the three terrains in the Khonsu region, that do not show any phase-bluing effect as evidenced in the Fig. 3.12 for the purpose of comparison. The mean spectral slopes of the three terrains were computed for each of the observations with varying phase angles and it appears that these local values correspond Chapter 3. Results

well to those of the general nucleus of 67P and share a similar phase-reddening effect. † Using the data from January-July 2016 for three selected terrains in the Khonsu region.

Bright features observed in the Khonsu region

The first detection of a bright feature in the Khonsu region was reported by Barucci et al., 2016 as detailed earlier in this chapter and here three more bright features that have been observed in the Khonsu region are presented. Attention has also been given to their temporal evolution. First, observational circumstances of the four bright spots are presented in Table 3.9. Then, each bright patch is studied in terms of their spectrophotometry and temporal evolution depending on the availability of the OSIRIS data.

• Bright feature K1 This bright feature has been observed since 29 January 2015 up to 07 May 2015, which amounts to a little more than three months. It is located close to the so-called pancake feature of the Khonsu region and seem to be resting on a top of an isolated boulder in a dark smooth terrain. The approximative estimates of physical dimensions of this feature indicate an on-going sublimation. Barucci et al., 2016 report the unambiguous detection of H 2 O ice on this feature using the VIRTIS-M infrared observations. Moreover, Deshapriya et al., 2016 report four bright features and this feature is included in their analysis. Fig. 3.14 puts into perspective the temporal evolution of this feature. The spectrophotometric plots given in Fig. 3.15 hint towards the on-going sublimation of the bright feature and its subsequent diminution of albedo. As of early March, K1 appears to be enriched in H 2 O ice and towards the April and May, a diminution of its quantity can be interpreted from the observation that the flat nature of the spectrophotometric curve tends to deviate and follow the trend of the typical nucleus in near-UV 3.9 for this feature. † This is an OSIRIS WAC observation. ‡ This is a cluster of smaller bright patches.

wavelengths. The error bars also get more scattered towards May, as seen in the case of the observation of 02 May 2015, indicating that the original H 2 O ice quantity has reduced significantly and that what has been sampled is a mix of residual H 2 O ice and typical organic material from the nucleus. Given the physical growth of the feature from 10 March to 25 March, it is possible that it was being replenished by H 2 O ice from a potential reservoir beneath.

• Bright feature K2

This bright feature appears to be resting on a boulder or a geological feature, referred to as 'Boomerang feature' by Deshapriya et al., 2016, similar to the case of K1. Indeed, this configuration is often observable in numerous regions (refer to the fig. 4 of Pommerol et al., 2015) on the surface of 67P. This feature is located at the bottom of the cliff wall that separates the Khonsu and Apis regions. This bright feature has been observed to be stable for longer duration compared to the other bright features observed on the comet. Its first detection dates to 23 January 2016 and it has been observed at regular intervals up to 24 June 2016 by OSIRIS cameras, whereas a NAVCAM image has been able to record it again on 29 July 2016, thus giving evidence for the longevity of this feature up to more than 6 months. This is probably due to the local illumination conditions at the feature, often subject to be cast in shadows due to the cliff wall towards the Apis region. Fig. 3.16 gives an idea of the evolution of this feature, driven by on-going sublimation during this time and additional details about corresponding observations could be found in the Table 3.9. When it comes to multi-filter observations, there are only two epochs of observations with multiple filters, which allows to perform spectrophotometric measurements on this feature. This corresponds to the surface observations done on 23 January and 27-28 January 2016 with about a 30-minute cadence. The corresponding spectrophotometric plots with a contextual view are given in the Fig. 3.17 and Fig. 3.18.

• Bright feature K3

K3 appears to be one of the features with the shortest life times. It has been observed only once and this corresponds to a post-perihelion observation on 10 February 2016.

The available spectrophotometric plot is given in the Fig.

Distribution and evolution of exposed bright features on the comet 67P

Given the context of bright features that have been hitherto presented in this chapter, I envisaged to implement a global study of all the exposed bright features that have been imaged on the nucleus of the comet 67P by the Rosetta mission. To achieve this task, I mainly relied on the OSIRIS NAC due to its better spatial resolution while complementing the search for the bright features with OSIRIS WAC for the time intervals where OSIRIS NAC images are absent. In some cases, I used Rosetta NAVCAM images too, for the same search, specially to constrain the observed life times of the features. Thus, this search spanned from data that had been recorded since early August 2014 , when the surface of the comet 67P was resolved for the first time, up to the end of the extended mission in September 2016 , when Rosetta touched down to the surface of the comet at the end of its extended mission.

When referring to bright features, that are composed of exposed H 2 O ice, it became clear that a proper definition was necessary in order to avoid potential ambiguities and deliver a clear message. This ambiguity was partially driven by the various local illumination conditions, that resulted in bright surface features, which did not have the spectrophotometric behaviour of H 2 O ice. In addition, the diurnal cycle of exposed H 2 O, also interfered with the unambiguous identification of exposed H 2 O ice, because the H 2 O frost, on the morning side of the comet showed spectrophotometric properties similar to H 2 O. Frost has a transient presence (about 2-3 minutes when 67P was near its perihelion as studied by Fornasier et al., 2016), before sublimating away as the surface gets insolated during each cometary rotation once the comet is inside the current snowline of the solar system. Because of this reason, frost features were excluded in the search of exposed bright features on the nucleus, despite the fact H 2 O frost naturally falls into our definition of exposed bright features. However, its spectrophotometric properties will be briefly presented and discussed in this context. Apart from this, there are volatile-rich terrains on the comet, that do not necessarily contain the exposures of ices as they are often well-mixed with desiccated refractory material on the comet or are sub-surface structures that appear as the surface gets eroded. In order to distinguish the exposed bright features from volatile-rich terrains, I attempted to visualise these effects in Fig. 3.21, where these different terrains are distinguished from each other using their spectrophotometric curves, although an analysis based on RGB images does not make it quite evident.

As seen in 3.21, the volatile-rich regions (blue dots in spectrophotometric plots), are characterised by a red spectral behaviour with two gradients. From near-ultraviolet to about 650 nm, they have a gradient that is higher than the one, they have from about 650 nm to nearinfrared wavelengths. Furthermore, they have a higher absolute reflectance than the nucleus (represented by black dots in the plot), for all the wavelengths concerned. Therefore, it can be concluded that these volatile-rich terrains have a higher albedo than the nucleus. The exposed bright patch, represented by the red dot in the spectrophotometric plots, has even a higher albedo, as its reflectances are greater than both of the volatile-rich terrain and the nucleus for all the wavelengths concerned. Given that the exposed bright features have a flat spectrophotometric curve in all visible wavelengths, their presence can, in this way, be distinguished from that of the volatile-rich terrains. Another characteristic of this type of volatile-rich terrains is that they appear in a bluish colour in false-colour RGB images made FIGURE 3.21: Top left -A NAC mono-chromatic full frame OSIRIS NAC image (F22 filter centred at 649.2 nm), revealing a part of the Khonsu region on the comet where volatile-rich (bluish material on RGB images) and exposed bright patches are present in the form of a cluster, right -an RGB image created using the NAC filters centred at wavelengths 882.1 nm, 649.2 nm and 480.7 nm for red, green and blue colours. This image also depicts 3 regions of interest chosen for the spectrophotometric analysis. Bottom left -spectrophotometric plots of the aforementioned regions of interest, right -a zoom onto the 3 regions of interest on the RGB image. The red box refers to an exposed bright patch (Deshapriya et al., 2016) containing H 2 O ice, whose reflectance has been sampled (1x2 pixels per a filter) with scrutiny in order to avoid saturated pixels and blue box corresponds to volatiles mixed with desiccated refractory material from the nucleus (2x2 pixels) , whereas black box (2x2 pixels) represents the typical nucleus. Note that the dimensions of the boxes are deliberately exaggerated in order to point out their respective locations on the terrain. The red and green features that sometimes appear adjacent to the shadows and near the frame edges in the RGB images are artefacts arising from the imperfect co-registration/receding shadows of the images. using filters centred at 480.7 nm, 649.2 nm and 882.1 nm, whereas the exposed bright features tend to have a rather dark bluish or whitish colour in the RGB images.

As such, this work is based on the exposed bright features that have flat spectrophotometric curves in visible wavelengths. Given the flat nature of the visible spectra of H 2 O ice presented in the Chapter 2, and the successful OSIRIS-VIRTIS study to unambiguously identify the presence of exposed H 2 O ice, based on OSIRIS spectrophotometric curves, it is suggested that all the exposed bright features presented in this study contain H 2 O ice.

Defining exposed bright patches

Two criteria were established that are used as the definition of an exposed bright feature.

1.

A ratio of at least 1.5 between the reflectance of bright feature and that of the typical nucleus for a given filter.

2. Presence of a flat spectrophotometric curve in reflectance.

The first criterion yields that the reflectance of the bright feature in question is greater than that of the nucleus for all the available filters used in spectrophotometry. Since the observed reflectance depends on the phase angle, if two different bright features observed at the same phase angle were to be compared, a higher ratio would correspond to an exposed feature, that would be fresher and less contaminated. The second criterion requires that the observations be made with multiple filters, at least with three, which allows to check the presence of a flat visible spectrum for the feature in question, in turn permitting the identification of H 2 O ice. Adhering to the above definition, the study led to the finding of 51 different features of exposed bright patches in the multi-filter observations where spectrophotometric analyses were possible. In the cases, where multi-filter observations were not available, it was possible to find 6 different features, that partially satisfied the definition, as they only met the first criterion. As such, the high ratio of reflectance between the exposed bright feature and the surrounding nucleus was used as a proxy for the presence of H 2 O ice. It is highlighted that, these 6 features that partially fit the definition are not exhaustive.

Five types of exposed bright features

Although the number of found exposed bright features totals to 57, this number does not represent the total number of individual features that were present on the nucleus, because a feature could appear in a variety of morphologies: a pair of features, a cluster of many scattered smaller patches, etc. Therefore, it is remarked that the total of 57 features is not an exhaustive measurement of all the individual bright patches that have been studied. In order to classify the studied features according to their morphologies, I followed the approach of three types of exposed bright features presented by Pommerol et al., 2015 using the early OSIRIS observations of the comet 67P, and expanded it leading to five types. They are given in the Table 3.10 along with respective counts corresponding to individual feature type. As mentioned earlier, frost is included among the five types of exposed bright features because it simply fits to the above definition, although it is not included in the search for the features. 3.10) respectively and the black dot indicates the unique detection of CO 2 ice (Filacchione et al., 2016b) where H 2 O ice was detected few weeks later (Fornasier et al., 2016). This map is the cylindrical projection of a merged shape model resulting from the shape models SHAP4S (Preusker et al., 2015 andSHAP5 (Jorda et al., 2016) respectively for the northern hemisphere and the southern hemisphere.

Each of these five types will be detailed in the following sub-section with examples from OSIRIS images and spectrophotometric curves. The locations of all the 57 features can be visualised in the Fig. 3.22 with a colour code that refers to the type of the feature as they are displayed on a cylindrically projected map of the comet. This map makes it clear that the locations of studied exposed bright features follow an equatorial trend, while most of them are located in the southern latitudes of the comet. However, special care should be given interpreting this, as this could be an observational bias owing to the fact that it was the southern hemisphere that was observed the most during the mission.

Then, each exposed bright feature is presented in a form of a catalogue in the Table 3.11 along with respective observational circumstances. The numbers 1-51 refer to the cases with multi-filter observations that enabled spectrophotometric analyses where characteristic flat spectrophotometric curves of H 2 O ice could be used to confirm its presence. The numbers 52-57 refer to the observations with a single filter, in which case, the higher reflectance of H 2 O compared to the surrounding nucleus terrain was used as the proxy. It is recalled that the observations from 52-57 are not exhaustive. The column 'Detection' corresponds to the first observation of the feature, whereas the column 'Life' gives the number of days that the feature has survived. The days are calculated using the supplementary temporal observations of the same feature and subsequently imply a lower limit of real life time of the feature. An absence of life time means that the feature in question has only been observed once, hence being unable to determine a life time. The area measurements of the clusters of features are not given due to the fact that the measurements might carry errors due to the presence of a number of smaller individual features present on different morphological structures. TABLE 3.11: The catalogue of exposed bright features on comet 67P. Column description : identification number of the exposed bright feature, first detection time, the type of the feature (1)(2)(3)(4), area of the feature, lifetime of the feature, phase angle at the time of the first detection, spatial resolution, latitude, longitude, observing instrument and remarks which refer to other mentions of the same feature by different publications. The remarks column should be read with the following legend. B: Barucci et al., 2016, Da: Davidsson et al. (in prep.), De: Deshapriya et al., 2016, Fi(a): Filacchione et al., 2016a, Fi(b): Filacchione et al., 2016a, Fo16: Fornasier et al., 2016, Fo17: Fornasier et al., 2017, O: Oklay et al., 2016, P: Pommerol et al., 2015. N and V in

Isolated patches on smooth terrains

The isolated bright patches that are found on smooth terrains (distinguished by the presence of a lot of dust), belong to this type. They begin to appear on the comet when the surface erosion becomes prominent as the comet approaches the perihelion. Indeed, the erosion process is activated by the sublimation of volatiles such as CO, CO 2 and H 2 O ices, whose rate gradually increases as the comet gets closer to the Sun in its in-bound orbit. In comparison to the consolidated cometary material on rugged terrains of the comet, smooth surfaces are relatively easier to erode since they have lower tensile strength (Groussin et al., 2015a).

As evident from OSIRIS data, cometary erosion was observed mostly on smooth terrains during the period prior to the perihelion passage of the comet.

Fig. 3.23 illustrates the evolution of three selected isolated bright patches located in Bes, Imhotep and Hapi regions of the comet. The panel A of the figure shows the evolution of an isolated bright patch that has a life time about a month. During its evolution, it is possible to notice that another companion patch appears beside it, although it sublimates away much rapidly. The bright patch is seen to grow in size as the increasing erosion leads to more of its exposure. The observations of the panels B and C corroborate well with erosion-driven scarp-retreating processes (El-Maarry et al., 2017) that led to their appearance. Imhotep and Hapi are regions characterised by a lot of smooth terrain and the observed evolution of bright patches clearly points to a lot of erosion taking place. Analysing the appearance of exposed bright patches on the Imhotep region, Groussin et al., 2015b concluded that the observed erosional rates cannot be solely due to sublimation of volatile species at the corresponding heliocentric distances and that it could be due to the low tensile strength (Groussin et al., 2015a) of the material or the processes like clathrate destabilisation or amorphous water ice crystallisation that would be required to produce the observed erosion rates.

My search yielded 14 such exposed bright features to be classified in this Type 1. Almost all of them were observed from early March to early August 2015, which correlates well with the decreasing heliocentric distance. It is important to note that the Rosetta spacecraft was obliged to stay away from the active nucleus following the perihelion passage from September 2015 up to January 2016, which prevented the nucleus from being imaged with satisfactory spatial resolutions to detect any isolated bright features on smooth terrains. Nevertheless, despite Rosetta spacecraft got closer to the nucleus later on, not a single Type 1 feature was observed up to the end of the extended mission in September 2016.

Isolated patches close to irregular structures

The features belonging to this Type-2 (Fig. 3.24) are located close to irregular structures such as : cliffs, boulders, terraces, etc. Owing to the shadows, often cast by their neighbouring structures, these features do not receive insolation for long periods of time, as opposed to their counterparts located on smooth terrains. Hence they are not observed repeatedly, which impedes the studying of their life times. In more favourable conditions, a life time of about 2 months can be attributed to one of these features. The observations indicate that these patches appear on consolidated cometary material, which are rather resistant to erosion. Therefore, a clear picture about the formation of this type of features is difficult to build. Nevertheless, there have been 18 patches of this type, corresponding to the largest number of bright features belonging to any single type. These seems to be no relation with the heliocentric distance, as these patches have been observed throughout the mission, which suggests that it is the local illumination conditions that are key to their sublimation processes. Panel b corresponds to about 3 weeks before the perihelion passage and there is a couple of bright patches that have already appeared on the smooth terrain probably due to the erosion that had been escalating at this time. The next panels from c through g illustrate the appearance of a family of transient outbursts and a long-lived outburst with a lifetime in the order of several hours, as the comet was reaching the perihelion of its orbit. The next panel h, where the area of interest is resolved about 4 months later, makes it clear that there have been significant changes on the surface morphology therein. The observable boulder field is therefore likely to be a direct result of the aforementioned series of outbursts. Finally, the panel i reveals the bright patch that has appeared on the boulder, some 2 months later.

Bright patches resting on boulders

This Type-3 corresponds to exposed bright patches that appear on boulders. Few examples of this type are given in the Fig. 3.24. Unlike other exposed bright features, it is possible to correlate potential activity sources for this type of bright patches. For instance, a cometary outburst could be correlated with an exposed bright patch that appeared in the Khonsu region. The first observation of this patch (Fig. 3.16) dates to January 2016 and it has been observed at regular intervals up to July 2016, despite its on-going sublimation. The outburst in question, took place on early August 2015, just prior to the perihelion passage and would have modified the would-be location of the boulder. Using the composition of images in Fig. 3.25, and analysing the images prior and posterior to the outburst, it is possible to notice considerable changes on the surface of the terrain, and a later appearance of a boulder and an exposed bright patch atop. This has been discussed by Deshapriya et al., 2016 and was attributed to this activity event that might have connected the sub-surface reservoirs of volatile ice layers to sustain the bright batch for the observed extended duration of about 6 months without a rapid sublimation.

Another example for a potential activity source related to an exposed bright patch appearing on a boulder can be found in the Bes region of the comet (Fig. 3.26). Comparing the topography and geometry of a field of boulders in images taken at regular intervals, it can be established that some activity source has triggered a displacement of a boulder between 07 February and 04 March 20165 in this region. A comparison between the panels a and b of the Fig. 3.26 leads to the identification of the boulder annotated 'a', between the acquisition times of the two images. It is also possible to notice that the boulder in question has an exposed bright patch that has appeared probably upon its displacement. One could further observe that its distance and orientation with respect to the centralised boulder, annotated 'c' have changed during the time elapsed between these two observations. The triggering source for this activity, is thus not possible to be determined due to the inadequacy of the observations. However, it is likely that the same activity source that resulted in the displacement of the boulder, was responsible for the appearing of the exposed bright patch on it. The following panels of the figure offer a perspective of the evolution of this feature, occasionally observed up to mid-September 2016, with improving spatial resolution (∼6 cm/pixel).

In March 2016, this patch corresponded to an area about 70 m 2 and by mid-September its area corresponded to about 7 m 2 , which gives an idea of the order of the sublimation that has been taking place during the 6 months since its detection.

Currently, I have been able to gather only the above two events for the activity related formations for the Type-3 of exposed bright features. This could be best explained by the absence of observations during the transient events that resulted in the appearance of exposed bright patches on the boulders. Nevertheless, based on these two cases studied here, the Type-3 features are suggested to be correlated to sources of cometary activity.

Clusters of patches

This Type-4 of exposed bright features has a unique morphology as they appear as clusters of individual patches in the order of 2-3 m of diameter, and hence are smaller than all the aforementioned features. These features have been observed mostly in the pre-perihelion period and their locations are correlated with cliff structures. By this way, it could be assumed that they are related to cliff collapse events, leading to the exposure of their internal material. In Imhotep region, Filacchione et al., 2016a studied two clusters of patches which they referred to as bright albedo patches, where they determined the presence of H 2 O ice with millimetre-sized grains, using the spectro-imager instrument VIRTIS-M. Another example is a cliff collapse in the Aswan area of the comet, as studied by Pajola et al., 2015;Pajola et al., 2016. Fig. 3.28 shows the evolution of a cluster of exposed bright patches in the Khepry region. This cluster was detected at the end of August 2014, and follow-up observations leading up to the end of November 2014 indicate the stability of these features probably due to the low sublimation rates, strongly influenced by the local illumination conditions. This cluster could have remained stable for even longer, but the lack of recurring observations limits monitoring its life time further. As shown in the bottom right panel of the figure, an observation of the same area on 25 March 2015 reveals that the entire cluster has completely sublimated away. Thus, it is possible to infer a lifetime of about 3 months for this feature, which is a lower-limit. Some of the individual features are numbered in the figure, in order to follow their stability over the given time scale. This cluster was first studied by Pommerol

Frost features

De Sanctis et al., 2015 reported the first detection of H 2 O frost on the nucleus of the comet as soon as Rosetta's arrival at the comet, at the heliocentric distance of about 3.6 AU during the in-bound phase of the orbit of the comet towards the Sun. Then, OSIRIS continued to observe the presence of H 2 O frost on the comet up to the end of the extended mission, when the comet was at the heliocentric distance of about 3.8 AU in its out-bound phase of the orbit. The frost presence was more pronounced during the perihelion passage (Fornasier et al., 2016) of the comet in August 2015. This can be explained by the increased H 2 O production rates due to the decreasing heliocentric distances. Frost could be observed at the local dawn on the comet, as the sun light returns after about 6 hours. As the shadows recede, frost formed from the re-condensation of H 2 O vapour, can be observed for a transient duration, prior to its complete sublimation. The duration, during which frost is being sublimated depends greatly on the the heliocentric distance, and Fornasier et al., 2016 estimated it to be about 3 minutes when the comet was close to its perihelion at about 1.24 AU away from the Sun. As such, this ephemeral phenomenon continues to take place as long as solar irradiation is intense enough to sublimate the frost. During its out-bound orbit, once the comet passes beyond the current snowline of the solar system, the diurnal cycle of H 2 O ceases to operate, with no further sublimation taking place, until the comet returns to the inner solar system and passes the snowline in its in-bound orbit, for its next perihelion passage.

It is possible that the four types of exposed bright features introduced earlier, contributed to the diurnal cycle of H 2 O on the comet, as they have been constantly sublimating, which could liberate the amount of H 2 O ice necessary to sustain the diurnal cycle of water on the comet.

Spectrophotometry of 5 types of features

I carried out spectrophotometric analyses for all the exposed bright features, which had multi-filter observations. This corresponded to 51 out of the 57 features and for each of the 5 feature types, a representative example is presented in Fig. 3.29 with accompanying spectrophotometric plots. Although frost features were not included in our catalogue of exposed bright features due to the reasons explained earlier, characteristic spectrophotometric plots are also given for frost.

From the flat nature of the reflectance curve for all the 5 types of bright features, it is now clear that they have flat visible spectra, which corresponds to volatile ices such as H 2 O , CO 2 , CO, etc. Although OSIRIS instrument is not capable of distinguishing among these potential volatile species, the preferred interpretation is the H 2 O ice due to three reasons. Firstly, there have been several (Barucci et al., 2016;Filacchione et al., 2016a;De Sanctis et al., 2015) studies based on VIRTIS-M spectroscopic data, that confirmed the presence of exposed H 2 O ice features on the comet. Secondly, CO 2 has been the only other volatile species that has been identified locally (Filacchione et al., 2016b) by means of spectroscopy on the comet and its presence was very transient given the nature of CO 2 as a super-volatile species. Thirdly, the observed life times of the features are evident to the fact they observations could not be of super-volatiles which would have rapidly sublimated, thus leaving H 2 O as the only candidate to be the observed sources. According to this analysis, it is suggested that the observed ices are most likely to be H 2 O ice.

Albedos of exposed bright features

In an attempt to determine the albedos for these features, I adopted the linear-exponential phase curve of bright features presented in Hasselmann et al., 2017, who used the OSIRIS WAC data acquired during the Rosetta's fly-by of the comet from 09-10 April 2016. In their work, these authors used the images acquired using WAC F18 filter, centred at 613 nm, while I used the data acquired using NAC F22 filter, centred at 649.2 nm, which is the closest to the latter. I discarded the effect of phase-reddening on bright features under the assumption that its effect is negligible given their low spectral slopes values. It is important to note that the data that corresponded to high phase angles (usually > 80 o ), were not included due to the limited applicability of the aforementioned phase curve of bright features. Adapting the aforementioned phase-curve, the morphological parameters for the photometric phase curve, which depend on the phase angle (α), and the wavelength (λ), could be obtained according to the Eq. 3.3, where A is the amplitude of the opposition surge, d is the width of the opposition surge, b is the phase-curve albedo without opposition surge and k is the angular coefficient of the linear part (values presented in Hasselmann et al., 2017 are used here). These morphological parameters can then be used for the phase correction.

r l-e (α, λ) = Ae -α/d + b -kα (3.3)
Then, the photometrically corrected radiance factor A eq (ᾱ,λ') at a mirror point fixed at ᾱ, or the so-called equigonal albedo [START_REF] Shkuratov | Optical measurements of the Moon as a tool to study its surface[END_REF], can be calculated through the Eq. 3.4. This leads to the calculation of the normal albedo A N (ᾱ=0 o ,λ'), when ᾱ equals 0 o as shown in the Eq. 3.5.

A eq (ᾱ, λ ) = r F,C(α,i,e,λ) r l-e (α, λ) r l-e (ᾱ, λ ) (3.4) A N (ᾱ = 0 o , λ ) = r F,C(α,i,e,λ) r l-e (α, λ) r l-e (ᾱ = 0 o , λ ) (3.5)
where, r F,C(α,i,e,λ) is the radiance factor already corrected for the illumination conditions using Lommel-Seeliger disk function which accounts for the illumination conditions due to topography. The normalisation at 0 o phase angle is represented by r l-e (ᾱ=0 o ,λ') , evaluated according to the Eq. 3.3.

The resulting normal albedos are presented in the Fig. 3.30. Interpreting the figure, it is possible to notice that the Type-1: isolated patches on smooth terrains have albedos in the range of 15%-30%, while the Type-2: isolated patches close to irregular structures share albedos in the range from 20%-50%. The Type:3, the patches resting on boulders have albedos in a more dispersed range from 15%-75% and the Type-4: the clusters of features have albedos in the range from 10%-50%. Thus, it becomes clear that Type-1 features have the lowest upper limit of albedos, whereas the Type-3 features have the highest upper limit of albedos.

Grain size distributions of H 2 O ice in cometary nuclei and implications

Thanks to space missions that have explored cometary nuclei, several grain size distributions are available. These grain sizes are directly coupled with their whereabouts in the cometary nucleus and hence they could give clues to the various physical and chemical processed that governed their evolution. In this sub-section, retrieved grain size distributions of H 2 O ice, using spectroscopic modelling are presented in Table 3.12. Most of these results come from the comet 67P, while those from 9P/Tempel (Deep Impact mission in 2005) and Prior to Rosetta's arrival to the comet 67P, the know-how of the grain size distributions of H 2 O ice in comets was limited to micrometer-scale. These results were based on the exhumed sub-surface H 2 O ice that got exposed following the impact experiment of Deep Impact mission on the comet 9P/Tempel (Sunshine et al., 2006) and the observations of hyper-active comet 103/P Hartley [START_REF] A'hearn | EPOXI at Comet Hartley 2[END_REF], returned from the EPOXI mission. Then, as early as the arrival at the comet 67P, Rosetta's VIRTIS spectro-imager detected an active diurnal cycle of H 2 O ice (De Sanctis et al., 2015), whose grain sizes, which were in the order of about 2 µm, corroborated the existing results for cometary nuclei. However, as more results came in from the comet 67P, the distribution of H 2 O grain sizes appeared to be diverse. Filacchione et al., 2016a assumed two potential H 2 O ice grain size distributions with a mean diameter of about 56 µm and 2 mm, followed by Barucci et al., 2016, whose results implied three grain size distributions for H 2 O ice. As such, there are four possible distributions whose presence has been revealed through spectroscopic modelling in cometary nuclei: ∅<10 µm,10 µm<∅<100 µm, 100 µm<∅<1 mm, 1 mm < ∅.

The presence of different grain sizes of H 2 O ice is interesting as it suggests different kinds of thermal histories depending on the grain size. Grains in the order of several µm have been the most common grain size and imply a pristine nature, that is, that the formation of these grains date back to the beginning of the solar system, as expected for the origin of most of the comets. Corresponding to the same order of magnitude, the grain sizes of the H 2 O frost found in the diurnal cycle of H 2 O on the comet 67P, are also the result of fast condensation of H 2 O vapour trapped in the upper surface layers, taking place per each cometary rotation. Similar-sized grains were detected following the impact experiment of the Deep Impact mission, which allowed to probe the condensed H 2 O in the upper surface layers of the comet 9P/Tempel. However, the exposures of H 2 O ice patches in the Imhotep region as observed by Filacchione et al., 2016a imply a bimodal distribution of grain sizes centred around 56 µm and 2 mm in diameter. Moreover, the results of Barucci et al., 2016, indicate the presence of three aforementioned distributions plus a forth distribution, in the order of several hundred µm. This variability of size is the H 2 O ice grain distributions, if confirmed, implies different growth mechanisms for the grains. The existence of millimetresized grains can be explained by the growth of a secondary population of ice grains due to H 2 O vapour diffusion towards internal deeper layers which are already rich in H 2 O ice or by the process of sintering6 of ice grains.

The required energy for such processes to take effect, comes from the thermal heat wave that propagates onto the interior of the comet and sublimates the deep-buried ice. In addition to the thermal heat wave, there could also be a heat component due to the transition of H 2 O ice from amorphous to crystalline phase, although less effective than the latter. Hence, these are the processes that are more dominant when the comet is close to the perihelion of its orbit. Indeed, Benkhoff et al., 1995, elaborating on the results of the KOSI experiment, which simulated the evolution of porous H 2 O ice in space, show that depending on the dust-to-ice ratio, more than 80% of sublimating H 2 O ice did not escape away through the dust-mantle, but rather got re-condensed and re-deposited at deeper layers, which could effectively contribute to the growth of ice grains in size. Thus, development of larger ice grains through the recurring nature of the thermal heat wave could substantially influence the thermal processing of a cometary nucleus. This in turn fills up the voids in the internal layers, which effectively reduces the porosity of the nucleus and in the process, increases the density. As a result, thermal conductivity increases as does the thermal inertia. The depth to which this densification could be effective has been studied by Kossacki, Szutowicz, and Leliwa-Kopysty ński, 1999 to be 1-10 m by means of integrated orbital simulations for the nucleus of the comet 46P/Wirtanen, which was the initial target of the Rosetta mission. Thus, these simulations, the results from KOSI experiment and observations of different cometary nuclei by in-situ spacecraft, suggest that the stratification on active comets is constantly evolving.

For the sake of comprehensiveness, it is mentioned that the unique detection of exposure of CO 2 ice on the nucleus of the comet 67P by Filacchione et al., 2016b, yielded a pair of distributions of grain sizes for CO 2 ice. Following their spectral modelling, they estimate: a) an intimate mixing based CO 2 abundance of 0.8% corresponding to grains of 800 µm with a goodness-of-fit χ 2 of 2.45, b) an areal mixing based CO 2 abundance of 0.1% corresponding to grains of 50 µm with a goodness-of-fit χ 2 of 2.05, over an area of 80x60 m 2 in the Anhur region, as this region came out of the long-lasting southern winter and started to be illuminated in late March 2015 (about 5 months prior to the perihelion passage). The authors prefer the solution with the areal mixing, whose goodness-of-fit χ 2 is slightly better.

The detection of exposed CO 2 ice implies seasonal effects reigning on the local surface during its orbit around the Sun. This can be explained as follows. As the comet passes through perihelion and recedes from the Sun, its surface temperatures begin to drop and in the case of objects like comet 67P, having a very irregular shape and an oblique rotational axis with respect to its orbital place, some regions experience sudden drops of temperature. This is the case of the Anhur region, which is located in the southern latitudes of the comet, subject to a short but intense southern summer and a long southern winter. As such, the comet passes beyond the perihelion in its out-bound orbit and as soon as the equinox is passed, the temperature in Anhur begins to drop substantially. Nevertheless, due to the low thermal inertia of the comet, the internal layers of the comet could stay relatively warmer 3.12: The ice grain sizes derived from spectroscopic modelling for different cometary nuclei, hitherto observed by in-situ spacecraft. The best mixing modality refers to the parameters obtained with the least χ 2 value between intimate (I) and areal (A) mixing. The references, S, A, DS, F and B refer respectively to Sunshine et al., 2006;[START_REF] A'hearn | EPOXI at Comet Hartley 2[END_REF]De Sanctis et al., 2015;Filacchione et al., 2016a;Barucci et al., 2016. The feature Types are also displayed where applicable, in order to find any correlations. than the surfaces that are not illuminated. As the comet would then be out-bound and heading towards the aphelion, its diurnal cycle of H 2 O would begin to cease as there would not be as adequate solar insolation to sublimate the H 2 O ice (around the current snowline of the solar system). However, the internal heat would be sufficient enough to sublimate CO 2 and thus, as the liberated CO 2 molecules try to escape the nucleus, they get trapped in colder outer dust mantle, where they would be re-condensed. Then, the CO 2 ice grains could grow in a similar way to H 2 O ice grains, as explained earlier and result in near milli-metric sizes as demonstrated by Filacchione et al., 2016b. These surface deposits could survive until the summer returns to their locations during the comet's following orbit. The fact that the exposures of CO 2 were detected soon after the equinox, when Anhur region was illuminated back as the comet was in-bound and heading towards the perihelion, corroborate well the above hypothesis.

It is interesting to remark that Fornasier et al., 2016, using OSIRIS data, detected two large (∼1500 m 2 ) H 2 O ice patches on the same location of the Anhur region, where exposures of CO 2 ice had been detected. The detection of H 2 O patches was about several weeks later to the detection of CO 2 ice on this area, generally covered by a dust mantle. In this context, these detections imply the differentiation of volatiles in the sub-surface layers of the nucleus, which further adds complexity to the interpretation of evolutionary processes that dictate the physics of cometary nuclei.

Chapter 4

Conclusions T he Rosetta OSIRIS data allowed me to implement a spectrophotometric study of the nucleus of the comet 67P during past three years. The main objective of my research was to understand the surface properties of the comet and their evolution over time as the comet approached and passed through the perihelion. I used the images acquired by OSIRIS instrument from August 2014 to September 2016 to analyse the surface morphologies of the nucleus of the comet. The copious amount of data corresponds to varying phase angles, heliocentric distances and altitudes, which translate into changing illumination conditions and spatial resolutions per an image. As detailed earlier in this thesis, there are three major scientific results at the heart of this thesis. They will be succinctly recapitulated here in this chapter and some conclusions driven from them will be presented.

The first major result corresponds to the work done to unambiguously identify the presence of exposed H 2 O ice on the nucleus of the comet 67P. This was an inter-instrumental work achieved in hand with the VIRTIS-M spectro-imager of the Rosetta spacecraft. The preliminary results of the Rosetta mission as soon as the Rosetta's arrival at the comet 67P, revealed a very dark body of a geometric albedo of 6.5 ± 0.2% at 649 nm (Fornasier et al., 2015) and this was quite an unexpected finding because comets were supposed to be made partially of volatile ices. This being the situation, the principal idea of this study was to use OSIRIS data to detect local exposures of H 2 O ice and use VIRITS-M data to probe the locations of exposed H 2 O ice, detected by the OSIRIS instrument. Although OSIRIS had a limited spectral resolution, its spatial resolution was much better than VIRTIS-M, which, on the other hand, had better spectral resolution, to verify whether the exposures detected by OSIRIS instrument, really corresponded to H 2 O ice. Thus, my contribution to this work was very crucial, as I began to look for potential candidate locations on the nucleus, where the H 2 O would be exposed, based on OSIRIS data. Since pure H 2 O ice corresponds to a very high albedo, I was examining the OSIRIS data for pixels with high reflectance values. Once potential candidate locations were found, I looked for multi-filter observational sequences, in order to be able to reconstruct the visible spectrum of the potential candidate locations. It is noteworthy that, this was not a trivial task, as I had to co-register the images of the observational sequence due to the shifts of the fields-of-view of consecutive images introduced by a time delay of several seconds switching between the filters of the filter wheel. Images were further corrected for the illumination conditions using Lommel-Seeliger disk function prior to carrying out spectrophotometric analyses on the terrains of interest. I was searching for a flat spectrophotometric curve as the proxy for the presence of volatile ices like H 2 O , which generally has flat visible spectra, absent of any absorption features. To be sure about the accuracy of this method as the proxy of exposed H 2 O ice, infrared spectral data was necessary to look for characteristic absorption bands of H 2 O ice. Thus, 13 potential candidate locations were found and their locations were transferred to the VIRTIS team to look for infrared absorption features in the VIRTIS spectra. Due to the fact that the spatial resolution of VIRTIS-M was lower than OSIRIS, I made sure to select the exposures that are as large as possible in OSIRIS data, which would favour the eventual spectroscopic detection by VIRTIS with a high signal-to-noise ratio. Following the spectroscopic analysis of VIRTIS-M, 8 out of the 13 candidate exposures, were identified to be exposures of H 2 O ice, using the absorption band at 2 µm. The VIRTIS-M non-detections of the 5 cases, does not necessarily mean the exposures found by OSIRIS, did not contain H 2 O ice. It might have been due to the fact that there had not been any VIRTIS-M data acquisitions at same period when candidate regions were imaged by OSIRIS or that the exposures of H 2 O ice had already been sublimated away at the time of VIRTIS-M data acquisition. Nevertheless, the spectral modelling that followed, suggested that the grain sizes of the exposed H 2 O ice features to be corresponding to three possible different ranges: several µm, several hundreds of µm and several mm. This size distribution implies different formation scenarios for the exposed bright features. The H 2 O ice abundances varied between 0.1 and 7.2%, mixed in areal or/and in intimate modalities with the typical dark terrain of the nucleus.

Then, I focussed my interest on the Khonsu region of the comet 67P, to implement a spectrophotometric study (Deshapriya et al., 2016) due to several reasons. Khonsu being a southern region on the nucleus, only got completely illuminated few months prior to the perihelion passage, which means that it offers a way to probe into a fresh surface, that has been in the dark for long time during the southern winter. In addition, this was a region with various interesting morphological features such as the so-called pancake feature, exposed bright features, boulders and smooth terrains. I computed spectral slopes for four different surface terrains: a) pancake feature, b) smooth terrain 1, c) smooth terrain 2, d) rugged terrain, taking into consideration the observations prior to posterior to the perihelion passage, in order to characterise the surface changes that had been taking place and the effect of phase-reddening. The spectral slopes were evaluated between the filters centred at 535 nm and 882 nm and the obtained results indicated phase-reddening for three out of four regions, with the exception of the smooth terrain 2, which corresponded to a phase-bluing effect. Succinctly put, I obtained 0.041±0.0082% /(100 nm) /degree as the slope for the preperihelion period and 0.108±0.016% /(100 nm) /degree as the slope for the post-perihelion period for the three regions displaying a phase-reddening effect. It is recalled that Fornasier et al., 2015 reported a phase-reddening value of 0.104±0.003% /(100 nm) /degree for the rotationally averaged nucleus of the comet, using OSIRIS data from late July 2014 to early August 2014, where the heliocentric distances ranged from about 3.68 AU to 3.59 AU. Their result matches well with the phase-reddening value I calculated for the above three terrains for the post-perihelion passage. Furthermore, spectral slopes for the pre-perihelion period from April to August 2015 yield a value of 0.042% /(100 nm) /degree from the work of Fornasier et al., 2015. This is well consistent with the value of 0.041±0.0082% /(100 nm) /degree that I calculated for the same period for the three selected terrains in the Khonsu region and that the value of Fornasier et al., 2015 takes into consideration the rotationally average nucleus. As such, my spectral slope analysis corroborates the work of Fornasier et al., 2015 andFornasier et al., 2016. Now, if one were to contemplate about the deviation of spectral slopes of the smooth terrain 2, I have shown that this terrain was subject to intense erosion which removed a lot of surface dust material, which resulted in the manifestation of a contrasting phase-blueing effect in the pre-perihelion period, which can be expressed by a gradient of -0.0603±0.0268 % /(100 nm) /degree. During the post-perihelion period, this terrain is characterised by a slight phase-reddening effect with a gradient of 0.0585±0.0220 % /(100 nm) /degree, corresponding to about half the value recorded for the average nucleus in post-perihelion period. This implies that either there has been a re-deposition of dust on the eroded surface or the eroded surface has had less subsurface volatiles left. The general interpretation of this spectral slope study yields that although the comet could appear homogeneous in a global level, there could be local heterogeneities based on the types of terrains explored. As the comet goes past the perihelion, some of these heterogeneities become more pronounced as they undergo different levels of erosion depending on the type of material they are made of. This was the case for the 'smooth terrain 2' which was subject to erosion more than the other terrains and led to the observation of sub-surface layers rich in volatiles such as H 2 O , hence the phase-blueing effect. Therefore, this stands as an indirect evidence of the abundance of volatiles inside the nucleus of the comet, although they are not directly observable on the surface of the nucleus. It is the cometary activity, that offers a peek to the volatile-rich sub-surface material as the comet approaches the perihelion.

My work on the Khonsu region, further yielded the detection of four exposed bright features, attributed to the presence of H 2 O ice depending on their reconstructed featureless flat visible spectra from spectrophotometry. For one of such features, it is interesting to observe a correlation with an outburst during the perihelion passage, that would have modified the local terrain, thus resulting in the later appearance of exposed bright feature which was stable for about 6 months in the post-perihelion period before complete sublimation.

Continuing on the spectrophotometric work, I decided to implement a global study of exposed bright features on the comet, in order to constrain their distribution, morphologies and albedos as well as to monitor their evolution. This involved searching for exposed bright features in the entire OSIRIS image archive and maintaining a database in a systematic way. In order to unambiguously identify the exposures of bright features that would correspond to H 2 O ice, I introduced a definition based on spectrophotometry. This led to finding 57 exposed bright features on the nucleus of the comet 67P that are attributed to the presence of H 2 O ice as presented in [START_REF] Deshapriya | [END_REF]. These features spanned a wide range of sizes from several meters to more than 150 meters. In an attempt to better understand the formation and origins of these features, I categorised them under four morphological types: 1. Isolated patches on smooth terrains, 2. Isolated patches close to irregular structures, 3. Patches resting on boulders, 4. Clusters of patches. Although a fifth category of 'frost' is also presented and discussed because it adheres to the spectrophotometric definition of exposed bright features, it is not included in the search of exposed bright features due to the fact that its formation and origin are understood thanks to the diurnal H 2 O cycle of the comet. This study suggests that, although the nucleus of the comet 67P appears to be dark in general and characterised by a red spectral behaviour, there are indeed localised H 2 O ice sources on the comet. It is the cometary activity, escalating towards the perihelion passage that leads to exposing volatile ices like H 2 O . It is suggested that the isolated H 2 O ice patches located in smooth terrains in regions like Imhotep, Bes, and Hapi, result from the frost deposition, as a consequence of the cessation of the diurnal water cycle on the comet as it recedes from the perihelion. As the comet returns to perihelion again on its following orbit, exposed icy patches are revealed by the sublimation-driven erosion, that successively removes the thin dust layers, deposited over them earlier. More intense activity sources such as cometary outbursts are capable of revealing fresher, less contaminated H 2 O ice, that has been preserved with consolidated cometary material inside the nucleus, as evidenced from exposed patches resting on boulders. The normal albedo values derived for the patches corroborate this. The mapping of the studied features reveals a distribution constrained between -30 o and +30 o of cometary latitude. This also leads to the observation that the majority of the exposed features are concentrated in the southern latitudes of the comet, although it is not possible to completely rule out an observational bias.

Furthermore, I focussed on investigating the temporal evolution of exposed bright features, in order to monitor their life times before complete sublimation. During the in-bound phase of the orbit, it was possible to observe increasing surface erosion on smooth terrains on the comet, which revealed exposed bright features. As such, the feature sizes began to grow revealing irregular shapes of such exposed features. Imhotep, the region containing largest portion of smooth terrains on the comet, was host to a several large exposed bright features, that rapidly evolved from features corresponding to about 30 m, up to irregular arcshaped features measuring up to more than 150 m in length within three weeks just before the perihelion passage. Several more evolutions of exposed bright features on smooth terrains were studied in Bes, Anhur and Hapi regions. Evolution monitoring additionally led to identifying correlations with two activity sources that would have triggered the appearance of exposed bright features. One such case corresponds to an outburst emanating from the Khonsu region close to the perihelion passage, which ended up significantly modifying the local surface terrain and a later appearance of an exposed bright feature that appears to be resting on a boulder in the post-perihelion period. This feature was then observed from January 2016 up to July 2016, indicating stability despite on-going sublimation. The other event is a displacement of a boulder that triggered the appearance of an exposed bright feature that appears to be resting on it. This feature is observable from March 2016 up to September 2016 (towards the end of the mission), without complete sublimation. The monitoring of temporal evolution for some features was hindered due to the absence of recurring observations, prior to their sublimation and hence it is not possible to obtain a life time. Nevertheless, the obtain life times span a wide range and it is therefore clear that these life times are a function of local illumination conditions, dominated by the irregular shape of the comet as well as seasonal effects. Thermal studies, based on a shape model are necessary to quantify the liberated volatile masses and sublimation rates in order to better understand the evolutions of exposed bright features.

The spectrophotometric analyses implemented in this thesis depends heavily on the quality of the co-registration of images. Hence, additional care has been given, when a) working with images acquired close to the surface (implying a faster movement of the spacecraft, resulting in a more pronounced shift of fields-of-view between a given pair of consecutively acquired images), b) sampling from features that cover relatively small number of pixels, in order to avoid any potential contamination into the sampling of exposed bright features. Generally speaking, although the Rosetta mission was aimed to answer several fundamental questions of cometary science, having taken part at latest Rosetta Science Working Team Meeting, where there were many open discussions, it appears that there are more questions today than earlier, in order to better understand and explain the aspects like formation, behaviour and the organic material of comets. The only way forward would be to support laboratory experiments to simulate the observed cometary spectra, so as to constrain the parameters such as types of mixtures, grain sizes, porosities, temperatures, etc. and emphasise the necessity of a cometary sample-return mission.

Chapter 5

Perspectives and outlooks T he Rosetta space mission, in itself and the returned scientific know-how, have signifi- cantly furthered our understanding of comets and small body populations. However, it has also raised several questions that are fundamental to interpreting some of the obtained results. For example, we are not still confident on the explicit nature of the so-called dark material found in abundance all over the cometary surface of the comet 67P. It is usually referred to as a mixture of a dark refractory polyaromatic carbonaceous component and opaque minerals. Despite numerous spectroscopic studies (Capaccioni et al., 2015;Ciarniello et al., 2015) conducted using infrared data of the VIRTIS instrument, and laboratory experiments (Quirico et al., 2016;[START_REF] Rousseau | Laboratory simulations of the Vis-NIR spectra of comet 67P using sub-micro meter sized cosmochemical analogues[END_REF] to retrieve the primordial chemical components, we are still far from understanding them completely. Quirico et al., 2016 attributed the OH group in carboxylic acids, FeS sulphides and Fe-Ni alloys as potential candidates for the observed spectrum by VIRTIS, based on laboratory experiments. This being the latest understanding of the dark material on comets, its fundamental building blocks still remain to be unveiled, despite the fact that the presence of this organic material has been established since the preliminary cometary missions to the comet 1P/Halley [START_REF] Combes | The 2.5-12 micrometer spectrum of comet halley from the IKS-VEGA experiment[END_REF], about three decades earlier.

Some of the existing knowledge about this unknown complex organic molecules stems from the the Stardust sample return mission, which played an important role as it returned dust particles collected from the comet 81P/Wild, to the Earth, to be analysed in sophisticated terrestrial laboratories. Stardust dust samples, quite ironically to its mission name, indicated the presence of dust grains larger than the micrometre-size grains that scientists expected to find in the samples [START_REF] Sandford | Organics Captured from Comet 81P/Wild 2 by the Stardust Spacecraft[END_REF]. This was based on the idea that comets were supposed to be made from the material in cold outer regions of the solar system, much likely to host interstellar grains that are older that the solar system. However this contradictory finding led to the idea that these grains have formed in the inner parts of the solar system, at relatively high temperatures and have had a mixing as they got radially distributed, prior to being integrated into the cometary nuclei. One of the significant findings of Stardust was the discovery of glycine (NH 2 -CH 2 -COOH) in the dust samples [START_REF] Elsila | Cometary glycine detected in samples returned by Stardust[END_REF]. This was the first time that an amino acid was discovered in a comet. The discovery of glycine carries a paramount importance in the research of molecular building blocks that would have allowed the appearance of reproductive organisms on Earth some 1.2 Gy ago [START_REF] Butterfield | Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes[END_REF], as glycine is an amino acid used by living organisms to produce protein [START_REF] Elsila | Mechanisms of Amino Acid Formation in Interstellar Ice Analogs[END_REF]. Hence, it could be postulated that such exogenous molecules vital to the terrestrial life, were delivered to the Earth by the impactors like meteorites, comets and asteroids. As such, Stardust mission offered insights into the material that emanate from cometary nuclei.

The latest of all the cometary space missions, Rosetta, further lengthened the inventory of identified molecules of comets with the advanced scientific instruments which analysed in-situ the material coming from the cometary nucleus. Most of the identified species came directly from the ROSINA mass spectrometer, which analysed the coma of 67P. One of the striking ROSINA results was the measure D/H ratio of H 2 O of the comet 67P, which appeared to be about three times larger than the value found on the terrestrial oceans. As depicted in Fig. 1.9 and elaborated in Chapter 1, this implies a strong heterogeneity on the JFCs, preluding to the idea that H 2 O on Earth did not come from comets like 67P, but rather likely from primitive asteroids [START_REF] Altwegg | 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio[END_REF]. Furthermore, ROSINA led to the finding of a variety of molecules in the coma of 67P as seen in the Fig. 5.1. As was seen in the samples returned by Stardust mission from 81P/Wild, the amino acid glycine was detected in the coma and it remains one of the major discoveries of ROSINA. Then, the unanticipated discovery of molecular Oxygen (O 2 ), was the first of its kind on a comet as reported by Bieler et al., 2015. Although Oxygen (O) is the third most abundant element in the universe after Hydrogen (H) and Helium (He), it is rare to be found in its molecular form of O 2 due to its high reactivity, which leads to it being combined rapidly with other species. The same study further found that the observed O 2 /H 2 O production ratio of the coma did not vary with the heliocentric distance of the comet, suggesting that the primordial O 2 was incorporated into the nucleus of 67P during its formation, ruling out the potential O 2 production mechanisms such as photolysis1 and radiolysis2 [START_REF] Oza | Dusk/dawn atmospheric asymmetries on tidally-locked satellites: O<SUB>2</SUB> at Europa[END_REF] both of which could generate limited amounts of molecular Oxygen in the upper layers of the comet that would already have been eroded and depleted of, since the time comet 67P has been a JFC. As such, the discovery of primordial molecular Oxygen strongly implies amendments to some of the existing models of the formation of the solar system (Rubin et al., 2015b). Molecular Nitrogen (N 2 ) is another species that was found in the coma of the comet 67P by ROSINA (Rubin et al., 2015a), that could pose constraints on the temperatures of the region where the cometesimals of 67P formed. N 2 is considered to be the most abundant N-bearing species in the protosolar nebula, from which the giant planets accreted and inherited the molecular Nitrogen, observed on them today. In addition, Pluto and Triton, which are supposed to have formed in the same colder outer regions of the protosolar nebular as the JFCs, possess N 2 -dominated atmospheres. Yet, until the aforementioned findings by ROSINA in the comet 67P, N 2 had not hitherto been observed in comets, yet considered to be the most primordial objects of the Solar system. The N 2 /CO ratio found by Rubin et al., 2015a appears to be lower than that corresponding to the protosolar nebula and hence suggests that the cometary grains formed at relatively low temperatures below ∼30K. Using the instrument VIRTIS-H, Bockelée-Morvan et al., 2015 reported the H 2 O and CO 2 production rates which were driven by different patterns in the early observations of Rosetta. H 2 O production was prominent in the neck region of the comet while it followed a pattern driven by the local illumination conditions, thus leading to the observation of H 2 O mainly in illuminated regions, while the contribution from the non-illuminated regions were weak. Nevertheless, the CO 2 production rates did not show any correlation with the local illumination conditions, as CO 2 outgassing was observed at similar rates in both illuminated and non-illuminated regions. This implies that the CO 2 sublimation took place at deeper layers of the nucleus, below the diurnal skin depth.

COSIMA collected dust particles coming from the coma and characterised them using its secondary ion mass spectrometer. [START_REF] Hilchenbach | Mechanical and electrostatic experiments with dust particles collected in the inner coma of comet 67P by COSIMA onboard Rosetta[END_REF] report to have collected more than 31,000 dust particles in a range of physical sizes with COSIMA and a heterogeneous composition is suggested with potential species such as silicate-containing pyroxene and olivine as well as iron sulphides [START_REF] Hilchenbach | Comet 67P/Churyumov-Gerasimenko: Close-up on Dust Particle Fragments[END_REF]. Additionally, COSIMA identified Carbon, Sodium and Magnesium ions in abundance among the dust particles it gathered, thanks to its in-situ mass spectrometer. Another in-situ laboratory of the Rosetta mission was GIADA, which analysed the grains and dust received at its collector, thus being sensitive to the dust environment of the comet. Using GIADA data, Fulle et al., 2017 find a mean dust-to-ice mass ratio of about 7.5 inside the nucleus of the comet. Now it is clear that a significant progress has been made during recent decades about the understanding of the formation of comets thanks to space missions. This break-through was possible mainly because of the presence of in-situ instruments that could analyse the species contained in the cometary nucleus. As such, all the aforementioned discoveries have been fundamental in refining existing models of the formation of the solar system and introducing new constraints. Next steps would be to address the questions raised during these discoveries such as to identify the complex organic material present on the nucleus of comets. This is a context equally shared by asteroids, for their elementary compositions still remain Chapter 5. Perspectives and outlooks to be unambiguously identified. In light of this challenge, several sample return space missions are lining up to respond to this concern, by having sample return projects. The basic idea is, therefore to successfully collect samples from small bodies and safely bring them to the Earth for eventual investigations in terrestrial laboratories. As such, I would reflect transferring my skills and know-how gained by working with the Rosetta space mission to take part in the analysis of data from on-going space missions like OSIRIS-Rex, Hayabusa2 and potentially CAESAR [START_REF] Squyres | The CAESAR New Frontiers Mission: 1. Overview[END_REF]. Following parts of this chapter are dedicated to briefly introduce these missions and their relevance to the exploration of solar system.

The OSIRIS-Rex (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a NASA space mission to study the asteroid (1019550) Bennu and return samples from its surface to Earth [START_REF] Lauretta | The OSIRIS-REx target asteroid (101955) Bennu: Constraints on its physical, geological, and dynamical nature from astronomical observations[END_REF]. The spacecraft was launched in September 2016 and is supposed to reach its target in August 2018, spend two years studying the asteroid with the delivery of samples expected in September 2023. ( 1019550) Bennu is a PHA-classified primitive carbonaceous asteroid belonging to the taxonomic class B, which implies a composition principally dominated by organic material. Given this spectral classification, it could potentially host amino acids and possibly volatile species that are vital to the development of life by the synthesis of proteins. The returned organic samples would hence carry a great importance in the study of source organic compounds that eventually led to the appearance of life on Earth. OSIRIS-Rex possesses a suite of three cameras (OCAMS), three spectrometers: a) OVIRS in the 0.4-4.3µm range, b) OTES in the thermal 4-50 µm range, c) REXIS in X-ray wavelengths and a LASER altimeter (OLA) as the scientific payload besides the sampling apparatus, which will be used to map the asteroid in infrared, visible and X-ray wavelengths and produce detailed shape models of the asteroid. Using the instruments that are functional in the visible and infrared wavelengths, the surface of the asteroid will be studied and scrutinised, which will support in selecting a landing site for the sample collection, estimated to be in the range of 60 g to 2 kg. The instruments will further be used to derive physical properties such as visible albedo, density, thermal inertia, phase function, etc. as well as to find whether the asteroid has experienced any aqueous alteration and to identify any traces of activity, which might prelude to a cometary nature as evoked earlier in the Chapter 1. The recent discoveries of exposed bright features and outgassing of objects in the main belt has alluded to a potential population of active asteroids / main belt comets [START_REF] Hsieh | The Strange Case of 133P/Elst-Pizarro: A Comet among the Asteroids[END_REF][START_REF] Nathues | Oxo Crater on (1) Ceres: Geological History and the Role of Waterice[END_REF]. One of the most awaited results is the measurement of D/H of (1019550) Bennu, because, following one of the major results of the Rosetta mission [START_REF] Altwegg | 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio[END_REF], which indicated that the terrestrial H 2 O would not have been sourced by the comets such as 67P, it is more likely that it was the organic-rich primitive type 'B' asteroids that would have brought H 2 O to the Earth. Contrary to the meteorites, whose original organic signatures which trace the processes taking place in interstellar medium and the protoplanetary disk, are modified as they enter the terrestrial atmosphere, unaltered samples from primitive asteroids could help retrieve the organic chemistry that would have taken place during the time of the formation of these objects. Given this, the expertise, skills and techniques developed during this thesis would be useful for the characterisation of the surface of (1019550) Bennu, as early as OSIRIS-Rex's arrival phase of the mission. Indeed, the spectrophotometric techniques and accompanying code could prove of substantial support, together with the utilisation of ancillary SPICE data, in order to encounter this endeavour. Above all, a deep understanding of the eventual landing site from where the samples will be collected, is essential to identify local compositional heterogeneities, thus offering a detailed contextual insight.

The mission Hayabusa2 [START_REF] Watanabe | Hayabusa2 Mission Overview[END_REF] is another sample return asteroid mission that is currently underway. This JAXA space mission was launched in December 2014 and in July 2018, it will arrive at its target, (162173) Ryugu with the objective of returning asteroid samples to the Earth in December 2020. ( 162173) Ryugu is a primitive asteroid of the taxonomical class C, containing organic material, supposed have been hardly modified since the formation of the solar system. The spacecraft is equipped with four remote-sensing instruments: ONT -A scientific imaging system functioning in the range of UV to near-IR wavelengths, NIRS3 -An infrared spectrometer, TIR -A thermal imager, LIDAR -A LASER altimeter. Hayabusa2 also carries four landers: (one MASCOT and three MINERVA) that will be systematically deployed upon the beginning of the scientific mission to implement in-situ studies. Three sample acquisitions are planned during the time Hayabusa2 spends at (162173) Ryugu. First two sample acquisitions are scheduled to take place in October 2018 and February 2019, while the third sample acquisition has been planned in coordination with an impactor which will be used to create a crater on the surface. As such, Hayabusa2 will be able to access and collect fresh material which haven't undergone any space weathering as samples. The mission is thus capable of broadening the present know-how on the primitive material available at the formation of the solar system and help achieve a bigger picture of the realm of small bodies.

In a personal point of view, past three-year period has been an enriching learning experience where I could develop a lot of new skills and grow a sense of scientific culture and critical thinking. As the mission Rosetta was still on-going during the first year of my thesis, most of this time was dedicated to data reduction, preparation of data cubes for spectrophotometric analyses and the analysis of large number of images. As such, I was regularly coordinating with the members of the planetology group of LESIA and attending the Full Team Meetings of the instrument OSIRIS that were being held about each three months during the mission. As the thesis progressed, I presented the results I had been obtaining, to the participants of these meetings, which brought together scientists and engineers from different laboratories working with the OSIRIS instrument. These occasions allowed me to exchange with colleagues from different laboratories and compare my results with theirs. This path led to the publication of my first scientific paper at the end of my first year of thesis and my involvement with several other scientific publications, in which I participated with data reduction, analysis and discussion. Much of my second year of this thesis was dedicated to the analysis of OSIRIS data and preparation of a second scientific paper which got published during my third year of the thesis. I was, further attending OSIRIS Full Team Meetings and international congresses to present my latest results in the form of oral and poster presentations, during my second and third years of thesis. As such, over the time, I developed an expertise on the photometric data reduction and spectrophotometric analysis, that could be transmitted to other space missions such as OSIRIS-Rex and Hayabusa2. As described earlier, both of these missions have imaging instruments in their spacecraft and hence they present a potential interest to work on in the near future as they arrive at their respective target asteroids. In addition, these missions also have spectrometers that operate in infrared wavelengths, with which spectroscopic studies could be carried out to identify the species present on the surface and perform thermal studies. This presents a possibility to expand my skills to spectroscopic analyses too, which is a widely-used powerful technique used in planetary science.

Working with remote-sensing data obtained by spacecraft requires a decent understanding of observation geometries during data acquisitions. This is where the SPICE toolkit Chapter 5. Perspectives and outlooks comes handy as it offers the ancillary data files, usually referred to as 'kernels'. In order to compliment my spectrophotometric studies and derive the pixel-based illumination angles were necessary for the illumination correction using Lommel-Seeliger disk function. Hence, I familiarised myself with the SPICE toolkit to create data cubes containing diverse information such as incident angle, emission angle, phase angle, whether a pixel is illuminated or not, etc. per a pixel per an image acquisition. The SPICE kernels also were used to reconstruct the orbit of the Rosetta spacecraft and the comet in order to better understand the observation orientations. As such I developed an important know-how on working with spacecraft boresight, getting field-of-view of an instrument for an instantaneous moment. I then used the resulting ancillary data products to analyse and interpret the data.

In the context of the Rosetta mission, one outlook would be to implement inter-instrument studies for the exposed bright features and try to identify the presence of volatiles other than H 2 O . Since the locations and life times of all such studied features are now published in [START_REF] Deshapriya | [END_REF], it is possible to look in more details for the infrared spectra that might have been recorded by VIRTIS-M corresponding to both the locations and times of OSIRIS observations. It is recalled that there has been only a single detection of exposed CO 2 ice on the nucleus of 67P, around March 2015, some five months prior to the perihelion passage of the comet. Since super-volatiles are likely to have sublimated much earlier, analysing the locations of exposed bright features since the August 2014, could potentially identify supervolatile species like CO, SO 2 , etc. On a side note, the infrared segment of VIRTIS-M ceased to function after May 2015, due to a failure of a cryo-cooler and hence only the data that have been acquired up to May 2015 could be used for spectroscopic analysis of bands in infrared wavelengths. However, species like SO 2 having an absorption band around 290 nm, remain still potentially identifiable by the visible segment of VIRTIS-M. Thus, the identification of any super volatiles on the locations of exposed bright patches would be first of their kind on a cometary nucleus.

The identification of exposed bright features was purely carried by visual inspection of images and applying spectrophotometric analyses on co-registered data cubes. Therefore, the task of analysing images that have been taken from August 2014 to September 2016 was quite laborious and time-consuming. As such, an automatic detection of such features could prove extremely useful and efficient. Nevertheless, this task is not evident due to the various illumination effects that are of both sporadic and spurious nature. This posed a problem in detecting exposed bright features at the initial stages of the analysis, for such illumination effects hindered the process of identifying real exposed bright features, whose presence is due to the exposures of volatiles. However, analysing images over the time, I gained an insight to distinguish illumination effects from real features. In this context, a machinelearning architecture would be of great resource in order to automatically identify real exposed bright features. Having recently taken a doctoral course on artificial intelligence and machine-learning, I reckon this to be a feasible task given the time needed to invest for it. Machine-learning is a powerful tool, that is becoming widely used in astronomy to facilitate repetitive and laborious tasks, through pattern recognition and development of an initial insight which will lead to knowledge and this could be improved by training. For example, successful applications of artificial intelligence based on convolutional neural networks have been implemented to detect exoplanets [START_REF] Shallue | Identifying Exoplanets with Deep Learning: A Five-planet Resonant Chain around Kepler-80 and an Eighth Planet around Kepler-90[END_REF] and Martian craters [START_REF] Palafox | Automated detection of geological landforms on Mars using Convolutional Neural Networks[END_REF]. Hence, a similar approach to detect exposed bright features on planetary imaging data could prove vital in the light of forthcoming data from both ongoing and future planetary missions.

The detection of exposed bright features of H 2 O ice on the surface of the comet 67, their sizes (from several meters to more than 150 meters), distribution, life times and correlations to activity sources are essential to understanding this primitive object of the solar system. Although what is achievable can be limited to an individual instrument like OSIRIS, inter-instrument collaborations could effectively help to put different pieces of the puzzle together. Such collaborative efforts would be key to understanding the origin and evolution of highly elusive bodies like comets, as have shown [START_REF] Agarwal | Evidence of sub-surface energy storage in comet 67P from the outburst of 2016 July 03[END_REF] As a concluding remark, I would like to mention that my recent work has also inspired in me an interest in the solar system ices. The study of cryogenic ices like H 2 O , CO 2 , CH 4 , N 2 , CO, O 2 , NH 3 , C 2 H 4 , CH 3 OH is very important in the study of the geological material of the cold outer solar system objects. Recent solar system research has furthered existing knowledge on bodies like Titan and Pluto, consisting of such ices, thus giving rise to a variety of geo-physical phenomena. Hence, in order to interpret the observations on such objects, laboratory experiments too are necessary, especially to understand behaviour of cryogenic ices under different mixtures.

Introduction

Upon its rendezvous with the Jupiter-family comet 67P/Churyumov-Gerasimenko (hereafter 67P) in August 2014, the Rosetta spacecraft carried out various studies about the cometary nucleus and coma using its onboard scientific instruments until the end of the extended mission in September 2016. Rosetta encountered 67P during the inbound phase of its orbit and followed its evolution past perihelion, as it was on its way to aphelion. The perihelion passage of the comet took place on 13 August 2015 at a heliocentric distance of 1.24 AU. During this phase the spacecraft was required to remain at a safe distance of hundreds of km from the nucleus because of the increased cometary activity. However, Rosetta managed to get closer to the surface many times during its mission and to perform some fly-bys as well. Throughout the mission, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS; Keller et al. 2007) proved to be a fundamental instrument aboard Rosetta, providing a detailed characterisation of the cometary nucleus in exceptional spatial resolution. The OSIRIS instrument was composed of two cameras, called Narrow Angle Camera (NAC) and Wide Angle Camera (WAC). The NAC was equipped with 11 broad band filters in the wavelength range of 269-989 nm and was generally used for the observation of the nucleus, while the WAC had 14 narrow band filters in the wavelength range from 246-631 nm and was intended for the observation of the dust grains and gaseous species in the coma.

While all previous cometary space missions have been brief fly-bys that only enabled a transient acquisition of data, Rosetta was able to further scientific research in an unprecedented manner by observing the comet at high spatial resolution continuously for more than 2 yr. The OSIRIS cameras permitted the observation of an irregular nucleus made up of two cometesimals forming a bilobate shape that hosts a plethora of heterogeneous geomorphological features (Sierks et al. 2015;Thomas et al. 2015). 67P has a low geometric albedo of 6.5 ± 0.2% at 649 nm and its surface shows a large colour heterogeneity (Fornasier et al. 2015). Both seasonal and diurnal changes were identified in terms of spectrophotometry and spectral slope analyses (Fornasier et al. 2016).

Although comets are supposed to be made of ice and dust, OSIRIS data have revealed that ice is not observed ubiquitously on the comet. Results by Fornasier et al. (2016) indicated that the cometary surface is covered by an optically thick layer of dust that prevents the direct observation of subsurface ice on the comet, which corroborates previous theoretical (Fanale & Salvail 1984) and experimental (Sears et al. 1999) models about cometary nuclei. When 67P approached perihelion, there was a substantial rise of cometary activity that progressively removed this layer of dust, allowing the observation of an ice-rich subsurface layer. These authors proposed that the dust layer got thicker following the perihelion passage as the cometary activity gradually ceased. Such is the evidence that the cometary surface is rich in volatile ices often mixed with dust, whose presence can be validated with spectrophotometric, spectral slope analyses, and false-colour RGB images (see Oklay et al. 2017). As shown in spectrophotometric plots of Fig. 1, such volatile-rich terrains (blue dots) are characterised by a red spectral (gradual increase of reflectance with the wavelength) behaviour in their absolute reflectance (I/F). They are characterised also by a higher absolute reflectance than that of the typical nucleus (black dots) for all the wavelengths. Consequently, their albedo is higher than that of the typical nucleus. As far as the relative reflectance is concerned, these terrains have spectrophotometric characteristics similar to the typical nucleus from near ultraviolet wavelengths up to around 650 nm, and then they deviate from the typical nucleus towards infrared wavelengths, diverging with a lower gradient. Another distinction of these volatile-rich terrains is that they appear in a bluish colour in RGB images, as evidenced in Fig. 1, which also reveals the presence of exposed bright patches that can be distinguished from volatile ices mixed with dust by virtue of their characteristic flat spectrophotometric behaviour (represented by red dots therein). Although the RGB image is not ideal for distinguishing between the two, their spectrophotometry clearly identifies the exposed volatile ices. Nevertheless, a careful inspection of the RGB images shows that the exposed ices are identifiable with a dark bluish or whitish colour in RGB images. A36, page 2 of 14

The OSIRIS instrument has revealed exposed bright features that have been attributed to the presence of H 2 O ice in several studies accompanied by multi-filter observations to perform spectrophotometric studies. Analysing the data in the inbound orbit of the comet from 3.6 to 3.0 AU, shortly after the arrival of Rosetta at 67P, Pommerol et al. (2015) reported various features of metre-sized exposed H 2 O ice, belonging to three different categories. Barucci et al. (2016) performed a collective study of eight exposed bright features initially identified with OSIRIS spectrophotometry, coupled with complementary infrared data from Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M) spectro-imager, to confirm the presence of H 2 O ice using the H 2 O ice absorption band at 2 µm. Filacchione et al. (2016a) studied two bright features using VIRTIS-M infrared spectra, attributing their presence to H 2 O ice, making use of the H 2 O ice absorption bands at 1.05, 1.25, 1.5, and 2.0 µm. Filacchione et al. (2016b) put forward the presence of exposed CO 2 ice identified for the first time on a cometary surface using the absorption bands at 1.97, 2.01, 2.07, 2.7, and 2.78 µm of VIRTIS-M infrared spectra. Following about a month from this detection, Fornasier et al. (2016) reported the presence of two large bright patches (about 1500 m 2 each) almost at the original CO 2 location, which indicates the local heterogeneity of the distribution of volatile ices in the comet. Oklay et al. (2016) discussed several previously studied exposed bright features and studied several new features as well. Deshapriya et al. (2016) focussed their study on four exposed H 2 O ice features on the Khonsu region on the southern hemisphere of the comet. Pajola et al. (2017) observed the freshly exposed interior of the comet 67P, following a wide collapse and estimated an albedo of 40% (lower limit) for the exposed material.

In this context, we report a comprehensive study of exposed bright features observed by the OSIRIS instrument and characterised by flat spectrophotometric curves as seen in Fig. 1, during the whole time spent at 67P. In Appendix A, we catalogue such features from early August 2014 (3.6 AU inbound) through the perihelion passage in August 2015 (1.24 AU) up to the end of September 2016 (3.8 AU outbound), taking into account their morphology and temporal evolution as the heliocentric distance of the comet was changing. In addition, we made use of the images recorded by Rosetta Navigation Camera (NAV-CAM) to refine the temporal evolution of some bright features. The objective of this study is to better constrain the morphologies hosting these bright features, to investigate potential triggers for their appearance and to monitor their chronological evolution when multiple observations are available. Therefore, this paper is structured as follows. In Sect. 2, the data set and methods are presented; Sect. 3 details the catalogue of exposed bright features under investigation. Then, in Sect. 4, we present our study on the exposed bright features, which is followed by their spectrophotometric analysis and albedo calculations in Sect. 5. A general discussion is presented in the Sect. 6 and, finally, Sect. 7 is dedicated to the conclusions drawn from this work.

Data and methods

We made use of the data available in level 3B of the OSIRIS standard data reduction pipeline throughout this work. These data are based on level B data, which underwent several corrections for bias, flat-field, geometric distortion, and calibration in absolute flux as detailed in Küppers et al. (2007) and Tubiana et al. (2015), and are expressed in the form of spectral radiance with W sr -1 m -2 nm -1 as units. Then the data are converted to the radiance factor (I/F) in level 3B according to Eq. ( 1), which is written as Radiance factor (I/F) = I(i, e, α, λ)

F(λ) , (1)
where I is referred to the observed scattered spectral radiance, and F is the solar spectral irradiance at the heliocentric distance of the comet, divided by π. The symbols i, e, α, and λ correspond to the incidence angle, emission angle, phase angle, and wavelength, respectively. The solar irradiance has been calculated at the central wavelength of each filter to be consistent with the derivation of the absolute calibration factors.

Because the image acquisitions are not made simultaneously, but with a lag of few seconds to switch the filter wheel, the images of a given sequence do not show exactly the same field of view; a sequence of observations can be made with a minimum of 2 filters up to 11 filters in the case of NAC. This is due to both the motion of the spacecraft and the cometary rotation during the image acquisition. In order to take this effect into account and perform accurate spectrophotometry, we adopted the image co-registration algorithm as in Deshapriya et al. (2016). We consequently obtained a data cube with dimensions corresponding to the number of filters available in the observation sequence.

We then computed the ratio between the radiance factor and the Lommel-Seeliger disk function (Fairbairn 2005) to account for the various illumination conditions prevailing during the observations. The Lommel-Seeliger disk function D is defined in Eq. ( 2). The incident and emission angles required for the calculation were calculated using a shape model of the comet.

D(i, e) = 2 cos(i) cos(i) + cos(e) . (2)

Catalogue of exposed bright features

During the early observations of the nucleus of 67P, Pommerol et al. (2015) introduced three types of exposed bright features whereas our study expands the types of distinguishable bright features to five as follows:

1. Isolated patches on smooth terrains 2. Isolated patches close to irregular structures 3. Patches resting on boulders 4. Clusters of patches 5. Frost Each of these types is described in detail in Sect. 4. We highlight that we have mentioned frost because it shows the characteristic spectrophotometric properties of the H 2 O ice in OSIRIS multi-filter observations. Nevertheless, we did not include frost in our catalogue of bright features owing to its ephemeral nature (lifetime of few minutes around the perihelion passage; see Fornasier et al. 2016) and inherent distinction in the process of formation compared with exposed H 2 O ice observed on 67P. Therefore, we make it clear that our catalogue of bright features does not contain frost, but only first four types of the above list.

In the preparation of this catalogue, we established a definition of an exposed bright feature attributed to H 2 O ice by adhering to the following criteria :

1. A ratio of at least 1.5 between the reflectances of bright features and the typical nucleus for a given filter. 2. The presence of a flat spectrophotometric curve in reflectance.

A36, page 3 of 14 (Fornasier et al. 2016). This map is the cylindrical projection of a merged shape model resulting from the shape models SHAP4S (Preusker et al. 2015) and SHAP5 (Jorda et al. 2016) for the northern hemisphere and southern hemisphere, respectively.

Table 1. Four types of bright features appearing in the catalogue and their respective counts. As per the first criterion, a ratio of 1.5 would imply that the reflectance of the bright feature is greater than that of the nucleus for all the available filters. If one were to compare different observations made at the same phase angle, a higher ratio would imply that the bright feature in question is fresher and less contaminated. In this study we found cases in which the ratio was sometimes 6 and 13 (for a bright patch in Bes region as shown in Fig. 5f) for different features, implying the presence of fresher and less contaminated H 2 O ice. It is noteworthy that most of the pixels in these cases were saturated and the actual value of the ratio would have been even greater. The second criterion requires that observations be multi-filter sequences, imaged at least with three NAC filters. This allows us to verify whether the candidate feature possesses the flat spectrophotometric curve in reflectance and relative reflectance plots, which is characteristic to the H 2 O ice.

Following the above criteria we found 51 different features of bright patches in the multi-filter observations in which spectrophotometric analyses were possible. In addition to this, we analysed the observations where a single filter was available and present six more features adhering only to the first criterion. In these observations, we used the higher reflectance of the feature compared to the surrounding as the proxy for the H 2 O ice. However we note that the latter is not an exhaustive list. We considered that an individual feature could be composed of an individual patch or a number of patches (2 or several smaller patches in the case of clusters). Therefore, the total of 57 features is not an exhaustive measurement of all the individual bright patches we studied. Table 1 lists the four types of bright features catalogued along with their respective counts. Figure 2 shows the locations of bright patches on a cylindrically projected map of the nucleus, favouring equatorial regions for most of the bright features, while the majority of these features are found on the southern latitudes of the comet. However, one should be cautious in interpreting this observation, for this might be an observational bias as it was the southern hemisphere that was mostly observed during the mission. The catalogue of exposed bright features is presented in the Appendix A with corresponding observational circumstances. The numbers 1-51 refer to the multi-filter observations enabling spectrophotometric analyses in which characteristic flat spectrophotometric curves of H 2 O ice could be used to confirm its presence, whereas the numbers 52-57 refer to the observations with a single filter in which case higher reflectance of H 2 O compared to the surrounding nucleus terrain was used as the proxy. We remark that the observations from 52-57 are not an exhaustive list. The column "Detection" corresponds to the first observation of the feature and the column "Life" gives the number of days that the feature has survived. These days are calculated using the other temporal observations of the same feature and hence they imply a lower limit of real lifetime of the feature. It is also noteworthy that the lifetime of 0 indicates that the feature in question has only been observed once; hence it is not possible to derive a lifetime for such features. Also the area measurements of the clusters of features are not given because the measurements may carry errors due to the presence of a number of smaller individual features present on different morphological structures.

A36, page 4 of 14

Types of bright features

Isolated patches on smooth terrains

This category of bright features encompasses isolated individual bright patches that are located on smooth terrain. These bright patches are likely to be formed by the sublimation-driven erosion (sublimation of volatiles such as CO, CO 2 , and H 2 O ices) on the cometary surface that was gradually increasing when 67P got closer (inbound) to perihelion. Compared to the consolidated cometary material on rugged terrains of the comet, smooth terrains represent dusty material with low tensile strength (Groussin et al. 2015a) that is easily subject to erosion. Hence, cometary erosion was observed mostly on the smooth terrains. In Fig. 3, we illustrate the temporal evolution of isolated bright patches located in Bes, Imhotep, and Hapi regions of the comet. Panel A of Fig. 3 shows an isolated bright patch on the smooth terrain that survived for about a period of 1 month. This patch annotated with "a" is joined by an ephemeral companion patch annotated with "b" in the window A2. The growth of its size is due to increasing erosion.

On the Imhotep region, where a vast majority of smooth terrains are present, large irregular bright patches appeared [START_REF] Groussin | Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko[END_REF]) just prior to the perihelion passage as erosion was escalating. At the same time on the Hapi region, erosion was also at play (Davidsson et al., in prep.) leading to the observation of isolated bright features. In addition to these A36, page 5 of 14 large features, it is noteworthy that several smaller bright features have also been observed on Imhotep towards its boundary, albeit on smooth terrain a few weeks before the perihelion passage. The erosion-driven scarp-retreating processes (El-Maarry et al. 2017) that triggered the appearance of bright patches are clearly observable in the panels B and C of Fig. 3. These patches significantly differ from other patches in this category by their arc-shaped irregular structure and size (length of the arc shape), although they are observed isolated in smooth terrains too. It is very interesting that the patch annotated with "c" in the panel B, almost doubles its dimension within a period of a week, and 1 week later this patch achieves a dimension that is more than five times its size 2 weeks earlier. Besides, this patch "c", the panel B reveals the presence and growth of other subordinate patches in the smooth terrains of Imhotep. Panel "C" brings out the accelerated growth of another patch where a small 7 m patch emerges and develops into a much larger arc-shaped structure measuring about 80 m in the course of little more than 3 weeks. [START_REF] Groussin | Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko[END_REF] showed that this observed erosion in Imhotep could not be solely due to sublimation of volatile species such as H 2 O and CO 2 at their corresponding heliocentric distances and that it could be the low tensile strength of the cometary material (Groussin et al. 2015a) or the processes such as clathrate destabilisation or amorphous water ice crystallisation, which would result in the observed rates of erosion.

In our catalogue of bright patches, we have 14 different features under this category. One interesting remark in this category is that except for one feature, all the other isolated patches on the smooth terrains have been observed from early March to early August 2015. This implies that all of these patches have appeared before the perihelion passage. However, there is an observational bias coupled with this remark, as Rosetta was obliged to have distant orbits around the comet in order to prevent potential damage during the period of perihelion passage due to rising cometary activity. Rosetta resumed its closer orbits around the comet in January 2016 and we have not been able to trace any more isolated patches on the smooth terrains through the end of the mission in September 2016. Therefore this observation matches well with the erosion-driven activity that was escalating during the same period.

Isolated patches close to irregular structures

The isolated individual bright features that are located close to varying morphological structures fall into this category. These patches are often located under shadows of the neighbouring morphological structures and hence might be potentially able to survive for longer durations than their counterparts located on smooth terrains (Fig. 4). However, their lifetimes cannot be better constrained as they have not been imaged with favourable spatial resolutions and because of insufficient recurring observations. In the most favourable scenario, we are able to determine a lifetime of about 2 months for a patch in this category. Depending on the local solar zenith angle and neighbouring irregular structures, these features could be illuminated ephemerally. In the cases in which these features and their vicinity were favourably observed, these patches seem to appear on consolidated cometary material.

A36, page 6 of 14 Because of the consolidated nature of the material that hosts these features, they are hard to erode and hence the origin of these isolated patches is difficult to constrain.

We catalogue 18 patches of this type, which represents the largest number of bright patches belonging to any category. The appearance of such features does not seem to have any correlation with the heliocentric distance because they have been observed throughout the mission.

Patches resting on boulders

There have been a number of bright patches observed resting on the surface of boulders throughout the course of the mission (Fig. 4). Their presence on 67P was firstly highlighted by Pommerol et al. (2015) during early observations of the comet by Rosetta. In our framework, we catalogue a large number of such bright patches and suggest potential activity sources that might have triggered their appearance, based on OSIRIS observations.

There have been a few cases in which some cometary activity could be correlated to these boulders. Concerning a bright patch observed from January 2016 to July 2016 in the Khonsu region, a cometary outburst could be correlated with its location. This outburst took place in early August 2015 as the comet was approaching the perihelion of its orbit. As evident from Fig. 8, the available images of the location of the boulder taken before and after the perihelion show significant changes on the terrain, thus resulting in the appearance of the boulder with the bright patch on it as discussed by Deshapriya et al. (2016).

In addition to this, a displacement of a boulder can be observed in the Bes region. Comparing the geometry of a family of boulders in this region (Fig. 5), it can be established that some activity source has triggered a movement of a boulder between 7 February and 4 March 2016 (better constrained using NAV-CAM images). Examining Fig. 5a andb, one could notice that the boulder annotated "a" has a bright patch on it and both its distance and orientation relative to the centralised boulder "b" have changed, which can also be validated using the co-ordinates of the boulders at aforementioned two dates. However, we are not able to determine the source of activity that triggered the displacement of this boulder. Nevertheless it is very likely that this same source has given rise to the appearance of the bright patch resting on the boulder. Further observations leading to the final stage of the mission reveal this patch in further detail because the spatial resolution significantly improved. The final observation of this patch dates to mid-September 2016 with a spatial resolution of about 6 cm per pixel and reveals several individual bright components forming an irregular shape. In March 2016, this patch appeared to be about 70 m 2 and as the spatial resolution improved its structures could be resolved. On the final observation in mid-September, the patch appears to have an area of about 7 m 2 , indicating the nature of sublimation during the 6 months since its first detection. Although we are not able to trace any kind of activity for other patches resting on boulders probably because of the absence of observations during the transient activity time, supported by the above two cases, we suggest that they are correlated to sources of cometary activity.

Clusters of patches

There have been several observations of clusters of bright patches that are individually smaller in size compared to the aforementioned features. These clusters have mostly been observed in the pre-perihelion period and they are located near cliff structures. Therefore these clusters could be related to cliff collapse events, in which the resulting debris exposes water ice contained and preserved inside since their formation, as the major cliff collapse in Aswan area has been explained by Pajola et al. (2015Pajola et al. ( , 2016)).

Figure 6 shows a cluster of smaller bright features in the Khepry region. First observed at the end of August 2014, this cluster remained very stable for about 3 months. We numbered some of the individual patches to emphasise their stability in the sequence of images. This cluster might have remained stable for longer, but it is difficult to conclude owing to the inadequate temporal coverage of images. An observation dating back to 25 March 2015 indicates that the entire cluster has sublimated away, which indicates the influence of increasing solar irradiation on the clustered individual patches. This cluster was firstly studied by Pommerol et al. (2015) and Barucci et al. (2016) attributed its presence to H 2 O using VIRTIS-M infrared spectra.

Frost

Frost has been observed since the arrival of Rosetta (De Sanctis et al. 2015) at 67P (3.6 AU) by VIRTIS spectro-imager and has since been continuously observed by OSIRIS up to the end of the mission (3.8 AU) with a pronounced abundance towards the perihelion passage (Fornasier et al. 2016). This frost could be observed at the local dawn on the comet, where the sun light returns after about 6 h. Thus, the shadows cast by different morphological structures get shorter as the sun moves towards the local azimuth, revealing the frost formed over the cometary night due to condensation of previously sublimated water ice. Frost could be observed for some minutes before being rapidly sublimated away, depending on the heliocentric distance of the comet. This process continues as long as the solar irradiation is strong enough to sublimate the frost. Once the comet is beyond the current snowline of the solar system, the diurnal water cycle of the comet ceases, leading to no further frost sublimation until the comet returns inside the snowline for its next perihelion passage. This diurnal cycle of water was more pronounced as 67P got closer to the perihelion, as water production rates on the comet rose due to the increasing solar irradiation. Hence, close to the perihelion, both inbound and outbound, frost was ephemerally visible on a given location during the local cometary morning.

It is also possible that the four types of bright features introduced earlier, contributed to the diurnal cycle of water on the comet as they have been constantly sublimating, thus liberating the amount of water ice necessary to sustain the diurnal cycle of water on the comet. 8. Activity sources in the Khonsu region including a powerful outburst that modified the local terrain and later triggered the appearance of a bright patch resting on a boulder. Panel A shows the area of interest in the Khonsu region about 3 months prior to the perihelion passage. Panel B corresponds to about three weeks before the perihelion passage and there are a couple of bright patches that have appeared on the smooth terrain probably due to the erosion that had been escalating at this time. The following panels from C through G illustrate the appearance of a family of transient outbursts and a long-lived outburst with a lifetime of the order of several hours, as the comet was accelerating towards the perihelion of its orbit. The next panel H, where the area of interest is resolved about 4 months later, provides evidence that there have been significant changes on the surface morphology. The observable boulder field is therefore likely to be an aftermath of the aforementioned outbursts. Finally the panel I reveals the bright patch that appeared on the boulder, some 2 months later.

Spectrophotometry and albedo of five types of features

We carried out spectrophotometric analyses for all the exposed bright features, which had multi-filter observations (51 out of 57 features). For each of the feature types, we selected a representative example from our catalogue along with frost. Reflectance (I/F) plots and relative reflectance plots (normalised to the filter at 535 nm) for each of the five types are presented in Fig. 7. The red dots correspond to the bright feature indicated with a red arrow, whereas the black dots are representative of the typical nucleus sampled from the vicinity. For each given panel, a NAC full-frame image is given followed by a zoom-in to the feature and the corresponding spectrophotometric plots. For a given wavelength, the I/F plots give an idea of the albedo of the feature compared to the typical 67P nucleus, which has a red spectral behaviour (increase of reflectance with the wavelength) due to the presence of the desiccated organics on the comet (Capaccioni et al. 2015). The different values of the I/F of the exposed bright features in comparison to the I/F of the typical cometary nucleus are connected to their varying ice contents. The relative I/F curves of the bright features have a flat behaviour, as their relative reflectance stays more or less the same as the wavelength varies, thus suggesting the presence of ices. Although these ices could be composed of, for example H 2 O, CO 2 , CO, and CH 4 , we are not able to explicitly identify their composition with OSIRIS observations alone without complimentary spectroscopic data in the infrared range.

One cannot use the heliocentric distances corresponding to these observations to rule out the volatile ices that would have already sublimated because the comet could have irregular local insolation conditions dominated by seasonal and diurnal effects because of its very irregular shape. However there have been several studies (Barucci et al. 2016;Filacchione et al. 2016a;De Sanctis et al. 2015) based on VIRTIS spectroscopic data that confirm the presence of H 2 O ice on the comet compared to only one localised identification of CO 2 ice (Filacchione et al. 2016b). Thus, H 2 O and CO 2 are the only ices that have been spectroscopically identified on the comet 67P up to now. Given this context and OSIRIS spectrophotometric observations, we suggest that the observed ices are most likely to be H 2 O ice.

In order to compute geometric albedos for the exposed bright features, we relied on the phase curve of bright features presented in Hasselmann et al. (2017) using the OSIRIS WAC data of the Rosetta fly-by of 67P from 9 to 10 April 2016. During our calculation the effect of phase reddening of bright features was not considered under the assumption that its effect is negligible given the very low spectral slope values of such exposed bright features. We also note that the observations that corresponded to high phase angles (usually beyond 80 • ) were discarded due to the limited applicability of the aforementioned phase curve of bright features. The isolated patches on smooth terrains had albedos that ranged from 15% to 30%, while the isolated patches close to irregular structures had albedos varying from 20% to 50%. The albedos for the patches resting on boulders varied in the range of 15% to 75%, whereas the clusters of patches had albedos that A36, page 10 of 14 ranged from 10% to 50%. It is evident from this analysis that the isolated patches on smooth terrains have the lowest upper limit of the albedos while the patches resting on boulders have the highest upper limit of albedos.

Discussion

We catalogue four types of exposed bright features on the comet. Type 1 are isolated patches located on smooth terrains of the comet and are highly correlated with the erosion that escalated towards the perihelion passage of 67P. Consequently almost all such features were observed within a period of 5 months before the perihelion passage. During this period we observe arc-shaped structures in Imhotep and Hapi regions and significantly large roundish patches in Anhur/Bes regions (Fornasier et al. 2016) all appearing on smooth terrains. It appears that these smooth terrains are covering the volatiles that are closest to the cometary surface. Assuming a dust-fall based formation for the smooth terrains during the previous perihelion passage, this type of volatiles could be formed through the cessation of the diurnal water cycle of the comet as the frost would not be sublimated any more as a result of gradually increasing heliocentric distances as the comet went pass beyond its perihelion. Nevertheless, we observed cometary frost and its sublimation even at 3.8 AU in the outbound orbit during the final stages of the mission. Hence, we could conclude that the cessation of the water cycle on the comet takes place somewhere between 3.8 AU and the aphelion of the comet 5.68 AU in the outbound orbit. Consequently, a thin frost layer would be deposited on the nucleus, which would then be subject to dust fall, which would successively cover the frost layer. As the comet returns to the perihelion, passing again the snow line, when the incoming solar irradiation is adequate enough for sublimation to work, aforementioned frost layers could be observed as the dust would be lifted off and driven away by the sublimation-driven erosion. Further, the albedos of the features in this category suggest some mixing probably with cometary dust since they have the lowest upper limit of albedos among all the types of features. Type 2 are isolated patches close to irregular structures, which have been observed throughout the mission without any preferable heliocentric distance. Because of the local terrain and topography, these features seem to be often cast in shadows, consequently making them rarely observable. Type 3 are patches resting on boulders, which have been observed throughout the mission as well. For two patches of this Type 3 category, we find convincing evidence of cometary activity sources that have possibly triggered their appearance. These patches are also peculiar in the sense that they have longer lifetimes despite being regularly subject to insolation. Therefore, we could assume that some cometary activity source has modified the surface of the boulder, such that an internal reservoir of H 2 O ice could be exposed. In Fig. 5f we visualise the bright patch that has been imaged at the best spatial resolution, which is 6 cm per pixel. This is also the type of feature that corresponds to the highest upper limit of albedos among all the four types, which suggests that these patches are richer in ice content compared to the others. Type 4 are clusters of patches that show long-term stability at larger heliocentric distances in both inbound and outbound phases of the orbit of 67P. They are seldom observed close to the perihelion probably because they are too small to last long enough to be observed under escalated sublimation rates.

Figure 8 shows the surface evolution of a part of the Khonsu region bordering Imhotep and Apis regions. In each subframe the region of interest is indicated in a dotted square. As seen in Fig. 8A, this region of interest is mainly characterised by smooth terrain, few surrounding boulders, and a terrace. The transition into panel 8B indicates the presence of two small bright patches, revealed probably due to the escalated erosion since the observation was about a couple of weeks before the perihelion passage. One of these is under the shadow of a neighbouring boulder and the spectrophotometry of these two patches hints at the presence of H 2 O ice. Panel 8C brings out a distinguishable collimated outburst (Vincent et al. 2016), originating from the location of the aforementioned bright patch under the shadow. The image is stretched to outline the outburst and zoomed view of the originating region reveals the presence of another minioutburst as indicated by the red arrow. Panel 8D shows the same region imaged 9 h after the main outburst event and more smallscale outbursts are identifiable. Panels 8E and 8F show temporal variation of this family of outbursts and panel 8G depicts the long-lasting presence of the main outburst, some 12 h later the original detection. Imaged 4 months later, panel 8H shows what remains of the original terrain after the perihelion passage. The terrace structure has completely disappeared and we are able to identify several boulders scattered on the smooth terrain. About six weeks later panel 8I shows the presence of a bright patch (Deshapriya et al. 2016) resting on one of the boulders and in terms of the available data, we are not able to attribute any triggering activity source for this observation. The presence of this bright patch can be monitored for about 6 months as it was gradually being sublimated. Thus Fig. 8 shows the varying sources of activities and their successive aftermaths that took place in this terrain in the Khonsu region.

All the features presented in the catalogue in Appendix A are suggested to be attributed to H 2 O ice, relying on their spectrophotometry or photometry (in the event of absence of multi-filter observations). We also note the presence of frost on the comet throughout the mission, and there is a pronounced abundance towards the perihelion, where the H 2 O production rates increased.

Conclusions

The presented four different categories of exposed bright features on comet 67P have a spectral behaviour and temporal variations that are related to the presence of the volatile ices. However, based on the already available data corresponding to spectroscopic identification of ices on the nucleus of the comet 67P, we suggest that the ice we report here are most likely to be H 2 O ice. Whatever the trigger of the formation of these different categories, their observation suggests that H 2 O ice is omnipresent inside the comet, hidden beneath dust layers and inside the consolidated cometary material. The fact that about 75% of all the features were observed prior to the perihelion passage suggests that increasing insolation has led to their exposure in different forms of cometary activity sources such as erosion and outbursts. Indeed seasonal colour changes indicate that the cometary dust mantle got thinner approaching the perihelion passage (Fornasier et al. 2016). We mapped all the studied exposed bright features onto a cylindrical projection of the comet and almost all of these features fall into the equatorial regions (between -30 • and +30 • of cometary latitude). Further, we also remark that a majority of them are concentrated in the southern latitudes.

Our analysis also suggests that the isolated bright H 2 O patches found on smooth terrains are not as freshly exposed as the other bright features catalogued in the sense that the former A36, page 11 of 14 A&A 613, A36 (2018) was formed after the previous perihelion passage of the comet as an aftermath of the cessation of the diurnal water cycle as the comet was getting further away from the sun. On the other hand, we suggest that the H 2 O ice on bright patches resting on boulders and on those found in clusters following a cliff collapse origin are freshly exposed because they are most likely to have remained inside the consolidated cometary material until the activity event that triggered their exposure. The fact that these patches also resulted in saturated pixels in many observations while the patches on smooth terrains did not supports the idea that the H 2 O ice found on bright patches resting on boulders and on clusters is less contaminated than H 2 O ice found on smooth terrains. This suggestion is additionally supported by the albedo calculations because the isolated patches on smooth terrains have the lowest upper limit of albedos (30%), whereas the other three types have higher upper limits of albedo (50%, 70%). Therefore we conclude that activity sources such as sublimation-driven erosion could only contribute to the appearance of bright patches on smooth terrains, whereas more powerful activity sources such as outbursts are capable of giving rise to other bright features containing a higher quantity of H 2 O ice as seen on boulders and cliff collapses.

ABSTRACT

Our work focuses on the spectrophotometric analysis of selected terrain and bright patches in the Khonsu region on the comet 67P/Churyumov-Gerasimenko. Despite the variety of geological features, their spectrophotometric properties appear to indicate a similar composition. It is noticeable that the smooth areas in Khonsu possess similar spectrophotometric behaviour to some other regions of the comet. We observed bright patches on Khonsu with an estimation of >40 per cent of normal albedo and suggest that they are associated with H 2 O ice. One of the studied bright patches has been observed to exist on the surface for more than 5 months without a major decay of its size, implying the existence of potential sub-surface icy layers. Its location may be correlated with a cometary outburst during the perihelion passage of the comet in 2015 August, and we interpret it to have triggered the surface modifications necessary to unearth the stratified icy layers beneath the surface. A boulder analysis on Khonsu leads to a power-law index of -3.1 +0.2/-0.3 suggesting a boulder formation, shaped by varying geological processes for different morphological units.

Key words: techniques: photometric -comets: individual: 67P/Churyumov-Gerasimenko.

I N T RO D U C T I O N

The Rosetta spacecraft rendezvoused with the comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) in 2014 August and has been following the comet towards the later phases of the mission, producing copious amounts of scientific data. The Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), aboard Rosetta has been very useful to characterise the cometary nucleus in exceptional detail. With OSIRIS's eyes, namely Narrow Angle Camera (NAC) and Wide Angle Camera (WAC), it has been possible to study a very irregular object with E-mail: prasanna.deshapriya@obspm.fr (JDPD); Antonella.Barucci@obspm.fr (MAB); Sonia.Fornasier@obspm.fr (SF) a low visible albedo, featuring a variety of locally heterogeneous morphologies. Although previous visits to comets, all of which were fly-by missions, expanded our understanding (Barucci, Dotto & Levasseur-Regourd 2011) about comets, the OSIRIS imaging instrument (Keller et al. 2007) has furthered this research by allowing us to follow the temporal evolution of the comet continuously and to image the cometary nucleus in unprecedented spatial resolution. The NAC is equipped with 11 broad-band filters centred at different wavelengths from 269 through 989 nm with the intention of studying the cometary nucleus, whereas the WAC is dedicated to the study of gaseous species in the coma with 14 narrow-band filters centred at wavelengths ranging from 246 to 631 nm. OSIRIS has been mapping the cometary surface at numerous observing conditions and has been able to reach a spatial Since Rosetta's arrival at the comet, OSIRIS has been carrying out observations under careful scrutiny as the comet was approaching the perihelion of its orbit. In 2015 August, during the perihelion passage, Rosetta was obliged to recede from the comet due to its enhanced activity as a precautionary measure. However around 2015 December, Rosetta returned to the vicinity of the comet and resumed high-resolution observations and is expected to continue through the end of 2016 September, which will mark the end of the extended mission.

OSIRIS observations have revealed a bilobate nucleus accreted from a pair of cometesimals each having an irregular shape, despite sharing similar stratifications [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF]. The Northern hemisphere of the comet has been examined in much detail with the identification of a plethora of geomorphological features (Sierks et al. 2015, Thomas et al. 2015), whereas the Southern hemisphere of the comet got exposed to the sun light later on. With the illumination conditions favouring the detailed mapping of Southern hemisphere of the comet, 2015 May marked a cornerstone of the Rosetta mission as more southern regions of the comet fell into the visibility, thus allowing us to expand the existing mapping phases that had previously been restricted to the Northern hemisphere. This opened the possibility to explore and characterise the southern regions on the cometary nucleus that would be freshly exposed to the sun light after a long winter. Subsequently following the observations, a number of regions were identified in the Southern hemisphere with a diversity of geological features (El-Maarry et al. 2016). The Khonsu region (see Fig. 1), in particular, is unique due to its geomorphology and spectrophotometric properties with prominent bright spots (Barucci et al. 2016) G). Although a considerable amount of work has been done covering many other regions of the comet, Khonsu has not been previously explored in detail. On the other hand, this is also a region that has started to be freshly exposed to the sun following the long winter in the Southern hemisphere of the comet and studying Khonsu since its early stages of illumination in its current orbit could allow us to follow the potential variations of spectrophotometric properties of different surface features. The background of this work focuses on investigating the manifold morphological features observed in this unique region by exploring their spectrophotometric properties using the images that have been recorded from 2015 March up to 2016 June. OSIRIS NAC images have been used for this work, unless otherwise specified, in the case of WAC.

DATA R E D U C T I O N

The data used for this study are sourced from the level 3B of the OSIRIS standard pipeline where they have undergone the data reductions described in Küppers et al. (2007) and Tubiana et al. (2015). These account for the correction of bias, flat-field, geometric distortion and calibration in absolute flux up to the level 3 of the pipeline and data are available in spectral radiance in the format of Wsr -1 m -2 nm -1 . In the level 3B, the data are then converted into the radiance factor π .(I/F) using equation ( 1):

Radiancefactor(I /F ) = πI (i, e, α, λ) F (λ) , ( 1
)
Here I is referred to as the observed scattered spectral radiance, whereas F is the solar spectral irradiance at the heliocentric distance of the comet. The symbols i, e, α and λ correspond to the incident angle, emission angle, phase angle and wavelength, respectively. It is noteworthy that solar irradiance has been calculated at the central wavelength of each filter in order to be consistent with the derivation of the absolute calibration factors.

When OSIRIS is in operation during a multispectral observational sequence, the images are recorded successively and usually, there is a delay of about 10-15 s between each pair of consecutive images and there is also the movement of both the spacecraft and the comet, which translates into a slight shift of the fields of view between a given pair of consecutive images. The shift becomes more pronounced when the spacecraft is closer to the cometary nucleus as this is when the spacecraft will be moving faster. Therefore, this effect translates into the requirement of co-registering the images of the observational sequence in order to perform spectrophotometry. We treat this problem by adapting an algorithm that automatically identifies identical features in consecutive images and estimates the projective transformation matrix between each pair of consecutive images (ORB and RANSAC tools implemented by van der [START_REF] Walt | scikit-image: Image processing in Python[END_REF], leading to an adequate alignment of the images. This co-registration of images results in a multispectral data cube on which the spectral analyses can be carried out.

Next, we corrected the different illumination conditions that prevailed during the observations by using Lommel-Seeliger disc law (Fairbairn 2005), which is appropriate for this case, given the lowalbedo nature of the comet 67P/C-G. The disc function, D, is defined in equation ( 2) where i and e stand for incident and emission angles for a given pixel:

D(i, e) = 2 cos(i) cos(i) + cos(e) . ( 2
)
We retrieved these angles by generating synthetic images for a given set of observational conditions using the shape model SHP5 (Jorda et al. 2016) together with Optimized Astrophysical Simulator for Imaging Systems simulator (Jorda et al. 2010) that relies on Rosetta SPICE kernels (Acton 1996) in order to obtain the ephemeris for the observations in question. Then, the correction is achieved by taking the ratio between the previously calculated radiance factor and the disc function (equation 3):

I /F corrected = πI (i, e, α, λ) F (λ)D(i, e) . ( 3
)
Once this illumination correction is applied over the multispectral data cube, it is possible to perform the spectrophotometric analysis. In this context, we restrict our scope to the observational sequences featuring Khonsu region. Thus, using the data cube, we plot the corrected radiance factor for areas of interest against the wavelength. This allows the visualisation of the spectrophotometric curve of the selected areas and compare different morphological features on the surface, which could give insights about albedo properties of the selected areas. In addition, we provide relative reflectance plots where we have normalised the corrected radiance factor values to a given filter. We used the OSIRIS NAC F23 green filter, centred at 535.7 nm to be consistent with similar analyses performed by Fornasier et al. (2015), Barucci et al. (2016) and Oklay et al. (2016).

A NA LY S I S O F D I F F E R E N T S U R FAC E F E AT U R E S I N K H O N S U

Spectrophotometry

In order to investigate the surface properties of the Khonsu region, we selected four different types of surface features (we refer to El-Maarry et al. 2016 for a dedicated description of the terrain) observed at two different periods and studied their spectrophotometric behaviour. The observational parameters are given in the Table 1 for both cases and here we use illumination-corrected and properly co-registered multispectral cubes.

We defined the four regions of interest over the following surface features shown in Figs 2(a The selection of different box dimensions corresponds directly to the different spatial resolutions of the two observations and the necessity to cover an area that is large enough to contain the local physiographic varieties that are being studied.

Both multispectral cubes are prepared with 11 NAC filters and for each of the observations, we plot the median of our four samples containing the corrected radiance factor, against the filter wavelengths per each selected surface feature and perform a normalisation at 535.7 nm for the relative reflectance plot. The subsequent plots are depicted in Figs 2(c) and 2(d). From the observation ID 1, it is evident that the light smooth terrain displays the highest reflectance among the features considered, whereas the rugged terrain corresponds to the lowest reflectance. Although these features show subtle variations in their reflectance characteristics, they all share a red spectral behaviour. From the spectrophotometric curves of the observation ID 2, it is clear that the highest reflectance is shared by the part of the pancake feature and dark smooth terrains. A comparison of reflectance curves between the two observations suggests that some of the selected features have undergone a variation of reflectance between the two periods. The order of reflectances of all the features has been affected, except for the rugged terrain that Table 2. Spectral slope values for the selected four surface features (Figs 2a and2b) on Khonsu region during two observational sequences. Spectral slopes are evaluated for per cent/100 nm in the 882-535 nm range using boxes of 7×7 pixels and 18×18 pixels for the observations on 2015 May 2 and 2016 February 10, respectively. The difference of the sampling is due to the different spatial resolutions of two scenarios and the sizes of both of the boxes translate to 16×16 m 2 on the surface. The errors are calculated from the distribution of pixels defined for each feature. For example, in the case of a feature of the observation on 2015 May 2, the standard deviation of the 7×7 pixels is used as the error. As for the observation on 2016 February 10, it is the standard deviation of a sample of 18×18 pixels, except for the ID d, where the sample is a filtered 250 pixels from the default 18×18 pixels in order to avoid the pixels affected from the shadows. (2016), the terrain analysed here corresponds to their 'higher spectral slope' group, defined above 15%/(100 nm) and this group is associated with inactive regions. The aforementioned red spectral behaviour has been associated with the presence of organic compounds in the comet by Capaccioni et al. (2015) using VIR-TIS spectro-imager on-board Rosetta, which operates from 0.25 to 5 μm. The relative reflectance plots shown in Figs 2(c) and 2(d) yield that all the selected surfaces features show similar composition as their normalised curves almost coincide. The deviation of the relative reflectance curve in near-infrared wavelenghts, shown by the light smooth terrain in Fig. 2(d) can be attributed to the presence of the bluish material on the surface. We interpret the spectral behaviour of the selected features on the Khonsu surface to be consistent with the spectral behaviour of the typical nucleus of 67P/C-G as demonstrated in Barucci et al. (2016) and Fornasier et al. (2015). In Fig. 3, we visualise a few RGB (Red, green, blue filters) images for different observing conditions, realised using STIFF program [START_REF] Bertin | STIFF: Converting Scientic FITS Images to TIFF[END_REF], which produces colour images from astronomical data. These images reveal the presence of some bluish material (compared to the surroundings) on the surface of the Khonsu region during these observations corresponding to varying periods of Rosetta's escorting phase of the mission. Fig. 3(b), dated 2015 July 26, shows the abundance of the bluish material on the comet with an oblique view on Khonsu that relates to a period of about two weeks before the perihelion passage of the comet where the cometary activity progressively removed a part of the dust mantle, revealing fresh surface underneath (Fornasier et al. 2016). The same image also illustrates a transient cometary outburst, attributed to the increased cometary activity and this has been identified (ID 3) as a type-B outburst by Vincent et al. (2016). Fig. 3(d) shows a narrow field of view of the Khonsu region at a better spatial resolution with the presence of the bluish material. Several blue patches are identifiable and the associated spectrophotometric plots are presented in Fig. 4 which shows that this bluish material follows the general relative reflectance behaviour of the nucleus at near-ultraviolet wavelengths, whereas its relative reflectance seems to have lowered at near-infrared wavelengths. We computed the spectral slope for this bluish material, evaluated as the mean for a 20×20 pixels square box to be about 11%/100 nm (phase angle ∼65 • ) according to the definition established earlier. A similar behaviour corresponding to a lower spectral slope has been found on the regions of 'lower spectral slope' group introduced by Fornasier et al. (2015). This lower spectral slope ranges from 11 to 14%/(100 nm) at a phase angle around 50 • and includes Hapi, Hathor and Seth regions. This spectrophotometric behaviour has been attributed to a higher abundance of H 2 O ice mixed with the desiccated refractory material. With the work done by Oklay et al. (2016) in mind, this bluish material corresponds to the 'low spectral slope' group ranging from 8 to 13%/(100 nm) and can be potentially associated with some sources of activity, being active pits, alcoves, bright features and, in particular, the Hapi region.

ID

According to the spectral slope comparison presented in Table 2 for the two aforementioned observations at phase angles ∼60 • and ∼65 • , we noticed a slight phase-reddening effect for three out of four surface features considered. The only surface feature that does not show phase-reddening is the 'light smooth terrain' whose spectral slope diminishes as the phase angle increases. Although this seems an outlier at first glance, this deviation could be explained by the presence of bluish material, which dominates the spectral slope in the observation on 2016 February 10. The RGB image on Fig. 3(d) makes this more evident. From the above statistics, a slope of 0.35 ± 0.12%/(100 nm)/deg can be obtained, discarding the 'light smooth terrain' where a phase-reddening is absent. We also note that using OSIRIS observations, Fornasier et al. (2015) obtained disc-averaged spectral slopes, increasing from 11%/(100 nm) to 16%/(100 nm) in the 1. • 3-54 • phase angle range, leading to approximately 0.1%/(100 nm)/deg. Using VIRTIS data spread between 20 • and 120 • of phase angles, Ciarniello et al. (2015) obtained 0.07%/(100 nm)/deg in the wavelength range from 550 to 800 nm. The data of the aforementioned studies are sourced to pre-perihelion period of the comet (Fornasier et al. -2014 July/August andCiarniello et al. -from 2014 July to 2015 February), when 67P/C-G was in an inbound orbit at heliocentric distances of about 3 au. The results of Fornasier et al. (2015) correspond to the mean spectral slope of the entire nucleus, whereas our results are more specific to the analysed features of the Khonsu region, which demonstrate heterogeneous characteristics. However, as explained in Fornasier et al. (2016), the comet's sub-surface has been partially exposed, approaching the perihelion, as the increasing level of cometary activity has removed parts of the dust mantle, which results in the variation of the spectral slope.

Boulder analysis

Considering that a large quantity of boulders are present in Khonsu region, we decided to analyse the boulder distribution in order to get an insight into what might have been the origins of their formation. We used the observations made on 2015 May 2 when Rosetta was at ∼125 km from the comet, and identified the physiographic region of Khonsu (see Fig. 1). The resulting scale of 2.3 m pixel -1 of boulders inside the Khonsu region, we compared it to the power indices presented in Pajola et al. (2015). Their work offers three possible index ranges to explain the different boulder formation processes, i.e. a -5 to -6.5 power-law range, which can be attributed to collapses/pit formation and creation of depressions with subsequent escape of high-pressure volatiles and consequent high fracturing. A second power-law range, between -3.5 and -4.0, was suggested to be linked to gravitational events triggered by sublimation and/or thermal fracturing causing regressive erosion. A third power-law index range, between -1 and -2, was interpreted as the result of the evolution of the original material formed during both the collapsing or the gravitational event, not particularly renewed, or present in areas where continuous and high sublimation occurred or is still occurring (see fig. 8 of Pajola et al. 2016a). The geological analysis of this area (Lee et al., 2016) shows that different geological units are present here, such as talus deposits, fine material deposits, gravitational accumulation deposits as well as outcropping stratified terrains. Since the boulder analysis has been performed on the Khonsu physiographic region, we believe that the derived power-law index of -3.1 +0.2/-0.3 is indicative of all these different morphological units and the related formation processes, being slightly lower with respect to the -3.5 power-law index of purely gravitational deposits, but higher with respect to the -2.0 of collapsing events not particularly renewed by the retreating cliffs.

B R I G H T PAT C H E S I N K H O N S U R E G I O N

The detection of bright patches with high albedo characteristics on the comet 67P/C-G has triggered a growing discussion ever since Rosetta's arrival at the comet, when OSIRIS cameras started to observe them (Pommerol et al. 2015). They have been detected in different regions of the comet and we focus here on four bright patches that are evident in the OSIRIS images of the Khonsu region. Three of them fall into the category of isolated bright features that Pommerol et al. (2015) have introduced while one corresponds to a cluster of bright patches. Among these four, one patch observed from 2015 March to May has been associated within H 2 O ice by Barucci et al. (2016) with complimentary infrared VIRTIS spectral data.

Fig. 6 shows the four bright patches in question and their observational conditions are given in Table 3. For each bright patch, the spectrophotometric plots make it clear that the absolute reflectance of the bright patch is considerably higher than that of the typical nucleus extracted from the neighbourhood of the bright patch. This implies a higher albedo for the considered feature and a compositional difference. All the four bright patches presented here seem to be tending towards a flat spectral behaviour, which is compatible with the characteristics of eight spots studied by Barucci et al. (2016), indicating the presence of H 2 O ice. In this section, we treat each of the four bright patches individually by taking their temporal evolution of physical size and spectrophotometric properties into consideration, subject to the availability of data. 11 [269.3,360.0,480.7,535.7,649.2,701.2,743.7,805.2,882.1,931.9,989.3] 1d 2015-05-02_13h42 7.0 2.29 60. 6 11 [269.3,360.0,480.7,535.7,649.2,701.2,743.7,805.2,882.1,931.9,989.3 [269.3,360.0,480.7,535.7,649.2,701.2,743.7,805.2,882.1,931.9,989.3 [360.0,480.7,535.7,649.2,701.2,743.7,882.1,931.9,989.3 Barucci et al. (2016). As for the sub-figures a and b, we had to avoid the saturated centre pixels and hence sampling was done along the perimeter of the feature. Therefore, we remark that the absolute reflectances given in sub-figures a and b are underestimated and are to be considered as a lower limit.

Bright Patch 1.

First detected on 2015 March 10, this feature has been recorded multiple times in NAC, thus giving some insights about its temporal evolution in terms of size and spectrophotometric be-haviour. It was under observation up to 2015 May 7 and seems to have disappeared by 2015 May 22. We list in Table 4 the temporal variation of the size of this bright patch with some other observational conditions and Fig. 7 shows cropped images corresponding to some of the observations and spectrophotometric behaviour of the bright patch in comparison to that of the typical cometary nucleus.

From the obtained feature sizes during the observational span of about 2 months, it is clear that the feature had been undergoing a temporal evolution as explained by Barucci et al. (2016). One should also take into account the fact that the viewing geometry of each image is different and hence one must be cautious that the estimated dimension values might not correspond exactly to the real values of the feature at a given time and they should be considered as an approximative estimation. Given that this feature has a lifetime of about two months, we recall that Pommerol et al. (2015) have observed similar lifetimes for some of the bright patches they had catalogued. However, a direct comparison becomes tricky due to the different insolation conditions prevailed at corresponding heliocentric distances of the observations. We also highlight the spectrophotometric plots as given in Fig. 7 corresponding to the four selected sessions under observation. Their analysis is limited to the availability of filters for given circumstances, ranging from 3 to a favourable maximum of 11 filters depending on the session. The very high albedo (compared to the cometary nucleus) of the bright patch is emphasised in the two earlier observations (Figs 7a and 7b), while it seems to decrease towards the later (Figs 7c and 7d) observations. Barucci et al. (2016) have concluded that this bright patch contains H 2 O ice using the infrared spectral data acquired from VIRTIS spectro-imager on-board Rosetta on 2015 March 25 and the high reflectance in our spectrophotometric plots is consistent with their interpretation of H 2 O ice. The diminution of the reflectance can be explained as an aftermath of the sublimation of H 2 O ice during constant solar insolation for over a month, leading the dirty ice (mixed with the nucleus material) to be more exposed. As seen in Fig. 7(d), this results in an important diminution of reflectance in ultraviolet wavelengths. The evolution of the physical dimensions of the bright patch contributes to the idea of sublimation of H 2 O ice. As reported in Table 4, on 2015 May 7, we notice that this bright patch further continued to decay in its size and that it must probably have vanished completely between 2015 May 7 and May 22. We note that our analysis of the temporal evolution of this bright patch is consistent with that of Barucci et al. (2016) on the same spot (ID 3 among their eight white spots).

Bright Patch 2.

This bright patch can be seen sitting on a boulder or what appears to be a combination of a couple of boulders, resembling a boomerang. This configuration is often encountered at the surface of the nucleus, in various regions as shown, in particular, in fig. 4 of Pommerol et al. (2015). This 'Boomerang' feature is located towards the very steep scarp that draws the border between Khonsu and Apis region. We see evidence that this bright patch has been stable for quite a long duration, far longer than the mean survival duration of most of the bright patches studied on the comet 67P/C-G. Its first detection dates to 2016 January 23 while its latest detection dates to 2016 June 24, which implies a lifetime of more than five months. Here, we used the OSIRIS WAC images to better constrain the lifetime of this bright patch and its temporal physical evolution can be seen in the Fig. 8, while its corresponding observational conditions are detailed in Table 4. As evident from the presented data, multispectral observations for this bright patch are only available at the early stages of detection. The three WAC observations we have only allow us to track the presence of the bright spot, but only limited information can be obtained due the lower spatial resolution of the WAC and the unavailability of multispectral data. Nonetheless, the multispectral observational sequence on 2016 January 27, as seen on Fig. 6, provided us with the spectrophotometric curves. In fact, the albedo of this bright patch was so high that most of its centre pixels were saturated. Consequently, we had to carefully sample the pixels, avoiding any saturated pixels for the spectrophotometric calculations. Therefore, we chose a 3×3 pixel box towards the perimeter of the patch where the signal was high but not saturated. From the absolute reflectance plot, it is clear that the signal of the bright patch is more than six times higher than that of the surrounding terrain. Interpreting the flat nature of the relative reflectance plot, we suggest that the source of this bright patch could plausibly be H 2 O ice. The pixel saturation caused by the bright patch could be a clue to highlight the less contaminated nature of H 2 O ice.

An interesting fact about this bright patch is that it could be related to a cometary outburst in the following way. The coordinates of this bright patch, 13. • 32 of southern latitude and 196. • 47 of longitude, thoroughly match those of the outburst identified by the ID 7 with -12 • of latitude and 196 • of longitude by Vincent et al. (2016) in their catalogue of summer outbursts of 67P/C-G. This outburst dates to 2015 August 1 at 10h53 UTC and is likely to have triggered the appearance of the bright patch 2, after having altered the surface. In Fig. 9, we visualise the exact location with pre-and post-perihelion images in order to verify this idea. The figure clearly puts forward the idea that there has, in fact, been a considerable surface modification around the location identified by the coordinates. It is possible to discern few scattered boulders and outcroppings in the post-perihelion image. One can also notice that the terrace-like structure in the pre-perihelion image has vanished after the outburst event. Nevertheless, we note that this is only a correlation and the fact that we have not found any other mechanism to explain the surface modification does not rule out the possibility that this surface modification might have resulted from another mechanism.

Bright Patch 3.

This bright patch seems to be one of the short-lived bright patches as it has only been detected once. In fact, the follow-up observations did not show any more the existence of this bright patch. Given its smaller size, the preferable interpretation is that is has probably sublimated quickly. We remark that the spectrophotometric characteristics of this bright patch correspond to those of H 2 O ice as shown by Barucci et al. (2016), as deduced from the flat nature of the corresponding curve in Fig. 6. It is noticeable also that the curve deviates from the flat behaviour towards ultraviolet wavelengths, following a pattern similar to the typical nucleus. This can be explained by considering the bright patch to be composed of H 2 O ice and typical nucleus material. Since the size of the patch is quite small, there is probably the nucleus material dominating the signature of H 2 O ice when co-registered and integrated.

Bright Patch 4.

Observed on 2016 May 7, this bright patch seems to be a part of a cluster of bright patches scattered near the 'pancake' feature in Khonsu region. Its individual size is smaller than the average isolated bright patches and is of the order of the scattered small bright patches observed elsewhere (Khepry, Imhotep regions) in the comet. The NAC images used for the spectrophotometry of this bright patch correspond to a spatial resolution of about 20 cm per pixel and hence offer the possibility to discern the bright patches quite well. An observation of the same area, after about 6 weeks, suggests that most of the small bright patches have already disappeared, for there seems to be only a single small bright patch of the original cluster that remains, with some bluish material lying alongside. Supported by the spectrophotometric plots of Fig. 6, we interpret this bright patch to contain H 2 O ice. The high reflectance of this patch had, in fact, saturated the pixels in the centre, and we had to sample the pixels near its perimeter. From the location of this remaining bright patch, we could conclude that its existence has been favoured by the constant shadows. The existence of bright patches in a scattered form hints that their origin might be different than that of the isolated bright patches.

D I S C U S S I O N

Three of the studied bright patches on Khonsu are isolated, while one is a part of a cluster, although they might not share a common origin. As for the bright patch 2, we suggest that a cometary outburst had triggered it close to the perihelion passage, although it might have a different origin. This suggestion is supported by the fact that there has been a major surface modification after the outburst event. Furthermore, the bright patch 2 has been observed for a duration longer than five (Fig. 8) months. A bright patch with an area of about 80 m 2 , maintaining its size for about a month and then getting halved at the end of the following two months, requires a mechanism to retain the bright patch without quick sublimation. This behaviour could be explained by the existence of sub-surface icy layers, which could compensate the H 2 O ice being sublimated away. It is possible that the aforementioned outburst connected the plausibly underlying stratified ice layers in order for this process to work. We recall that a similar explanation was used by Barucci et al. (2016) to interpret the behaviour of their white spot 1. It is also noteworthy that the location of the bright patch 2 is often subject to shadows and hence the daily solar irradiation received at its location could be somewhat less than that for bright patch 1, whose location is not directly influenced by shadows as it is somewhat isolated on the smooth terrain on Khonsu. Therefore, we highlight that the location of the bright patch with regards to its neighbourhood is important to its survivability. The white spot 8 studied by Barucci et al. (2016) has been on the surface for more than four months and it was located between two boulders sitting next to each other, hence often casting shadows between themselves. In the case of bright patch 2 we present, it is next to the great scarp marking the Apis region, which results in constant shadows upon it.

Once again, we had to be cautious with the spectrophotometry of these bright patches, as often some of the centre pixels of the bright patches were saturated due to the high signal and we had to scrutinise the pixels before choosing them for the calculations. This implies an underestimation of the radiance factor of the ROSETTA orbiter, and it comprises a Narrow Angle Camera (NAC) for nucleus surface and dust studies, and a Wide Angle Camera (WAC) for the wide-field coma investigations (Keller et al. 2007). This imaging system has enabled extensive studies at high resolution (down to 10 cm px -1 , and even lower during the Rosetta final descent phase) of the nucleus, showing a peculiar bilobated shape with a surface characterized by a variety of astounding morphological regions including both fragile and consolidated terrains, dusty areas, depressions, pits, boulders, talus, fractures and extensive layering [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF]Sierks et al. 2015, Thomas et al. 2015a;Vincent et al. 2015). The Southern hemisphere had become visible from Rosetta only since 2015 March, two months before the Southern vernal equinox, and it shows a clear morphological dichotomy compared to the Northern one, with much less variety associated with the absence of wide-scale smooth terrains (El-Maarry et al. 2015a, 2016;Giacomini et al. 2016).

The 67P nucleus has a red spectral appearance with spectral properties similar to those of bare cometary nuclei, of primitive D-type asteroids like the Jupiter Trojans (Fornasier et al. 2007), and of the moderately red trans-Neptunian population (Capaccioni et al. 2015;Sierks et al. 2015). The surface is globally dominated by desiccated and organic-rich refractory materials (Capaccioni et al. 2015), and it shows some colour heterogeneities. Three kinds of terrain, ranging from the relatively bluer water-ice-rich mixture to the redder ones, associated mostly with dusty regions, have been identified by visible spectrophotometry from the first resolved images acquired in 2014 July-August (Fornasier et al. 2015). Local colour and compositional heterogeneities have been identified up to the decimetre scale (Feller et al. 2016) during the closest fly-by of 2015 February 14. Although water is the dominant volatile observed in the coma, exposed water ice has been detected in relatively small amounts (a few per cent) in several regions of the comet (De Sanctis et al. 2015;Pommerol et al. 2015;Barucci et al. 2016;Filacchione et al. 2016a;Oklay et al. 2016, Oklay et al. 2017), and in higher amounts (>20 per cent) in localized areas in the Anhur, Bes, Khonsu and Imhotep regions (Deshapriya et al. 2016;Fornasier et al. 2016;Oklay et al. 2017), and in the Aswan site (Pajola et al. 2017). Thanks to the unprecedented spatial resolution, Visible and In-fraRed Thermal Imaging Spectrometer (VIRTIS) and OSIRIS instruments have detected the indisputable occurrence of water frost close to the morning shadows, highlighting the diurnal cycle of water (De Sanctis et al 2015;Fornasier et al. 2016). Water frost is strongly correlated with the speed of the receding shadows and has an extremely short lifetime of a few minutes as observed while approaching perihelion (Fornasier et al. 2016).

Seasonal and diurnal colour variations of the surface of 67P's nucleus from inbound orbits to the perihelion passage have been reported by Fornasier et al. (2016). The nucleus became spectrally less red, i.e. the spectral slope decreased, as it approached perihelion, indicating that increasing activity had progressively shed the surface dust, partially showing the underlying ice-rich layer. A change in the physical properties of the outermost layer is also indicated by the evolution of phase reddening of the nucleus (i.e. the increase of spectral slope with phase angle) over time: the phase reddening coefficient decreased by a factor of 2 in the 2015 observations, approaching the perihelion passage, compared to the observations acquired on 2014 August, at 3.6 au inbound (Fornasier et al. 2016).

In this paper, we present a spectrophotometric and geomorphological analysis of two regions located in the Southern hemisphere of the 67P nucleus: Anhur and part of Bes regions (see Fig. 1 andEl-Maarry et al. 2015a, 2016 for the description of the 67P morpho- logical regions). These regions are more fragmented compared to other areas of the nucleus. They experience strong thermal effects because they are illuminated for a relatively short interval during the comet orbit, but close to the perihelion passage. These regions look particularly interesting as some large bright patches of exposed water ice have been identified pointing to local compositional heterogeneities. They are also highly active regions and sources of several jets, including the strongest outburst observed by Rosetta, which took place at the comet perihelion passage (Vincent et al. 2016a).

O B S E RVAT I O N S A N D DATA R E D U C T I O N

The images used for this study were reduced using the OSIRIS standard pipeline up to level 3B, following the data reduction steps described in Küppers et al. (2007) and Tubiana et al. (2015).

Those steps include correction for bias, flat-field, geometric distortion and absolute flux calibration (in W m -2 nm -1 sr -1 ), and finally the conversion in radiance factor (named I/F, where I is the observed scattered radiance and F is the incoming solar irradiance at the heliocentric distance of the comet), as described in Fornasier et al. (2015).

For the spectrophotometric analysis, the images of a given observing sequence were first coregistered taking the F22 NAC filter (centred at 649.2 nm) as reference and using a PYTHON script based on the scikit-image library (Van der [START_REF] Walt | scikit-image: Image processing in Python[END_REF]. To retrieve the radiance factor at pixel level precision for the high-resolution images acquired in 2016, we also used the optical flow algorithm to improve the image coregistration [START_REF] Farneback | High Accuracy Velocity Estimation using Orientation Tensors, Parametric Motion, and Simultaneous Segmentation of the Motion Field[END_REF]. Images were then photometrically corrected by applying the Lommel-Seeliger disc law:

D(i, e, a) = 2μ i μ e + μ i , (1)
where μ i and μ e are, respectively, the cosine of the solar incidence (i) and emission (e) angles. The geometric information about the illumination and observation angles were derived using the 3D stereophotoclinometric shape model (Jorda et al. 2016) and considering all relevant geometric parameters, such as the camera-distortion model, the alignment of the camera to the Rosetta spacecraft and the orientation of the spacecraft (with reconstructed orbit position and pointing) with respect to the 67P nucleus. RGB maps, in false colours, were generated from coregistered NAC images acquired with the filters centred at 882, 649 and 480 nm using the STIFF code (Bertin 2012).

The data presented here were acquired from 2015 March, when the Anhur-Bes regions became visible from Rosetta, through 2016 July. Details on the observing conditions are reported in Table 1.

G E O M O R P H O L O G Y

We used three OSIRIS NAC images to produce the geomorphological mapping of Anhur, Bes and Geb regions (El-Maarry et al. 2016) at different viewing angles. One NAC image (Fig. 2, top panel) was obtained on 2015 May 2 when the comet was at the equinox, and the other two (Fig. 2, middle and bottom panels) were acquired during the post-perihelion phase. We cross-checked the images acquired at different periods and different viewing angles to analyse possible geomorphological changes.

Mapping of top and middle panels of Fig. 2 are centred on Anhur and Bes regions, respectively, whereas the bottom panel displays both Anhur and Bes, and part of Geb. All the images show that outcropping consolidated terrains prevail and are sculpted by staircase terraces. This supports the inner stratification hypothesis [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF]. By comparing the different views it is clear that, assuming an onion-like layering of the nucleus [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF], Bes is in a shallower structural level with respect to Anhur and Geb that are dominated by an elongated canyon-like depression. The flank of the depressions displays a uniformly degraded sequence of strata. The long depression and the bounding cliffs and terraces extend from Anhur to Geb without any evident morphological interruption (Fig. 2, middle panel). Hence the Geb region is completely indistinguishable from Anhur at least from a structural point of view. For this reason, we will refer only to Anhur region. Both Anhur and Bes consolidated terrains are crossed in places by long fracture systems superimposed on the m-scale polygonal ones already described by El-Maarry et al. (2015b, 2016) on other regions of the comet nucleus.

The strata and cuesta margins in the Anhur region have apparently been dissected by several scarps with various kinds of deposits at their feet. In particular, we have distinguished talus and more heterogeneous gravitational accumulation deposits. They are both interpreted as made up of material dislodged from adjacent cliffs due to concurrent sublimation and gravitational processes (La Forgia et al. 2015;Pajola et al. 2015;Vincent et al. 2016b). We have also distinguished diamictons that are characterized by a highly heterogeneous texture with blocks and fine materials as the gravitational accumulations, but unlike the latter, diamictons are located far from the cliffs. The diamictons can be leftovers from erosive scarp retreats (Pajola et al. 2016) or the result of in situ degradation not yet been established for this comet, we studied two end cases. For the first one we assumed that the surface consists of an intimate mixture of dust and ice, with the two components mixed at submillimetre scales and assumed to be in thermal equilibrium (model A of Keller et al. 2015). For this case we used a Bond albedo of 0.05, i.e. a value five times higher than the one measured at 535 nm for 67P by Fornasier et al. (2015), in order to match the higher albedo observed for the ice-rich patches.

For the second end case, we considered a surface consisting of pure ice and refractory material well separated on small scales -but still spatially unresolvable by OSIRIS -and nonthermally interacting. For this case, the brightness observed by OSIRIS corresponds to the linear combination by surface of the two individual components. In this case, we used thermal model A with a Bond albedo for pure water ice snow of 0.7 [START_REF] Delsemme | Ices in the Solar System[END_REF]. For the purpose of comparison, we also modelled the behaviour of the dark background material with model B (Keller et al. 2015), which describes the comet surface as a two-layer system, in which a sub-surface dust-ice mixture is overlain by a desiccated dust coating with a Bond albedo of 0.01.

The results shown in Table 2, predict erosion rates ranging from about 0.25 mm d -1 to 5 mm d -1 for the facets on the shape model representative of the patche location. The higher erosion rate corresponds to the intimate mixing model, in which, due to the lower albedo, more input energy is available for sublimation. Conversely, the lower erosion rate corresponds to the end case in which pure ice is mixed with refractory material at geographical, unresolved scales. The erosion is computed for pure porous ice with a density of 533 kg m -3 , corresponding to the bulk density measured for 67P (Paetzold et al. 2015;Jorda et al. 2016;Preusker et al. 2015). Assuming a dust to water ice ratio of 4:1 (Rotundi et al. 2015), the total erosion would be in the range 1.3-24 mm d -1 , with an estimated ice loss rate ranging from about 0.14 to 2.5 kg d -1 m -2 . Considering that the observed permanence time of the ice patches was of the order of 10 d, the thickness of the porous dust/water ice layer is estimated to be between 13 and 24 cm for the intimate mixture case, while for an areal mixture of dust and ice, the original thickness would be between 13 and 36 mm. These ice-rich patches indicate a variation of the water ice content in the uppermost layers pointing to local compositional heterogeneities of the 67P nucleus. Fornasier et al. (2016) proposed that these compositional heterogeneities are related to the re-condensation of volatiles and sintering of the sub-surface material during previous perihelion passages. The uniqueness of the observed bright patches is probably related to a delicate balance between activity and dust removal leading to an icy surface for a limited duration.

Notably, the presence of a small amount of CO 2 ice was detected by the VIRTIS spectrometer in a 80 by 60 m area corresponding to the location of patch A on 2015 March 21-23 (at longitude 66 • and latitude 54.6 • ), ice that had sublimated in less than 3 weeks (Filacchione et al. 2016b), well before the detection of the bright patches with OSIRIS. Filacchione et al. (2016b) estimated the thickness of the CO 2 ice layer to be <5.6 cm and proposed that it was formed by the freezing of CO 2 at low temperature after the previous perihelion passage. In the OSIRIS archive, there are no images covering the Anhur region simultaneously to the VIRTIS CO 2 ice detection. That region is visible in images acquired on 2015 March 25 (Fig. 6, green triangle), located close to shadows and it has a spectrophotometric behaviour in the 480-881 nm region, which is quite different from the other selected regions, with a much lower spectral slope value consistent with the presence of some ice. However, this region is not brighter than its surroundings, indicating that the exposed fresh layer of CO 2 ice had partially sublimated and that the residual ice must be well mixed with the surface dust.

In Fig. 6, a bright structure is visible, however its spectrophotometry (red star symbol) indicates a red spectral behaviour indistinguishable from that of the comet dark terrain. Other bright and spectrally reddish regions have been observed at higher spatial resolution by OSIRIS (Feller et al. 2016). These features may be associated with brighter minerals and/or with different structure/grain size, and not with an enrichment in water ice of the surface composition.

Spectrophotometry of smaller bright spots

Besides the two large bright patches, several smaller and localized areas enriched in water ice have been identified in Anhur, close to shadows and/or in the pit deposits, and characterized by a relatively blue spectrophotometry and an increase in the reflectance . Several studies with the VIR-TIS spectrometer have proven that regions having a flat or less steep spectral behaviour than the surroundings in the visible range have a higher abundance of water ice mixed with the refractories (Barucci et al. 2016;Filacchione et al. 2016a). For comets 9P/Tempel 1 and 103P/Hartley 2, relatively bluer colours were also associated with a higher abundance of water ice (Sunshine et al. 2006(Sunshine et al. , 2012)).

In Fig. 7, the light blue symbol indicates a bright feature whose spectrophotometry is flat with a reflectance 5-6 times higher in the 360-480 nm range than the surrounding terrains, similarly to what has been observed in other ice-rich features. The bright but spectrally red feature observed in March 25 images is still present (green triangle in Fig. 7) and shows a spectrum similar to that of surrounding dark terrains (magenta square). The areas where the two large bright patches A and B have later appeared (orange circle and red star) look spectrally bluer than the surroundings, indicating a progressive local enrichment in the surface water ice abundance from the sub-surface layers.

A nice view of water-ice-enriched regions observed close to the Anhur morning shadows is given in Fig. 8 (left side). The spectrophotometry in these regions indicates a flat spectral behaviour, consistent with the presence of water ice, and, for the brightest region represented by the red star symbol an enhancement of the flux in the UV region possibly due to the presence of some water frost. For this region (in particular for the area represented by the green triangle), visible and NIR spectra were obtained 3 months earlier with the VIRTIS spectrometer and presented in Barucci et al. (2016), indicating the presence of some exposed water ice although only a small amount (less than 1 per cent).

Looking at the spectrophotometry in Fig. 8, we also notice an increase of the flux in the 700-750 nm region and at 930 nm, which has already been previously observed in different regions of the comet, and attributed to cometary emissions of H 2 O + and NH 2 (Fornasier et al. 2015). Notably, beside the cometary emission feature, no diagnostic absorption bands are observed in any of the spectrophotometric data presented here. In particular, we do not detect any absorption band associated with hydrated silicates like the one centred at 0.7 µm seen on the spectra of several hydrated asteroids (Fornasier et al. 2014), and of some trans-Neptunian bodies (de Bergh et al. 2004;Fornasier et al. 2004Fornasier et al. , 2009)). Thus the apparent absence of phyllosilicates in the Northern hemisphere smooth deposits which are spectrally bluer than the nearby areas. Fig. 12 shows this active pit in images acquired on 2015 June 5 (panel c), and on 2015 December 7 (panel d). The bluer colours of the pit floor may be produced by fresh material resulting from possible lateral outflow, as observed by Vincent et al. (2015) in some Northern hemisphere pits, and/or to floor contamination due to fallback ejecta (dusty-ice grains) which were not able to escape far enough from the pit source. 13). From the images acquired at higher resolution on 2016 January, we evaluate the length of this scarp to be about 140 m. It bounds a depressed area with an extension of 4000-5000 m 2 , suggesting a maximum volume loss of about 50 000 m 3 from sublimation. The collapse event most likely occurred due to the comet's enhanced activity close to the perihelion. At the foot of the scarp and within the newly formed depression, the deposits display more boulders compared to the original terrains at the scarp hanging wall. Thus this event generated a collapse of the material with the formation of new boulders.

The new scarp was observed on 2016 May 7 from a spacecraft distance of 11 km, corresponding to a spatial resolution of 0.2 m px -1 , with 7 NAC filters. It shows bluer colours than the surrounding area (Fig. 14). Notably the freshly exposed terrain indicated by the regions of interest (ROI) 2 and 3 (Fig. 14) is less red than ROI 1 located on the top of the scarp while the bright region in ROI 4 (blue square symbol), located close to the shadow of a ∼9 × 6 m boulder, has a flat spectrophotometric behaviour indicating the presence of some water ice. Several isolated bright spots with a reflectance similar to that of ROI 4 are also visible close to the other boulders located in the new scarp. We estimate the water ice content in ROI 4 using a simple areal mixture model. To do so, we first compute the normal albedo of the ROIs using the geometric information derived from the shape 7S model (Jorda et al. 2016), and the Hapke model parameters determined by Fornasier et al. (2015, their table 4) from resolved photometry in the orange filter centred at 649 nm. We assume that the phase function at 649 nm also applies at the other wavelengths. To simulate the observed reflectance of ROI 4, we create an areal mixture of two components: the comet dark terrain, represented by ROI 1 at the top of the new scarp, and water ice. The water ice spectrum was derived from the synthetic reflectance from Hapke modelling starting from optical constants published in Warren & Brandt (2008) and adopting a grain size of 30 µm, that is a typical size for ice grains on cometary nuclei (Sunshine et al. 2006; Capaccioni et al. 2015;Filacchione et al. 2016a). This assumption was necessary because it is not possible to constrain the ice particle size from OSIRIS observations. The model that best fits the reflectance of ROI 4 is the areal mixture of the comet dark terrain enriched with 17 ± 2 per cent of water ice, i.e. a value comparable to what has been found for patch B seen one year earlier in the Bes region.

Spectral slope evolution

The evolution of the spectral slope of the Anhur region is shown in Fig. 15. As the observations are obtained at different phase angles and the phase reddening has been found to be important for the 67P's nucleus and variable with the cometary activity (Fornasier et al. 2015(Fornasier et al. , 2016;;Feller et al. 2016), the analysis of the spectral evolution over time is not obvious. Several areas having a lower spectral slope value are evident in Fig. 15 (relatively bluer surfaces in the given colour scale) and associated with regions enriched in water ice and/or active. The comparison of images acquired at very similar phase angles clearly shows an increase of the spectral slope for images acquired post-perihelion at larger heliocentric distances. The 2015 May 2 and 2016 January 27 data were both acquired at phase angle 62 • , and the average spectral slope in the 535-882 nm range increased from S = 14.9 per cent/(100 nm) to 16.4 per cent/(100 nm) from 2015 to 2016 observations. Local spectral slope variations (notably regions with a much lower spectral slope value) are associated with the large water-ice-rich patches (on 2015 May 2 images) or to the exposed ice close to a scarp previously discussed (on 2016 January 27). Similarly, the data acquired at phase angle 87 • show the same trend, with the spectral slope S increasing from 15.9 per cent/(100 nm) to 17.5 per cent/(100 nm) from 2015 June to 2016 June data in the observed common region. Fornasier S106 S. Fornasier et al. et al. (2016) analysed the spectral slope evolution over time and heliocentric distance for the comet finding a progressive spectral slope decrease of the entire comet surface as it approached the Sun when the intense activity lifted off large amounts of dust, partially exposing the ice-rich terrain underneath, followed by a spectral slope increase in the post-perihelion orbit at heliocentric distance >2 au. The spectral slope evolution observed for the Anhur-Bes regions confirms the results reported by Fornasier et al. (2016) for the whole comet, notably the continuous changes of the physical properties of the uppermost layers of the nucleus related to the cometary heliocentric distance and to the level of activity. As observed for the whole comet, the Anhur-Bes regions also became spectrally redder at large post-perihelion distances when the activity was no longer capable of sustained removal of dust. Keller et al. (2017) estimates that up to 20 per cent of the dust particles lifted up by the cometary activity from the active Southern hemisphere during the perihelion passage may directly return to the nucleus surface with re-deposition in the Northern hemisphere, implying important mass transfer between the two hemispheres. However, even if the Southern hemisphere is depleted in large scale smooth terrains and is dominated by consolidated material (El-Maarry et al. 2016), a dust mantle covers its surface with a thickness varying with the cometary activity, thus explaining the red colours observed in the Bes and Anhur regions beyond ∼2 au.

C O N C L U S I O N S

In this work, we have investigated the geomorphology, the spectrophotometric properties, and the transient events of Anhur and part of Bes regions. They are most fragmented regions showing fractured and patterned rough terrains, and displaying talus deposits, elongated canyon-like structures, scarp retreats, diamictons and degraded sequences of strata indicating a pervasive layering. They receive an intensive insolation during the Southern summer. Because of this, the erosion rate due to the sublimation of water ice in the Southern hemisphere is up to 3-4 times higher than that in the Northern hemisphere (Keller et al. 2015, Keller et al. 2017). Thus Southern regions give potentially a better glimpse of the pristine cometary material than the Northern ones, where the dust coating is thicker.

The Anhur-Bes regions are highly active and are sources of several jets, including the strongest outburst event ever observed, as established in this paper and in Vincent et al. (2016a). Most of the jets' sources are found close to the Anhur-Aker boundary, i.e. close to cliffs. Anhur also hosts the only active pit observed in the Southern hemisphere.

The Anhur-Bes regions are sculpted by staircase terraces that support the nucleus stratification hypothesis formulated by [START_REF] Massironi | Two independent and primitive envelopes of the bilobate nucleus of comet 67P[END_REF]. Anhur seems highly eroded with the presence of several scarps dissecting the different strata. Interestingly, at the feet of scarps and cliffs, we observed both talus and gravitational accumulation deposits. These deposits often show a relatively bluer spectral behaviour than the surroundings, pointing to an enrichment in the surface water ice content, and in some cases a higher reflectance and a flat spectrophotometric behaviour, consistent with the presence of exposed water ice. These deposits are sometimes also sources of activity. These observations thus reinforce the hypothesis by Vincent et al. (2016b) that the fresh material falling from cliffs/scarps is volatile rich and may become active.

In the boundary between Anhur and Bes, two water-ice-rich patches were visible for about 10 d (between at least 2015 April 27 and May 7), shortly before the equinox, and they were observed one month after the unique detection of exposed CO 2 ice on the 67P's nucleus (Filacchione et al. 2016), in the location corresponding to one of the two bright patches (patch A). These ice-rich patches formed in a smooth terrace covered by a layer of fine deposits on the consolidated material. The fact that first the CO 2 ice and then the H 2 O ice was exposed, indicates a progressive stratification of different volatiles resulting from recondensation and sintering of the sub-surface material during previous perihelion passages, and clearly points to local compositional heterogeneities on scales of several tens of metres.

In this peculiar Anhur-Bes boundary, we also noticed a new scarp formed sometime between the perihelion passage and 2015 December. The scarp is about 140 m long and 10 m high, bounding a depressed area of about 4000-5000 m 2 , and generated a collapse of the material with the formation of new boulders. The strong activity throughout the perihelion passage, together with the observed local surface and sub-surface enhancement in volatiles in these areas presumably triggered the formation of this new scarp. Also in this case, the freshly exposed material collapsed from the scarp shows a bluer colour and a lower spectral slope indicating the presence of some water ice, reaching abundance of about 17 per cent in the shadows of some boulders located in the new depression.

The spectral slope evolution from 2015 April to 2016 June clearly indicates that the Anhur-Bes regions, as observed for other regions of the comet, became spectrally redder post-perihelion at heliocentric distances >2.0 au compared to the pre-perihelion data. This indicates continuous changes of the physical properties of the uppermost layers, in terms of composition and roughness, driven by the different levels of cometary activity and insolation. Close to perihelion the strong cometary activity thinned out the nucleus dust, partially exposing the underlying ice-rich layer, resulting in lower spectral slope values seen all over the nucleus as shown by Fornasier et al. (2016). This implies that water ice is abundant just beneath the surface on the whole nucleus, as also inferred by the study of the gaseous emission in the coma, with the H 2 O emission which is strongly correlated with the local illumination on the nucleus, and produced predominantly by local well-illuminated active areas (Bockelee-Morvan et al. 2015;Migliorini et al. 2016). Far from the Sun, as the cometary activity decreased, the dust mantle became thicker, masking the sub-surface ice-rich layer. This results in a steep spectral slope associated with a surface composition depleted in volatiles.

AC K N OW L E D G E M E N T S

OSIRIS was built by a consortium led by the Max-Planck-Institut für Sonnensystemforschung, Goettingen, Germany, in collaboration with CISAS, University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universitat Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB) and the ESA Technical Directorate is gratefully acknowledged. We thank the Rosetta Science Ground Segment at ESAC, the Rosetta Mission Operations Centre at ESOC and the Rosetta Project at ESTEC for their outstanding work enabling the science return of the Rosetta Mission. The authors thank

Comparing the spectral slopes obtained in late August 2015, shortly after perihelion passage, with those obtained in early August 2014 after phase reddening correction (12) (Fig. 1, figs. S1 to S3, and table S1), clearly indicates that the nucleus has become relatively bluer, i.e., the spectral slope has decreased as the comet approached perihelion.

Even though only the equatorial regions such as Imhotep ( 13) are common for both datasets (Figs. 1 and fig. S4), due to different seasonal insolation conditions, a decrease of about 30% in the mean spectral slope ( 12) from 3.6 AU to the perihelion passage is measured in the common areas. A decrease of the visible spectral slope was also reported from VIRTIS observations in the August-November 2014 timeframe [START_REF] Filacchione | The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. I) Prelanding mission phase[END_REF]. In a water-ice/refractory-material mixture, regions with bluer colors indicate a higher abundance of water ice [START_REF] Fornasier | Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft[END_REF]16), as demonstrated by VIRTIS observations (8,9), and as previously observed for 9P/Tempel 1 and 103P/Hartley 2 comets (1,2).

In addition to the change of color, the amount of phase reddening (the increase of spectral slope with phase angle) decreased by a factor of two when the comet approached perihelion in the 2015 observations compared to those from August 2014 [START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF][START_REF] Fornasier | Spectrophotometric properties of the nucleus of comet 67P/Churyumov-Gerasimenko from the OSIRIS instrument onboard the ROSETTA spacecraft[END_REF], indicating a change in the physical properties of the outermost layer of the nucleus. The relatively blue color of the surface and the decreased phase reddening effect observed at small heliocentric distances suggest that the increasing level of activity has thinned the surface dust coating, partially exposing the underlying icerich layer. This result is in agreement with the variation of the sublimation rate with heliocentric distance r reported by [START_REF] Fougere | Three-dimensional direct simulation Monte-Carlo modeling of the coma of comet 67P/Churyumov-Gerasimenko observed by the VIRTIS and ROSINA instruments on board Rosetta[END_REF], considerably steeper (∝ r -4.2 ) than the one obtained from modeling considering the shape of the nucleus and seasonal effects [START_REF] Keller | Insolation, erosion, and morphology of comet 67P/Churyumov-Gerasimenko[END_REF]. This steepness can be explained by the observed thinning of the surface dust layer that facilitates the sublimation during the approach to perihelion. Observations at 2.2 AU outbound (fig. S3) show that the comet's colors redden again as the activity decreases and is no longer capable of sustained removal of dust.

The increase in water ice visibility is observed on the whole surface indicating that the composition in terms of dust to ice ratio must be similar at large scales all over the nucleus. This means that even the smooth areas commonly thought to be covered with material that fell back on the surface ( 18) must be water ice-rich.

We observed and monitored 2 bright patches, of about 1500 m 2 each (Table S2), located in a smooth area of the Anhur/Bes (13) regions (Fig. 2, and fig. S4), on images acquired on 27 April-2 May 2015. One of the two patches, B in Fig. 2, was still present in an image acquired on 7 May, yielding a survival time of at least 5-10 days. These features are considerably larger than the meter-scale bright spots previously detected on 67P comet (10). The VIRTIS spec-trometer detected the presence of CO 2 ice in region A on 21-23 March 2015 that had sublimated in less than 3 weeks [START_REF] Filacchione | Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko[END_REF]. On 12 April the bright patches were not present yet but these regions were spectrally bluer than the surrounding ones, indicating a higher water ice abundance (Fig. 2). On 27 April and 2 May the bright patches were clearly visible, and showed a relatively flat spectrum, with a maximum reflectance 4 to 6 times brighter than the surrounding regions. A water ice surface abundance of ~25% linearly mixed with the comet dark terrain is needed to match the patches reflectance [START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF]. Subsequent images covering the Anhur region were acquired on 4-5 June, revealing that the water ice had fully sublimated from the surface, leaving a layer spectrally indistinguishable from the average nucleus (Fig. 2).

We compute the sublimation rate for this period by applying the thermal model described in [START_REF] Keller | Insolation, erosion, and morphology of comet 67P/Churyumov-Gerasimenko[END_REF] for both intimate and areal mixtures of refractory material and ice [START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF]. The estimated ice loss rate ranged from 1.4 to 2.5 kg day -1 m -2 for the intimate mixture, and from 0.14 to 0.38 kg day -1 m -2 for the areal mixture cases. By noting that the permanence of the ice patches was about 10 days, we estimate a solid-ice equivalent thickness between 1.5 and 27 mm for the ice patches. The actual thickness of this layer will be up to a factor 10 higher due to porosity of the dust/ice mixture.

The appearance and disappearance of water ice patches shows that the level of activity is varying on time scales that are short compared to seasonal changes of the illumination. These ice-rich patches indicate a variation of the water ice content in the uppermost layers pointing to local compositional heterogeneities with scales of 10s of meters on 67P's nucleus. The huge width-to-depth ratio observed in the ice patches would suggest a near-surface solar-driven processing being responsible for enhancement of local ice abundance, resulting from the recondensation of volatiles and sintering of the subsurface material during previous perihelion passages. Laboratory results from KOSI (KOmetenSImulation) experiments had shown that a considerable fraction of sublimating ice can be redeposited instead of being released through the dust mantle [START_REF] Benkhoff | Energy analysis of porous water ice under space-simulated conditions: Results from the KOSI-8 experiment[END_REF]. Numerical simulations show that a hardened layer may form beneath a fine dust mantle [START_REF] Kossacki | Comet 67P/Churyumov-Gerasimenko: Hardening of the sub-surface layer[END_REF], hard layer that was detected by the Philae lander in the Abydos site [START_REF] Spohn | Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko[END_REF]. The composition of the icy patches may be representative of the comet's near-surface. Occasional local removal of the overlying mantling material could expose the underlying layer, leading to an icy surface for limited periods of time.

Approaching perihelion, the nucleus has shown considerable diurnal color variations on extended areas and the occurrence of water frost close to morning shadows. This is evident on the Imhotep region, where morphological changes were observed [START_REF] Groussin | Temporal morphological changes in the Imhotep region of comet 67P/Churyumov-Gerasimenko[END_REF]. Areas just emerging from the shadows are spectrally bluer than their surroundings (Fig. 3A while 40 min later, once fully illuminated, their spectral slope has increased (Fig. 3B). This phenomenon, observed during other color sequences acquired in June-July 2015, and seen at dawn on different areas on both lobes of the comet (12) (fig. S5 and movie S1), is periodic. We interpret the relatively blue surface at dawn as the presence of additional water frost that condensed during the previous night.

We also observe the presence of fronts of bright material in the illuminated regions close to rapidly traveling shadows cast by local topography (Fig. 3). A water rich fringe near shadows at the Hapi/Hathor transition was also observed by VIRTIS (7). The bright fronts move with the shadows. Modeling of the illumination (fig. S6 and movie S2) shows that the extent of these bright features directly correlates with the shadow travel speed, being wider where the shadow speed is faster, and narrower where the shadow speed is slower.

These fronts are about 6 times brighter than the mean comet reflectance. Their spectrum is globally flat with a flux enhancement in the ultraviolet (Fig. 4 and fig. S7), similar to what observed on blue regions of comet Tempel 1 (1). An abundance of about 17% water frost linearly mixed with the comet dark terrain is needed to match the reflectance of the bright fronts [START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF]. We interpret these bright features as surface frost, formed when water vapor released from the subsurface recondenses after sunset, that rapidly sublimates when exposed to the Sun. Molecules in the inner coma may also be back-scattered to the nucleus surface and recondense, contributing to the frost formation [START_REF] Davidsson | A practical tool for simulating the presence of gas comae in thermophysical modeling of cometary nuclei[END_REF][START_REF] Crifo | Improved gas-kinetic treatment of cometary water sublimation and recondensation -Application to comet P/Halley[END_REF]. From the shadow travel speed and the extent of the frost fronts we calculate an ice permanence time of about 3 min [START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF]. From thermal modeling using intimate mixture we estimate a water-ice sublimation rate of about 4 g m -2 min -1 , from which we infer a total ice content of about 12 g m -2 for the frost layer, which is extremely thin (thickness ~10-15 μm of equivalent solid ice, Table S3). If we assume an areal mixing, the sublimation rate and total ice content are 8 times lower. In the absence of direct temperature measurements of the frost it is impossible to discriminate whether areal or intimate mixture represent the correct thermal model (1). In either case, the exposed ice is depleted on short time scales. In the case of geographical mixing, however, sublimation rates at the time of the observations would be so low, that a frost permanence time of 3 min would imply a frost layer of the order of ~1 μm solid-ice equivalent. Since it is questionable whether such a thin layer would be optically thick, it appears more likely that the frost patches are intimately mixed with the refractory material. Similarly to the observed diurnal color variations, the recondensation of frosts is also a periodic phenomenon that takes place close to topographic shadows, as indicated in Figs. 3 and4.

The long term observations of 67P provide information on the composition of the outermost layers of the nucleus. They reveal that ice is abundant just beneath the surface on the whole nucleus, which, most of the time, is covered by a thin layer of dust. Hence the apparent surface composition is globally dominated by anhydrous refractory materials (6).

The increasing cometary activity while approaching the Sun progressively thins the dust surface layer partially exposing the ice-richer sub-surface, yielding nucleus's colors that are bluer relative to those at large heliocentric distances. OSIRIS observations show that mixtures having high abundance (up to ~30%) of water ice are occasionally locally present on the nucleus and that the lifetime of exposed ice is short, of the order of few minutes to few days. However, no dust-free ice patches were observed even during the peak of activity near perihelion, indicating that water ice and dust are well mixed within the resolution limit of the images (~2 m pixel -1 ). Most of the bright features are observable only at high spatial resolution and under particular insolation conditions, i.e., close to the morning shadows. Similar phenomena presumably take place on other comets, explaining why cometary nuclei are so dark even if they have significant water ice abundance. The extended bright patches observed in the Anhur/Bes regions indicate a local enrichment of water ice, pointing to compositional heterogeneities in the uppermost layers of comet 67P.

Introduction

Space exploration has triggered major progress in our understanding of comets beginning in March 1986 with the exploration of comet 1P/Halley by an armada of missions including the ESA Giotto mission [START_REF] Reinhard | The Giotto mission. Its scientific investigations[END_REF]. With the arrival of the ESA Rosetta mission at the comet 67P/Churyumov-Gerasimenko (67P/C-G) on July 2014, comets appear more complex and fascinating than ever. All the visited comets show a low visible albedo and heterogeneous surface (Barucci et al. 2011). However 67P/C-G and some other periodic comets reveal the presence of intriguing bright spots on the surface (Sunshine et al. 2006;Sunshine et al. 2012;and Li et al. 2013;Pommerol et al. 2015). To better understand the properties and composition of the comet 67P/C-G is one of the major objectives of the ESA Rosetta mission, and all on-board instruments have so far contributed with high quality and a precious quantity of data.

Since the orbital insertion of the Rosetta spacecraft, the comet nucleus has been mapped by both OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System), and VIRTIS (Visible InfraRed Thermal Imaging Spectrometer) acquiring a huge quantity of surface images in different wavelength bands and spectra, and producing the most detailed maps at the highest spatial resolution of a cometary nucleus surface. The OSIRIS imaging system (Keller et al. 2007) is composed of the Narrow Angle Camera (NAC) designed to study the nucleus with 11 large band filters at different wavelengths from the ultraviolet (269 nm) to the near-infrared (989 nm), while the Wide Angle Camera (WAC) is devoted to the study of gaseous species in the coma with a set of 14 narrow band filters ranging from the ultraviolet to visible wavelengths. The OSIRIS imaging system was the first instrument capable of mapping a comet surface at high resolution, reaching a maximum resolution of 11cm/px during the closest fly-by that occurred on February 14, 2015, at a distance of ∼ 6 km from the nucleus surface. VIRTIS [START_REF] Coradini | Virtis: An Imaging Spectrometer for the Rosetta Mission[END_REF]) is composed of two channels: VIRTIS-M, a spectro-imager operating both in the visible (0.25-1.0 µm) and infrared (1.0-5.0 µm) ranges at low spectral resolution (λ/δλ=70-380), devoted to surface composition, and VIRTIS-H, a singleaperture infrared spectrometer (1.9-5.0 µm) with higher spectral resolution capabilities (λ/δλ=1300-3000) devoted to the investigation of activity.

The OSIRIS images of 67P/C-G show a highly shaped, irregular bilobed comet, with a dark, dehydrated, and morphologically complex surface characterized by several terrain types, including numerous diverse geomorphologic features (Sierks et al. 2015). The comet's surface is highly heterogeneous with different geological terrains showing smooth, dust-covered areas, large scale depressions, brittle materials with many pits and circular structures, and exposed consolidated areas (Thomas et al. 2015a;El-Maarry et al. 2015;El-Maarry et al. 2016). Pits have also been connected to activity, possibly accompanied by outbursts (Vincent et al. 2015). The complex surface of comet 67P/C-G shows regions covered by different layers of dust on both lobes, including areas with evidence of transport and redistribution of dust materials (Thomas et al. 2015b). Temporal variations of morphological structures have also been observed on the smooth terrains of the Imhotep region (Groussin et al. 2015), as well as in other regions (Fornasier et al. 2016), in particular when the comet was close to perihelion. The comet shows albedo variation of up to about 25% and spectrophotometric analysis (Fornasier et al. 2015) identified at least three groups of terrains with different spectral slopes (computed in the 535-882 nm range). These differences have been associated with the local composition variation, but since many different surface characteristics overlap, this makes the interpretation difficult. Oklay et al. (2016a) also studied surface variegation on the comet, detecting local color inhomogeneities connected to active and inactive surface regions.

The first results by VIRTIS (Capaccioni et al. 2015) about the spectral analysis showed the presence of a broad absorption feature around 2.9-3.6 µm present across the entire observed region and compatible with carbon-bearing compounds (opaque minerals associated with organic macromolecular materials) with no evidence of ice-rich patches. Later on, De Sanctis et al. (2015) detected the first evidence for the presence of H 2 O ice as part of a diurnal cycle on the neck of the comet, while Filacchione et al. (2016a) identified H 2 O ice on two gravitational debris falls in the Imhotep region exposed on the walls of elevated structures. The latter was interpreted as being possibly extended layering in which the outer dehydrated crust is superimposed over water ice-enriched layers. During the first mapping phase of 67P/C-G nucleus, completed in August-November 2014 (heliocentric distances between 3.6 and 2.7 AU), VIRTIS-M achieved a complete mapping of the illuminated regions in the equatorial and northern hemisphere, which enabled us to retrieve the first compositional maps by using VIS and IR spectral parameters (Filacchione et al. 2016b). During the same period, coma observations performed by VIRTIS-M (Migliorini et al. 2016) and VIRTIS-H (Bockelée-Morvan et al. 2015) channels have traced the H 2 O vapor emission, which occurs preferentially above the illuminated regions of the northern hemisphere.

As limited evidence of exposed H 2 O ice regions has so far been collected, the aim of this work is to investigate in depth the composition of the 67P/ C-G surface, combining the high spatial resolution images of OSIRIS and the high spectral resolution of VIRTIS for detecting and emphasizing interesting ice spectral signatures. Over 100 meter-sized spots were identified by Pommerol et al (2015), possibly associated with the presence of H 2 O, on the basis of laboratory experiments, but with no con-firmation of the real presence of ice. Deshapriya et al. (in preparation) are collecting a catalogue of large bright spots that are present on the surface of the comet by analyzing the OSIRIS images and spectrophotometry data. For this work we selected the largest spots, good candidates to search for H 2 O ice that could be detected at the lower angular resolution of VIRTIS. We identify large features with high albedo and low spectro-photometric slope with OSIRIS, we compute accurate coordinates, and we analyze them on the basis of the VIRTIS spectra. Over the large number of spots identified by OSIRIS, 13 of them were checked by VIRTIS. Eight of them show clear evidence of H 2 O ice in their spectra.

In this paper we report on the analysis of the eight bright spots for which we obtained a positive detection of H 2 O ice in VIRTIS data. In Section 2, the OSIRIS data and the performed analysis are presented and, in Section 3, the VIRTIS data, while in Section 4 the spectral modeling of the selected spots is described. In Section 5, a detailed analysis of the spots and surrounding area is reported, while in Section 6, a possible evolution of the area is discussed. The main aim of this work is to confirm the unambiguous presence of H 2 O ice by spectral analysis.

OSIRIS data

Bright spots have been observed on 67P/C-G at various locations on the nucleus throughout the cruise phase of the Rosetta mission. First detections of these spots date back to as early as August 2014 and they continued to appear in numerous forms, be it an ensemble of a plethora of small bright spots, or much larger individual or twin bright patches. The abundance of these spots reached a peak during the perihelion passage of comet 67/P C-G in August 2015, resulting in the largest white patches ever detected on the cometary nucleus. The first objective of this study is to explore the nature of these spots in terms of spectrophotometry.

We started by selecting potential bright spots found on OSIRIS NAC data with good spectro-photometry and spatial coverage. In the event of an observational sequence of several filters, owing to the fact that both the spacecraft and cometary nucleus are constantly in motion, the recorded images do not necessarily show exactly the same field of view. Owing to the time difference between a pair of consecutive images being around ten seconds, there is a small shift in the fields of view. Hence we have to take this shift into account when stacking up images and creating a data cube for spectral analysis. To achieve this, we adopted an algorithm that automatically identifies identical features in consecutive images and estimates the affine transformation matrix between each couple of consecutive images (ORB and RANSAC tools implemented by van der [START_REF] Walt | scikit-image: Image processing in Python[END_REF]. Then we analyzed the data cubes to identify bright spots. The basic criterion to identify these spots was their high reflectance compared to the typical nucleus in the filter wavelengths observed. We note that the data used are from 3B level of the standard OSIRIS data reduction pipeline, which accounts for the correction for bias, flat field, geometric distortion, solar flux, and calibration in absolute flux (Tubiana et al. 2015). Thus the data available in the reduced files are in the form of radiance factor that corresponds to the ratio between the observed scattered radiance (I) from the comet and incoming solar irradiance (F λ ) at the heliocentric distance of the comet, being referred to as the I/F value Article number, page 2 of 13page.13 Fig. 2. NAC OSIRIS images (first column) for the eight spots reported in Table 1 with a zoom on the spot (second column). The images have been taken with F22 filter (at 649.2 nm). The arrows indicates the spots that have been analysed using boxes of 3x3 pixels. The measured I/F(α) of the bright spots are reported in red, while the surrounding area is reported in black (third column). The relative reflectance (normalized to F23 at 535 nm) of the indicated bright spot in red and the surrounding area in black are represented in the fourth column. organic-rich typical comet's nucleus material, we set the following criteria for the bright spots to qualify as final candidates : i) higher albedo properties than the typical nucleus I/F and ii) flat spectral behavior, compared to the typical nucleus on relative reflectance plots.

The above method enabled us to filter potential candidates and discard certain previously catalogued bright spots that had shown higher albedo properties, but not necessarily having flat spectral behavior when normalized, thus failing to meet the second requirement. In this case, the high albedo properties were probably due to the illumination conditions during the observation.

We selected 13 spots (or cluster of spots) reported in the map (Fig. 1) as the best sample to be analysed by spectroscopy with VIRTIS. In this paper we present only eight spots (Table 1) for which VIRTIS spectral analysis gave positive detection of H 2 O ice signatures. The OSIRIS images of the area and the zoom of the selected eight bright spots were reported in Fig. 2, together with the measured I/F and the relative spectro-photometry reflectance. As shown in Fig. 2, the spots 4 (depending on the shadow), 6, and 7 belong to a cluster.

VIRTIS data

The search of VIRTIS-M spectra in correspondence with the bright albedo features identified on OSIRIS images needed a deep data mining of the dataset. Since the entire nucleus was imaged with very high redundancy from a wide range of distances, local times and illumination/viewing geometries, the research has been performed starting from georeferenced data. For each individual pixel in each VIRTIS-M observation, many geometry parameters, including longitude, latitude, incidence, emission, phase angles, distance, and local solar time for the pixel center and four corners, were computed by means of SPICE (Acton, 1996) routines that are able to reconstruct these quantities starting from spacecraft and comet attitude and trajectory kernels. Once computed and validated by the VIRTIS team, these geometry (.GEO) cubes are released through ESA's PSA archive and made publicly available. The nucleus shape model used for

Spectral modeling

To derive the properties of the H 2 O ice detected in the bright spots on the surface of the comet, a spectral analysis was performed using Hapke's radiative transfer model (Hapke, 2012), as described in Ciarniello et al. (2011). Unfortunately we are still not able to infer the real composition of the organic-rich dark terrain present on the comet surface. The broad absorption band centered at 3.2 µm, and the difficulty of its interpretation, have been largely discussed by Quirico et al. (2016) on the basis of present knowledge of the composition of cometary grains and all the components available in laboratory data. The mixture presented in this paper was consequently modeled by means of two spectral end members: crystalline water ice, simulated by using optical constants measured at T=160 K between 1 and 4 µm (Warren et al., 1984, Mastrapa et al., 2008, Mastrapa et al., 2009, Clark et al., 2012) and a Dark Terrain unit corresponding to the average spectrum of the comet's surface after the application of photometric correction (Ciarniello et al., 2015), as shown in Fig. 3. The quantitative analysis is based on the spectral shape of the diagnostic absorption bands of H 2 O ice. The absolute level of reflectance of the model is multiplied by a free parameter to fit the data, to account for uncertainties on the radiometric and photometric accuracy as well as errors on the local geometry information, owing to unresolved shadows and roughness. In some cases, the measured spectra present a fictitious slope where a high signal contrast is measured between adjacent pixels, like regions near shadows. This is due to the increasing FWHM of the point spread function toward longer wavelength. To account for this effect, a slope is added to the model to fit the measured spectrum, where it is required. Because of its artificial origin it should not be the subject of interpretation. Before fitting, the observed spectra are corrected for spikes and instrumental artifacts. Thermal emission is modeled and removed simultaneously to the spectral fit, as in Protopapa et al. (2014). The best-fitting result is obtained by applying the Levenberg-Marquardt method for nonlinear least squares multiple regression. During the fitting procedure, the spectral bands are weighted for the Poissonian noise as calculated in [START_REF] Raponi | Spectrophotometric analysis of cometary nuclei from in situ observations[END_REF]. We have modeled areal and intimate mixing modalities: in the areal mixing, the surface is modeled as patches of pure H 2 O ice and dark terrain; in the intimate mixing model the particles of the two end-member materials are in contact with each other. The model's free-parameters are the percentage and grain size of the water ice. Further details about spectral modeling are given in Filacchione et al. (2016a) and in [START_REF] Raponi | [END_REF]. Most of the icy regions can be described by both areal and intimate mixture, which can be alternative. In the intimate mixing case, the model always requires larger abundance and grain size than the areal mixing case to compensate for the lack of the multiple scattering contribution, which is low because of the interaction of light with the dark terrain. The two alternative solutions indicated with A in Table 4 are the extremes of a possible set of solutions in which both kind of mixtures can be present simultaneously. For the spectra indicated with S in Table 4 (spots 4 and 6), we have only one possible solution in which the two kind of mixtures both contribute to the production of the model, and two populations of grain size are modeled at the same time, confirming the result of Filacchione et al. (2016a). Figure 4 shows spectra and images for each spot with positive identification of ice using VIRTIS-M data. The best fitting model (in red) for each spot is shown as the areal mixing case, or the areal-intimate case according to the parameters, as indicated in Table 4.

To highlight the effect of variation in abundance and grain size of the water ice, in Fig. 5 we show, as an example, the best fit obtained for spot 3, in the areal mixture case, varying the H 2 O ice abundance and the grain size. An additional slope has been set as a free parameter to obtain the best fit. The parameters used and the resulting goodness-of-fit of the models are shown in Table 5.

Analysis of the exposed H 2 O ice spots

We analyze the OSIRIS NAC images in the area where the spots have been identified and, in addition, we investigate all available observations to evaluate the lifetime of the H 2 O ice spots. All spots are almost in equatorial or near-equatorial locations.

Spot 1. This bright feature is located in the southern hemisphere close to the Khonsu-Imhotep boundary and the cometary equator. The bright spot appears as a freshly exposed cliff on a rough region of rocky appearance bounding an alcove. It is likely a remnant of a collapsed sector of a former pit. The first detection of this feature occurred on 5 June 2015, when it measured 57 m, whereas later on, on 27 June 2015, it measured about 36 m.

Spot 2. Located in the Anhur region in the southern hemisphere of the comet, this bright feature measures about 45 m. Oweing to the oblique observing geometry and many shadows cast in the neighboring terrain, it is difficult to give a clear depiction of the surroundings. Nevertheless the feature seems to correspond to a flat terrace at the centre of a roundish area (possibly a collapsed pit) in a region of consolidated materials. It shows a very low albedo for a bright feature, although it stands out from that of the typical nucleus. The OSIRIS NAC observations of this bright spot are recorded from 4 June 2015 up to 11 July 2015. As for spot 1, this feature is also surrounded by many shadows caused by the rugged terrain of Anhur.

Spot 3. This spot appears as a bright boulder close to a pancake-like feature (apparently composed of three broad layers), which is a morphologically unique feature right in the center of the Khonsu region (El-Maarry et al. 2016). The bright spot has a good temporal coverage in terms of OSIRIS NAC observations. It has been observed on several occasions from the end of March to the beginning of May in 2015. The observations suggest a diminution of its size from about 18 m in late March by about 8 m in April, up to complete disappearance in May. By applying illumination correction using the Lommer-Seeliger disk law, the reflectance of the bright spot increased by a factor of 2 for the sequence on 25 March, unlike other cases of bright features, with the exception of spot 8. Spot 5. This bright spot faces the rounded feature with a diameter of 500 m in Imhotep, which is interpreted by Auger et al. 2015 as a fractured accumulation basin. Similar to the bright patches of spot 4, this patch also seems to be formed by a freshly exposed area of a highly fractured material. This bright spot was observed on 22 November 2014 and measures about 10 m. It has also been noted (Pommerol et al. 2015) that this bright feature was spotted as early as 5 September 2014 by OSIRIS NAC. It has also been included in the Oklay et al. (2016a) study. On further analysis of the images, it is possible to find the same feature in some images on 23 August 2014, but with a smaller size.

Spot 6. This is a cluster of small bright spots located in the Khepry region at the base of a scarp bordering a roundish flat terrace covered by dust deposits at the Babi region margin. Again the material, where the bright patches are located, appears consolidated, brittle (El-Maarry et al. 2015), and dissected by pervasive fractures. The bright patches themselves seem to be either on freshly exposed outcropping material or on boulders. These spots have been observed many times from late August 2014 through to the end of November 2014 by OSIRIS NAC. The first observation on 26 August 2014 indicates the presence of around some 20 small bright spots with sizes ranging from 1.5 m to 3 m along with few spots of about 6 m in size. The following observations on 5 September reveal that there has not been any significant change in the bright patches in terms of their sizes and population. There were more observations on 16 September and three days later (Pommerol et al. 2015), suggesting the stability of the bright patches over time. Later on, an observation sequence on 29 October 2014 suggests that the cluster has reduced to only four bright spots, each measuring about 1 m, but it is not possible to rule out the possibility that some small bright spots may be under a shadow and hence not observable. Another sequence, on 22 November, reveals the cluster of bright spots present on observations in early September, leading to the conclusion that the observation on 29 October was subject to shadows. This cluster of bright spots seems to have been stable for at least three months.

Apart from this cluster of bright patches in late 2014, few observations dated from 25 March 2015 indicate the re-emergency of small bright patches at the same location. Despite the shadowy terrain, it is possible to discern up to three small bright patches, each about 5 m in diameter. Perhaps it could be inferred that this locality near the cliff at the Khepry/Babi border is active in terms of bright patches. Some mechanism may have triggered an outburst of icy material underneath the surface of this region allowing the cluster of bright patches to become apparent, possibly gravitational falls of boulders (Pajola et al. 2015) with consequent exposure of patches at the scarp foot.

Spot 7. This is a cluster of small bright patches that might correspond to a series of boulders at the base of a small terrace bounded by scarps. This feature appears on the OSIRIS NAC observations of 30 September 2014. It is located on a slope adjacent to the smooth plain in Imhotep, pointing to the neighboring Apis region. The location itself is somewhat shadowy and is camouflaged by the surrounding terrain, making it challenging to observe the full extension of this feature. VIRTIS-M data support the presence of H 2 O ice for this bright feature located in Imhotep region with a size of about 5 m. The corresponding positive VIRTIS-M observations date back to early September 2014, suggesting that this bright feature has been on the cometary surface even as early as the beginning of September. Therefore this feature could have a lifespan of at least one month.

Spot 8. This Imhotep-based bright feature is recorded in several epochs. The bright spot is close to the isolated accumulation basin and to the small roundish features interpreted by Auger et al. 2015 as ancient degassing conduits. It is located at the base of what appears to be an open trench surrounded by steep small scarps. Therefore the full view of this feature is somewhat hampered by the constant casting of shadows and the viewing geometry. Nevertheless the multiple observations offer partial views of the feature that seems to lie on a consolidated flat area with fractures and tiny staircase borders. For example on the image recorded on 30 September 2014, it appears to be composed of two segments with one measuring 3 m, while the other measures 6 m approximately. We note that the absolute reflectance of this bright spot increased by a factor of 3 upon applying the illumination correction using Lommer-Seeliger disk law for the obser-Article number, page 9 of 13page.13 Fig. 5. Simulated reflectance spectra are shown to highlight the effect of variation in abundance and grain size of the water ice. In both plots, the black curve is the spectrum of spot 3, and the red curve is the best fit for areal mixture case already reported in Fig. 4. The green and blue curves are simulated by varying one of these parameters, as indicated: the panel on the top shows the effect of the variation in abundance of H 2 O ice (0%, 1.3%, 2.6%) fixing the grain size at 750µm, and the panels on the bottom show the variation in grain size (75µm, 750µm, 7500µm), fixing the H 2 O ice abundance at 1.3%. The gaps in the spectral ranges are not taken into account, as in Fig. 3. vation sequence on 5 September 2014. A similar effect has only been noticed in the case of spot 3, where the factor was 2. The earliest detection of this feature dates to 25 August 2014 and the corresponding positive VIRTIS-M recordings date to mid-December 2014, suggesting a stable existence of almost four months on surface for this bright feature.

Temporal evolution of the bright spots

Although only a subset of the bright spots detected by OSIRIS can be analyzed by VIRTIS, the unambiguous detection of the spectral signatures of H 2 O ice in eight of these bright spots is a clear confirmation of their icy nature. In section 5, we detail evidence of spots with long life time. Here (Fig. 6) we add a comparison for a cluster of bright spots (spot 6) in Khepry with Table 5. Parameters used to perform the models shown in Fig. 5. The first line represents the best fit (reported in Table 4) shown in red in both panels of Fig. 5. The abundance and grain size of water ice in areal mixture are fixed as: 0% and twice the abundance retrieved for the best fit, and one tenth and ten times the grain size retrieved for the best fit. The additional slope (free parameter of the model) and the resulting goodness are indicated in the observations at an interval longer than two months, which confirms the stability of this cluster at that time of the mission.

From the VIRTIS and OSIRIS data, we can measure several parameters: estimation of the amount of water ice on each spot, its local temperature, a timescale and an extent of erosion. These were measured at different times and should consequently be considered only as first order indications. They are theoretically linked together, so that we can use them to estimate whether ice behaves as expected for each spot. In Table 6 we report the mass release rate of H 2 O (Prialnik et al. 2004), for each temperature, from the surface of 67P/C-G estimated in each of these locations as follows

Q H 2 O = P H 2 O (T ) m H 2 O 2πk B T , (4)
with m H

P H 2 O = Ae -B/T , (5)
with A=356 10 10 Nm -2 and B=6141.667 K for water (Fanale and Salvail, 1984).

For each spot, the temperature changes because of local diurnal variations of the solar input, seasonal effects, shadowing, or self-heating. These effects are difficult to estimate since they depend on the local illumination geometry and thermo-physical properties. A low thermal inertia, as derived by VIRTIS (Capaccioni et al. 2015) and MIRO [START_REF] Schloerb | [END_REF], results in Article number, page 10 of 13page.13 quite large day-night and seasonal variations of the temperature. These may influence the survival of ice-rich spots in a way that may be unpredictable. However, we find that the behavior of the eight spots found by the OSIRIS-VIRTIS study is in good agreement with the expected thermal behavior of H 2 O ice.

Although the size and timescales measured for each spot may be considered with caution owing to the errors like those induced by shadows, they allow for another estimate of the mass release rate (Prialnik et al. 2004) through Q H 2 O = ∆l/∆t, with the local density of water ice, ∆l the typical extent of erosion of each spot, and ∆t its erosion timescale. If we take the example of spot 1, the expected timescale to erode this feature at the observed extent and for a temperature of 203 K would be 50-100 hr. We emphasize that given the numerous uncertainties this should be considered as a first order approximation. However, this timescale is such that the feature should be stable against day-night variations of the temperature, since it would take more than one cycle to erode it. In addition, the lower temperatures of the night would prevent such a rapid erosion. The total mass of water released by the erosion of spot 1 is M = Q H 2 O ∆t f ∆S with ∆S the eroded surface and f the water ice fraction inferred from VIRTIS data modeling. The feature containing 1% of water ice seems to have decreased from 57 m to 36 m in about 22 days, i.e. ∼1500m 2 were eroded in ∼2×10 6 s. At 203 K, this translates into ∼10 4 kg of water ice being sublimated, which is not possible to reach with surface ice alone. We thus have to assume that 1) icy grains may be scattered to the nucleus surface and recondence, creating a cycle maintaining water ice at the surface (Crifo, 1987;Davidsson and Skorov, 2004), 2) the average temperature is much lower than the temperature measured by VIR-TIS, 3) ice-rich subsurface layers contribute to maintaining the surface ice. Case 1 is beyond the scope of the simple calculations we perform here. For case 2, we estimate that surface ice (contained in a 1mm layer) would be able to reproduce the observed behavior if the average temperature is ∼175 K: large day-night variations of the temperature would thus be necessary. For case 3, if we assume that subsurface layers have the same properties as the bulk of the comet, a layer of the order of ∼10 cm would be required to explain the observed mass release. We note that in 67P/C-G, the diurnal and seasonal skin depths ( , with a the semi major axis, c the specific heat, G the gravitational constant, κ the thermal conductivity, M the solar mass, P spin the spin period, and ρ the bulk density; Prialnik et al. 2004) vary from 1 mm to 9 cm and 80 cm to 7 m respectively, depending on local thermo-physical properties, as computed by Leyrat et al. (2015).

Spot 2 is larger than spot 1, but contains an order of magnitude less H 2 O ice as inferred from spectral modeling. This feature's evolution is found to be in good agreement with the expected thermal behavior of water ice, as well as the inference from its low albedo that this spot is close to the end of the sublimation phase.

Spot 3 is the hottest feature measured in this study. If we assume that the ice-rich boulder has the same properties as the bulk of 67P/C-G, the 18 m feature being eroded in ∼3×10 6 s results in a mass release rate of 2.82×10 -5 kg m -2 s -1 , i.e. two orders of magnitude less than the rate computed from the measured temperature. We should thus assume that the temperature is much lower most of the time at this spot, or the boulder properties vary from those of the bulk of the comet (a local density of 800 kg m -3 for example would be required). Alternatively, the very high temperature observed for spot 3 is likely the result of an areal mixture of icy and ice-free terrain within the VIRTIS pixel. Indeed, for a given observed albedo and temperature, the mixing of ice and dust at a grain level plays an important role: ice intimately mixed with dust will be hotter and shorter-lived than a patch of pure ice surrounded by dust. Given the low temperatures encountered for spots 4 to 8, it is expected that these features should be long-lived, as observed by OSIRIS and VIR-TIS. At a lower temperature of 180K (spot 6), the sublimation rate is only 0.625×10 -5 kg m -2 s -1 , and decreases exponentially at lower temperatures (spots 4, 7, and 8).

The appearance of meter-sized spots, which are mostly only illuminated for a short fraction of the day, remains constant over time. These features would be more affected by seasonal variations than diurnal variations of the temperature, since water ice is mostly stable at the measured temperatures. Cometary activity, triggered below the surface by other volatile species, may locally influence the surface properties, such as the distribution of dust, and expose fresh H 2 O ice at the surface.

All the bright spots are on consolidated dust free materials, either on boulders or on freshly exposed outcropping regions that often display penetrative fractures. This suggests that H 2 O ice can mainly be found on the consolidated substratum exposed along scarps or detached in the form of boulders. Some of the bright spots have been in place for weeks and months, while others seem related to diurnal variation.

Conclusions

Comet 67P/Churyumov-Gerasimenko shows a surface rich in heterogeneous geological structures and surface morphological variations that show color and albedo variations across the surface. The high-resolution images obtained by OSIRIS enable us to identify a large quantity of bright spots of different size and located in areas with different properties and high albedo.

In this paper, we present for the first time a complementary study of data acquired by the OSIRIS and VIRTIS instruments. A major objective of this paper is to firmly detect the presence of H 2 O ice on the comet's surface. We confirm the presence of H 2 O ice on eight new spots and we model the spectra with H 2 O ice and dark material.

Comparing the coordinates of the detected eight H 2 O ice spots with those of 67P/C-G dust jets, five spots (4, 5, 6, 7 and 8) have been found to lie in the same approximate position of the jets identified by Vincent et al. (2016a) and one (spot 3) among the outbursts observed in the cometary summer (Vincent et al., 2016b). Observational evidence showed that the majority of dust jets also arose from rough terrains and fractured walls rather than smooth areas (Vincent et al., 2016a). Some of these detected H 2 O ice spots have also been compared by Oklay et al. (2016b) to those of comets 9P and 103P.

The detection of H 2 O ice signatures by VIRTIS on eight of the 13 locations given by OSIRIS data does not mean that the other spots do not contain ice on their surface and this can be explained by not simultaneous observations, unfavorable instrumental signal-to-noise conditions, spatial resolution on the surface, different illumination/viewing geometry, and by the fact that VIRTIS-M channel was unavailable after 4 May, 2015 owing to the failure of the active cooler.

The main results of this work can be summarized as follows:

-We presented for the first time a complementary analysis of H 2 O ice is present on the surface substratum where solar illumination plays an important role with seasonal and diurnal variations. During the perihelion orbit passage of the comet, the Rosetta spacecraft was at a greater distance and the available surface OSIRIS images were at lower resolution. Starting in March 2016, the comet is observed again from close distances. With analysis of other available data (in particular from OSIRIS), we will study the surface changes after the perihelion passage to better understand the surface evolution of the comet.

FIGURE 1 :

 1 FIGURE 1: Average monthly walking distances from March 2017 to September 2018, reflecting the daily dose of walking involved with the commute to and fro between home in Paris 18 and LESIA in Meudon.

  Figure credit: De-Meo, 2010. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 A: The mass distribution of asteroids across the main belt up to Jupiter Trojan asteroids, classified as per the corresponding taxonomic classes. Represented in grey background is the total mass embedded within 0.02-AU bins. Each colour refers to an asteroid taxonomic class referred to in the legend to the right. The horizontal line at the 10 18 kg indicates the threshold values achieved hitherto 1980s. The modern improvements remain consistent to the latter and additional details are revealed at lower masses. B: The compositional mass distribution of asteroids as a function of their size across the main belt up to the Jupiter Trojan asteroids. The derivation of mass is explained in DeMeo and Carry, 2014. The distribution is divided into 7 groups to be consistent with A. The total mass of each group at each size is labelled and fractional mass contribution of each taxonomical class is represented by a pie chart. It is noted that the total masses of Hildas and Jupiter Trojans remain as a lower limit due to the unavailability of data at smaller sizes. Credit: DeMeo and Carry, 2014. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviii 1.4

7 1 . 5

 15 The figure depicts the Grand Tack model, explaining the early radial mixing of the asteroid belt through the inward and outward migration of Jupiter. Credit: Walsh et al., 2011. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.

  Figure excerpted from Jewitt, 2012. . . . . . . . . . . . . . . . . . . . . . . . . 18 1.9 Different D/H ratios in different objects of the solar system, excerpted from Altwegg et al., 2015. f is the enrichment factor, defined as the ratio [D/H] object /[D/H] PSN . In the PSN the value [D/H] PSN is equal to 2 x 10 -5 . . . . . . . . . . . . . . . . 21 1.10 A spectrum of 1P/Halley obtained by the IKS spectrometer aboard Vega-a spacecraft. Figure credit : Combes et al., 1988 . . . . . . . . . . . . . . . . . . . 23 1.11 Halley armada with their respective interception distances to the comet 1P/Halley. 24 1.12 Recovering the final location of the Philae lander using an OSIRIS image on 02 September 2016, during the last month of the extended Rosetta mission. Philae rests in on an oblique plane in a crevice of the small lobe of the comet. The definitive location provided scientists to better constrain the results derived from the data returned by Philae. Image credit : ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; ESA / Rosetta / NavCam . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 1.13 Presence of active pits in the Seth region, detected by OSIRIS NAC on 28 August 2014. The right panel with enhanced contrast identifies the fine structures in the shadow of the pit, interpreted as fine jets emanating from the pit, attributed to the activity of the comet. Figure credit: Sierks et al., 2015. . . . . 29 1.14 The variation of spacecraft-comet distance (black curve) and the heliocentric distance of the comet (blue curve) from August 2014 to September 2016. The red vertical line corresponds to the perihelion passage of the comet that took place on 13 August 2015. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 xix 1.15 The variation of observational phase angle (blue dotted curve) and the spacecraftcomet distance (black dotted curve) from August 2014 to September 2016. The red vertical line corresponds to the perihelion passage of the comet that took place on 13 August 2015, whereas the blue horizontal line corresponds to the 90 o phase angle, corresponding to terminator orbits of Rosetta around the comet 67P, which refer to orbits in the terminator plane, which is defined by the hypothetical line dividing the day-lit side and the dark night side of the comet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  Figure credit: El-Maarry et al., 2016. . . . . . . . . . . . . . . . . . . . . . 1.19 This image of the Rosetta spacecraft depicts its suite of scientific instruments and the Philae lander. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.20 The Philae lander and its scientific instruments. . . . . . . . . . . . . . . . . . 2.1 Top panel: Left image shows the NAC prior to the integration into the payload and the image on right shows WAC, ready to be integrated to the payload. Bottom panel: On the left is the off-axis optical system of NAC with its three mirrors, while that of WAC with two mirrors is displayed on the right. Anti-radiation plate is abbreviated as ARP in the ray-tracing schema and the figure is adapted from Keller et al., 2007. . . . . . . . . . . . . . . . . . . . . . . 2.2 Top panel: Transmission curves for OSIRIS NAC filters. Bottom panel: Transmission curves for OSIRIS WAC filters. Figure credit : Tubiana et al., 2015. . . 2.3 Top panel: the transmission curves of the NAC focus plates, the anti-reflection coated plates, together with the total reflectivity of the NAC mirror system. Bottom panel: the quantum efficiency of the NAC CCD as measured at laboratory on the flight model at room temperature (black solid line) and close to operational temperature (red dot-dashed line). Figure credit : Tubiana et al., 2015. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 The continuous gray line corresponds to the ground-based visible to near-IR spectrum of the comet recorded in 2007

  Figure credit : Fornasier et al., 2015 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xx 2.

  Figure credit: Rousseau et al., 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Images and spectrophotometric plots of eight exposed bright features detected by VIRTIS-M to contain H 2 O ice.

H 2 O

 2 abundance is fixed at 1.3%. . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Left: Location of the Khonsu region on the bi-lobate nucleus with other neighbouring regions (El-Maarry et al., 2016), right: a zoomed-in view on the Khonsu region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Variation of average spectral slope values with the phase angle. Spectral slopes are evaluated in the 535-882 nm range are in %/(100 nm). The data points that show a larger scatter from the regression line are due to the rotational phase of the comet, characterised by the local variations of reflectance in the observed disk. Figure excerpted from Fornasier et al., 2015. . . . . . . . 3.9 Contextual images and spectral slope maps, used for the spectral slope analysis. xxii 3.10 Spectrophotometric plots and the selected four different surface terrains in the Khonsu region for the data cube of 02 May 2015. The boxes in the top image have dimensions of 7x7 pixels, while the plots are derived from data cubes that are co-registered and corrected for the illumination conditions, using the Lommel-Seeliger disk function. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Spectrophotometric plots and the selected four different surface terrains in the Khonsu region for the data cube of 10 February 2016. The boxes in the top image have dimensions of 18x18 pixels, while the plots are derived from data cubes that are co-registered and corrected for the illumination conditions, using the Lommel-Seeliger disk function. . . . . . . . . . . . . . . . . . . . . . . . 3.

  Figure adapted from Deshapriya et al., 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 3.22 Map of exposed bright features observed on the comet 67P: The detections of H 2 O ice are presented in red, green, blue and yellow colours which correspond to the feature types 1-4 (Table

  respectively for the northern hemisphere and the southern hemisphere. . . . . . . . . . . . . . 112 3.23 Evolution of three isolated bright patches on smooth terrains located in Bes, Imhotep and Hapi regions. These belong to the Type 1 of the exposed bright features. Each panel shows the morphological evolution of a selected feature as the comet approached the perihelion. (Perihelion passage took place on the 13 August 2015.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 3.24 Examples for the Type-2 and Type-3 exposed bright patches. Top panel shows the Type-2 : Isolated patches close to irregular structures whereas the bottom panel shows the Type-3 : bright patches resting on boulders. . . . . . . . . . . 117 3.25 Activity sources in the Khonsu region which included a strong outburst that modified the local terrain and later resulted the appearance of an exposed bright patch resting on a boulder. Panel a shows the area of interest in the Khonsu region about 3 months prior to the perihelion passage. Panel b corre-

  Figure credit: Deshapriya et al., 2018. . . . . . . . . . . . . 3.27 Evolution of a cluster of exposed bright features in Khepry region, which indicates the stability of the features. The observations indicate a stability of about 3 months from the end of August to end of November 2014. 5 individual patches are numbered in the images so as to follow their evolution over the given time scale. The bottom right panel indicates that all the small patches on the cluster have already sublimated away by the end of March 2015, as comet was getting closer to the perihelion.

FIGURE 1 . 1 :

 11 FIGURE 1.1: Radial and vertical mixing of material in the protoplanetary disk due to the conversation of angular momentum. Localised convective cells are also identifiable above and below the accretion disk.Figure credit: Nuth and Johnson, 2012.

  FIGURE 1.1: Radial and vertical mixing of material in the protoplanetary disk due to the conversation of angular momentum. Localised convective cells are also identifiable above and below the accretion disk.Figure credit: Nuth and Johnson, 2012.

  FIGURE 1.2: The compositional distribution of solar system bodies is succinctly recapitulated in the above figure which is structured in several horizontal layers, with the exception of comets that span much larger heliocentric orbits than the range depicted therein. The colouring trend from brown to blue with the heliocentric distance represents the transition of composition dominated by silicates to volatile ices. The populations of small bodies are represented as a function of their semi-major axes versus their inclinations to the ecliptic plane with a colour code for the population density. The approximate distribution of taxonomic classes for the main belt asteroids are given as well as some sample spectra for different small body populations. The horizontal gray bar refers to the average albedo as a function of heliocentric distance and the bar above it corresponds to the effective black-body temperature as a function of the heliocentric distance, calculated for an albedo of 0.1. Figure credit: DeMeo, 2010.

  FIGURE 1.3: A: The mass distribution of asteroids across the main belt up to Jupiter Trojan asteroids, classified as per the corresponding taxonomic classes. Represented in grey background is the total mass embedded within 0.02-AU bins. Each colour refers to an asteroid taxonomic class referred to in the legend to the right. The horizontal line at the 10 18 kg indicates the threshold values achieved hitherto 1980s. The modern improvements remain consistent to the latter and additional details are revealed at lower masses. B: The compositional mass distribution of asteroids as a function of their size across the main belt up to the Jupiter Trojan asteroids. The derivation of mass is explained in DeMeo and Carry, 2014. The distribution is divided into 7 groups to be consistent with A. The total mass of each group at each size is labelled and fractional mass contribution of each taxonomical class is represented by a pie chart. It is noted that the total masses of Hildas and Jupiter Trojans remain as a lower limit due to the unavailability of data at smaller sizes. Credit: DeMeo and Carry, 2014.

FIGURE 1 . 4 :

 14 FIGURE 1.4: The figure succinctly depicts major aspects of the dynamical history of the small bodies in the solar system according to the Grand Tack model[START_REF] Walsh | A low mass for Mars from Jupiter's early gas-driven migration[END_REF] and the Nice model[START_REF] Morbidelli | Chaotic capture of Jupiter's Trojan asteroids in the early Solar System[END_REF][START_REF] Gomes | Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets[END_REF][START_REF] Tsiganis | Origin of the orbital architecture of the giant planets of the Solar System[END_REF]. Although these models may not simulate the exact evolution of the solar system, they appear to be viable hypotheses. They connect the epochs of radial mixing, mass removal and planetary migration, leading to the present image of the solar system with its planets and populations of small bodies. Credit: DeMeo and Carry, 2014.

FIGURE 1 . 5 :

 15 FIGURE 1.5: The figure depicts the Grand Tack model, explaining the early radial mixing of the asteroid belt through the inward and outward migration of Jupiter. Credit: Walsh et al., 2011.

FIGURE 1 . 6 :

 16 FIGURE 1.6: The above figure depicts the relationship between semi-major axis and eccentricity for some of the small bodies located in the outer solar system. Neptune's orbit is marked as a vertical dotted line. The blue populations are in mean motion resonance with Neptune, which results in their stability over the age of the solar system. The most significant blue population is located around 40 AU of semi-major axis with 3:2 mean motion resonance with Neptune where Pluto and many plutinos belong. Figure credit : David Jewitt.

FIGURE 1 . 7 :

 17 FIGURE 1.7: The observational, dynamical and composition criteria on which the definition of a comet is based. Jewitt, Morbidelli, and Rauer, 2007

FIGURE 1 . 8 :

 18 FIGURE 1.8: The population of active asteroids / MBCs (black dots) that display various mass-losing mechanisms compared to main belt asteroids (orange dots) and short-period comets/ SPCs (blue dots). The dotted lines correspond to the semi-major axis of Mars, orbit related to the 2:1 mean motion resonance with Jupiter and the semi-major axis of Jupiter, when viewed from left to right. Figure excerpted from Jewitt, 2012.

FIGURE 1 . 9 :

 19 FIGURE 1.9: Different D/H ratios in different objects of the solar system, excerpted from Altwegg et al., 2015. f is the enrichment factor, defined as the ratio [D/H] object /[D/H] PSN . In the PSN the value [D/H] PSN is equal to 2 x 10 -5 .

FIGURE 1 .

 1 FIGURE 1.10: A spectrum of 1P/Halley obtained by the IKS spectrometer aboard Vega-a spacecraft. Figure credit : Combes et al., 1988

FIGURE 1 . 11 :

 111 FIGURE 1.11: Halley armada with their respective interception distances to the comet 1P/Halley.

FIGURE 1 . 12 :

 112 FIGURE 1.12: Recovering the final location of the Philae lander using an OSIRIS image on 02 September 2016, during the last month of the extended Rosetta mission. Philae rests in on an oblique plane in a crevice of the small lobe of the comet. The definitive location provided scientists to better constrain the results derived from the data returned by Philae. Image credit : ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; ESA / Rosetta / NavCam

FIGURE 1 . 13 :

 113 FIGURE 1.13: Presence of active pits in the Seth region, detected by OSIRIS NAC on 28 August 2014. The right panel with enhanced contrast identifies the fine structures in the shadow of the pit, interpreted as fine jets emanating from the pit, attributed to the activity of the comet.Figure credit: Sierks et al., 2015.

  FIGURE 1.13: Presence of active pits in the Seth region, detected by OSIRIS NAC on 28 August 2014. The right panel with enhanced contrast identifies the fine structures in the shadow of the pit, interpreted as fine jets emanating from the pit, attributed to the activity of the comet.Figure credit: Sierks et al., 2015.

Chapter 1 .FIGURE 1 . 14 :

 1114 FIGURE 1.14: The variation of spacecraft-comet distance (black curve) and the heliocentric distance of the comet (blue curve) from August 2014 to September 2016. The red vertical line corresponds to the perihelion passage of the comet that took place on 13 August 2015.

FIGURE 1 . 15 :FIGURE 1 . 16 :

 115116 FIGURE 1.15: The variation of observational phase angle (blue dotted curve) and the spacecraft-comet distance (black dotted curve) from August 2014 to September 2016. The red vertical line corresponds to the perihelion passage of the comet that took place on 13 August 2015, whereas the blue horizontal line corresponds to the 90 o phase angle, corresponding to terminator orbits of Rosetta around the comet 67P, which refer to orbits in the terminator plane, which is defined by the hypothetical line dividing the day-lit side and the dark night side of the comet.

FIGURE 1 . 17 :

 117 FIGURE 1.17: Left column gives different perspectives of the nucleus of the comet 67P with different orientations. Rotational axis of the comet is also indicated for better interpretation of the images. Left panel shows the same images with an overlaid texture to identify the regional boundaries, which correspond mainly to those located in northern latitudes. Figure credit: El-Maarry et al., 2015.

FIGURE 1 . 18 :

 118 FIGURE 1.18: The regions located at the southern latitudes of the comet along with their nomenclature. Left column offers different views on the regions located in southern latitudes, while the right column offers views similar as those on horizontal panels along with an overlaid texture to identify the regional boundaries. Figure credit: El-Maarry et al., 2016.

FIGURE 1 . 19 :

 119 FIGURE 1.19: This image of the Rosetta spacecraft depicts its suite of scientific instruments and the Philae lander.

  CONSERT : Comet Nucleus Sounding Experiment by Radiowave Transmission investigates the interior of the comet by means of reflection and scattering of radio waves Chapter 1. Introduction which are being sent to and fro between the two components of the experiments located in the spacecraft and the Philae lander.

FIGURE 1 .

 1 FIGURE 1.20: The Philae lander and its scientific instruments.

FIGURE 2 . 1 :

 21 FIGURE 2.1: Top panel: Left image shows the NAC prior to the integration into the payload and the image on right shows WAC, ready to be integrated to the payload. Bottom panel: On the left is the off-axis optical system of NAC with its three mirrors, while that of WAC with two mirrors is displayed on the right. Anti-radiation plate is abbreviated as ARP in the ray-tracing schema and the figure is adapted fromKeller et al., 2007.

FIGURE 2 . 2 :

 22 FIGURE 2.2: Top panel: Transmission curves for OSIRIS NAC filters. Bottom panel: Transmission curves for OSIRIS WAC filters. Figure credit : Tubiana et al., 2015.

FIGURE 2 . 3 :

 23 FIGURE 2.3: Top panel: the transmission curves of the NAC focus plates, the anti-reflection coated plates, together with the total reflectivity of the NAC mirror system. Bottom panel: the quantum efficiency of the NAC CCD as measured at laboratory on the flight model at room temperature (black solid line) and close to operational temperature (red dot-dashed line).Figure credit : Tubiana et al., 2015.

  FIGURE 2.3: Top panel: the transmission curves of the NAC focus plates, the anti-reflection coated plates, together with the total reflectivity of the NAC mirror system. Bottom panel: the quantum efficiency of the NAC CCD as measured at laboratory on the flight model at room temperature (black solid line) and close to operational temperature (red dot-dashed line).Figure credit : Tubiana et al., 2015.

FIGURE 2 . 4 :

 24 FIGURE 2.4: The continuous gray line corresponds to the ground-based visible to near-IR spectrum of the comet recorded in 2007 (Tubiana et al., 2011) and the NAC and WAC spectrophotometric data of the comet recorded on 28 July 2014 between 23:25-23:45 UT(Fornasier et al., 2015). It is noted that the comet was inactive during the acquisition of its spectrum in 2007, while the same is not true for the OSIRIS spectrophotometric data in question.

FIGURE 2 . 5 :

 25 FIGURE 2.5: Phase curve of the nucleus of the comet 67P from NAC observations (F23). The continuous line refers to the IAU HG model that best fits the data whereas the dashed-line corresponds to the linear fit to the data.Figure credit : Fornasier et al., 2015

FIGURE 2 . 8 :

 28 FIGURE 2.8: Stucture of an OSIRIS data cube produced using images acquired with four filters. For each spatial location of the image, it is possible to extract the intensity / reflectance for each wavelength of the filters and perform a spectrophotometric analysis.

FIGURE 2 . 9 :

 29 FIGURE 2.9: Orthographic renderings of the shape model SPG SHAP7 (along the primary axes of the body-fixed 67P/CG_CK coordinate frame). Indicated axes point towards the viewer, north is up for the +/-XY views, +Y is up for the +/-Z views. Figure credit: Preusker et al., 2017.

FIGURE 2 .

 2 FIGURE 2.10: Spectrophotometric analysis of 3 selected regions of interest in the image sequence dated 2015-05-02 15:09 UTC. Top panel: To the left is a full frame NAC image (filter F22, centred at 649.2 nm) of a sequence taken with 11 filters. The areas of interest are marked in distinct colours and to the right are their zoomed views, where squares are used to illustrate the 5x5 sampling of pixels used for the spectrophotometric analysis. Bottom panel: Spectrophotometric plots of the 3 regions of interest.

FIGURE 2 . 11 :

 211 FIGURE 2.11: Spectrum of H 2 O frost spanning from UV wavelengths to IR wavelengths. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis.Figure adapted from Hapke et al., 1981.

  FIGURE 2.11: Spectrum of H 2 O frost spanning from UV wavelengths to IR wavelengths. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis.Figure adapted from Hapke et al., 1981.

2. 8 .

 8 Data analysis techniques 67 identified a feature on the nucleus that corresponded to SO 2 .

FIGURE 2 .

 2 FIGURE 2.12: Characteristic spectra of (1)H 2 O , (2)CO 2 , (3)NH 3 and (4)SO 2 frosts. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis.Figure adapted from Wagner, Hapke, and Wells, 1987.

  FIGURE 2.12: Characteristic spectra of (1)H 2 O , (2)CO 2 , (3)NH 3 and (4)SO 2 frosts. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis.Figure adapted from Wagner, Hapke, and Wells, 1987.

FIGURE 2 . 13 :

 213 FIGURE 2.13: Contextual images and spectral slope maps, derived from OSIRIS NAC data in August 2014.

FIGURE 2 . 14 :

 214 FIGURE 2.14: Contextual images and spectral slope maps, derived from OSIRIS NAC data in June and July 2015.

FIGURE 2 . 3 Results 3 . 1

 2331 FIGURE 2.16: RGB images created using the same data cubes used to create the spectral slope maps in the previous section. OSIRIS NAC filters centred at 882.1 nm (F41), 649.2 nm (F22) and 480.7 nm (F24) as input channels respectively for red, green and blue colours have been used in their creation.

FIGURE 3 . 1 :

 31 FIGURE 3.1: Individual optical designs of VIRTIS-M and VIRTIS-H along with an image showing both of them being assembled together prior to the integration to the Rosetta spacecraft. Adapted from Coradini et al., 2007.

FIGURE 3 . 2 :

 32 FIGURE 3.2: Exposed bright features on a map of the comet 67P created using two shape models for northern and southern hemispheres respectively fromPreusker et al., 2015 and Jorda et al., 2016. All the 13 candidate features initially detected using OSIRIS as well as two other features (F) referred to as bright albedo patches (BAP) and studied byFilacchione et al., 2016a are populated in red across the map. The positive VIRTIS-M detections where the presence of H 2 O was confirmed, bear their respective numbers(1)(2)(3)(4)(5)(6)(7)(8). The figure is adapted fromBarucci et al., 2016

FIGURE 3 . 3 :

 33 FIGURE 3.3: Average spectrum of the surface of the comet 67P derived from VIRTIS data. The missing data from 0.8 -1.0 µm is due to a calibration artefact.Figure credit: Rousseau et al., 2018

  FIGURE 3.3: Average spectrum of the surface of the comet 67P derived from VIRTIS data. The missing data from 0.8 -1.0 µm is due to a calibration artefact.Figure credit: Rousseau et al., 2018

FIGURE 3 . 4 :

 34 FIGURE 3.4: Images and spectrophotometric plots of eight exposed bright features detected by VIRTIS-M to contain H 2 O ice. The first column refers to a contextual NAC full frame image (F22 filter centred at 649.2 nm) and the second columns gives a zoom on the feature in question, where an arrow points to the location of the feature. The reflectance (I/F) plots, photometrically corrected using the Lommel-Seeliger disk function are given in the third column and in the forth column are their relative reflectance plots, normalised at the F23 filter, centred at 535 nm. The red curves correspond to the bright feature, whereas the black curves represent the behaviour of the typical nucleus, as sampled close to the feature. All the samplings have been done 3x3 pixels centred at the feature. The detailed OSIRIS observational circumstances for each feature are given in theTable 3.2.

Fig. 3 .

 3 Fig. 3.5 shows spectra from 1-4 µm and images for each positive VIRTIS-M detection of H 2 O ice. The best fitting model for each spot is given as per the mixing parameters specified in the Table.3.4. Fig. 3.6 illustrates the variation of modelled spectrum as a function of abundance and grain size of H 2 O ice for the feature #3 in the areal mixing modality.

  Fig. 3.5 shows spectra from 1-4 µm and images for each positive VIRTIS-M detection of H 2 O ice. The best fitting model for each spot is given as per the mixing parameters specified in the Table.3.4. Fig. 3.6 illustrates the variation of modelled spectrum as a function of abundance and grain size of H 2 O ice for the feature #3 in the areal mixing modality.

FIGURE 3 . 5 :FIGURE 3 . 6 :

 3536 FIGURE 3.5: The infrared spectra of VIRTIS-M for the eight positive features. The left column represents the features from 1-4 and the right column represents the features from 5-8. Given are also the false colour RGB images created using the reflectances at 1.32 µm (channel B), 2.0 µm (channel G) and 4.0 µm (R). The reflectances for each feature are plotted in black, while the best fits For each spot, we report the VIRTIS-M observed reflectance (black curve), not corrected for phase angle, and their respective best fits are plotted in red. The missing data in the spectral ranges correspond to order-sorting filter-junction wavelengths that can produce unreliable features and, hence, have been dis-

FIGURE 3 . 7 :

 37 FIGURE 3.7: Left: Location of the Khonsu region on the bi-lobate nucleus with other neighbouring regions (El-Maarry et al., 2016), right: a zoomed-in view on the Khonsu region.

FIGURE 3 . 8 :

 38 FIGURE 3.8: Variation of average spectral slope values with the phase angle. Spectral slopes are evaluated in the 535-882 nm range are in %/(100 nm). The data points that show a larger scatter from the regression line are due to the rotational phase of the comet, characterised by the local variations of reflectance in the observed disk. Figure excerpted from Fornasier et al., 2015.

FIGURE 3 . 9 :

 39 FIGURE 3.9: Contextual images and spectral slope maps, used for the spectral slope analysis.

FIGURE 3 . 10 :

 310 FIGURE 3.10: Spectrophotometric plots and the selected four different surface terrains in the Khonsu region for the data cube of 02 May 2015. The boxes in the top image have dimensions of 7x7 pixels, while the plots are derived from data cubes that are co-registered and corrected for the illumination conditions, using the Lommel-Seeliger disk function.

FIGURE 3 . 11 :

 311 FIGURE 3.11: Spectrophotometric plots and the selected four different surface terrains in the Khonsu region for the data cube of 10 February 2016. The boxes in the top image have dimensions of 18x18 pixels, while the plots are derived from data cubes that are co-registered and corrected for the illumination conditions, using the Lommel-Seeliger disk function.

FIGURE 3 . 12 :

 312 FIGURE 3.12: Variation of the spectral slopes for both pre and post-perihelion periods. The sampling has been carried out in such a way that always same surface area is covered. The individual images have been co-registered to prepare the data cubes and are corrected for the illumination conditions, using the Lommel-Seeliger disk function, prior to computing the spectral slopes.

TABLE 3 . 7 :

 37 Comparison of the phase-reddening effect for individual terrains of the analysis for both pre-perihelion and post-perihelion periods. In bold face are mean values when all the three regions showing positive phasereddening effect are taken as a whole. (c) smooth terrain 2 -0.0603±0.0268 0.0585±0.0220Having explored the spectral slopes and phase-reddening effect in the Khonsu region

FIGURE 3 . 13 :

 313 FIGURE 3.13: Evolution of the spectral slope with the heliocentric distance and the phase angle. Spectral slope values are evaluated in the 535-882 nm range, averaged over one rotation, for observations acquired on August 2014 (red squares), on April-August 2015 (black circles) and on January-February 2016 (magenta stars). Excerpt courtesy of Fornasier et al., 2016.

FIGURE 3 . 14 :

 314 FIGURE 3.14: Temporal evolution of the bright feature K1 in the Khonsu region. An OSIRIS NAC context image along and a zoomed sub-frame image are given for each NAC observation listed in Table3.9 for this feature.

FIGURE 3 . 15 :FIGURE 3 . 16 :

 315316 FIGURE 3.15: Spectrophotometric plots for the bright feature K1, where its reflectance is represented in red, while that of the typical nucleus sampled from nearby terrain is represented in blue. For A, C and D sampling has been done by 2x2 pixels whereas for B, the sampling was 3x3 pixels.

  3.19 and it shows the flat nature of the curve, characteristic of H 2 O ice. Its reflectance appears to diminish in near-UV wavelengths, which might indicate the presence of a super-volatile component like SO 2 ice which has an absorption band at ∼ 290 nm. This could correlate well with the fact that K3 has rapidly sublimated due to the presence of a super-volatile at the observed heliocentric distance of ∼2.33 AU.• Bright feature K4 First observed on 23 March 2016 by OSIRIS NAC, K4 represents a cluster of features that have been observed up to 16 July 2016, indicating a life time of about four months. This is located in the smooth terrains of Khonsu regions found beside the 'pancake' feature. The morphology of the feature resembles a cluster of bright patches that are scattered around what appears to be an exhumed area in these smooth terrains. This kind of feature corresponds well to the other clusters of bright patches that have been observed in Khepry and Imhotep regions in terms of morphology and sizes. The spectrophotometric plots and contextual images of this feature are presented in Fig.3.20, which implies the presence of H 2 O ice given flat nature of spectrophotometric curve of this feature.

FIGURE 3 . 17 :

 317 FIGURE 3.17: Spectrophotometric plots for K2 (red data points) multi-filter observation on 23 January 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 2x2 pixels.

FIGURE 3 . 18 :

 318 FIGURE 3.18: Spectrophotometric plots for K2 (red data points) multi-filter observation on 27 January 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. Since several pixels towards the centre of the feature K2 were saturated, the sampling was done towards its edges. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 2x2 pixels.

FIGURE 3 . 19 :

 319 FIGURE 3.19: Spectrophotometric plots for K3 (red data points) multi-filter observation on 10 February 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 1x2 pixels.

FIGURE 3 . 20 :

 320 FIGURE 3.20: Spectrophotometric plots for K4 (red data points) multi-filter observation on 16 July 2016. OSIRIS NAC full frame images for the context as well as zoomed images to capture the details of the sampling areas are given too. The nucleus spectrophotometry (blue data points) has been sampled close near to the feature and all the samplings correspond to an area of 1x2 pixels.

  FIGURE 3.21: Top left -A NAC mono-chromatic full frame OSIRIS NAC image (F22 filter centred at 649.2 nm), revealing a part of the Khonsu region on the comet where volatile-rich (bluish material on RGB images) and exposed bright patches are present in the form of a cluster, right -an RGB image created using the NAC filters centred at wavelengths 882.1 nm, 649.2 nm and 480.7 nm for red, green and blue colours. This image also depicts 3 regions of interest chosen for the spectrophotometric analysis. Bottom left -spectrophotometric plots of the aforementioned regions of interest, right -a zoom onto the 3 regions of interest on the RGB image. The red box refers to an exposed bright patch(Deshapriya et al., 2016) containing H 2 O ice, whose reflectance has been sampled (1x2 pixels per a filter) with scrutiny in order to avoid saturated pixels and blue box corresponds to volatiles mixed with desiccated refractory material from the nucleus (2x2 pixels) , whereas black box (2x2 pixels) represents the typical nucleus. Note that the dimensions of the boxes are deliberately exaggerated in order to point out their respective locations on the terrain. The red and green features that sometimes appear adjacent to the shadows and near the frame edges in the RGB images are artefacts arising from the imperfect co-registration/receding shadows of the images. Figure adapted from Deshapriya et al., 2018

FIGURE 3 . 22 :

 322 FIGURE 3.22: Map of exposed bright features observed on the comet 67P: The detections of H 2 O ice are presented in red, green, blue and yellow colours which correspond to the feature types 1-4 (Table3.10) respectively and the black dot indicates the unique detection of CO 2 ice(Filacchione et al., 2016b) where H 2 O ice was detected few weeks later(Fornasier et al., 2016). This map is the cylindrical projection of a merged shape model resulting from the shape models SHAP4S(Preusker et al., 2015 and SHAP5 (Jorda et al., 2016) respectively for the northern hemisphere and the southern hemisphere.

FIGURE 3 . 23 :FIGURE 3 . 24 : 17 FIGURE 3 . 25 :

 32332417325 FIGURE 3.23: Evolution of three isolated bright patches on smooth terrains located in Bes, Imhotep and Hapi regions. These belong to the Type 1 of the exposed bright features. Each panel shows the morphological evolution of a selected feature as the comet approached the perihelion. (Perihelion passage took place on the 13 August 2015.)

  FIGURE 3.25: Activity sources in the Khonsu region which included a strong outburst that modified the local terrain and later resulted the appearance of an exposed bright patch resting on a boulder. Panel a shows the area of interest in the Khonsu region about 3 months prior to the perihelion passage. Panel b corresponds to about 3 weeks before the perihelion passage and there is a couple of bright patches that have already appeared on the smooth terrain probably due to the erosion that had been escalating at this time. The next panels from c through g illustrate the appearance of a family of transient outbursts and a long-lived outburst with a lifetime in the order of several hours, as the comet was reaching the perihelion of its orbit. The next panel h, where the area of interest is resolved about 4 months later, makes it clear that there have been significant changes on the surface morphology therein. The observable boulder field is therefore likely to be a direct result of the aforementioned series of outbursts. Finally, the panel i reveals the bright patch that has appeared on the boulder, some 2 months later. Figure credit: Deshapriya et al., 2018.

FIGURE 3 . 26 :FIGURE 3 . 27 :

 326327 FIGURE 3.26: The field of boulders in Bes region, where a displacement of a boulder was observed, leading to the appearing of an exposed bright patch on it between February and March 2016. Its sublimation-driven evolution can be characterised by means of the diminution of its effective size calculated from pixels resolutions. However, the reader is cautioned of any bias due to the geometry of the observation. Figure credit: Deshapriya et al., 2018.

  FIGURE 3.27: Evolution of a cluster of exposed bright features in Khepry region, which indicates the stability of the features. The observations indicate a stability of about 3 months from the end of August to end of November 2014. 5 individual patches are numbered in the images so as to follow their evolution over the given time scale. The bottom right panel indicates that all the small patches on the cluster have already sublimated away by the end of March 2015, as comet was getting closer to the perihelion.Figure credit: Deshapriya et al., 2018.

FIGURE 3 .

 3 FIGURE 3.28: A schema describing the diurnal cycle of H 2 O on the comet 67P.As the surface gets irradiated during the day by the Sun, a heat wave is triggered and it propagates to the interior layer of the comet. This results in the sublimation of volatile H 2 O ice and their escape as vapour through the porous matrices of surface layers. During the night, due to the low thermal inertia of the surface layers, some of the H 2 O vapour is trapped in these surface layers and re-condenses to become H 2 O ice, which will be the first to sublimate the following morning as solar photons resume to irradiate the surface. This cyclic process continues as long as solar irradiation is capable of sublimating the H 2 O ice, thus directly depending on the heliocentric distance of the comet. As such this cycle is active as long as the comet is inside the current snowline of the solar system. Figure credit: De Sanctis et al., 2015.

3. 3 .

 3 Distribution and evolution of exposed bright features on the comet 67P 123

FIGURE 3 . 29 :

 329 FIGURE 3.29: Four types of studied exposed bright features are presented in the panels A through D, while the panel E refers to frost features. Left most column shows a full frame NAC observation (imaged using F22 filter centred at 649.2 nm), presenting the context for the location of the selected bright feature. The following column offers a zoomed-in view of the bright feature, where a red arrow points to its location. Next column shows the corrected reflectance (using Lommel-Seeliger disk function) plots with the wavelength, where the red dots correspond to the exposed bright feature and the black dots corresponds to the typical nucleus, sampled from the vicinity of the feature in question. It is noted that the sampling areas correspond to 1x2 pixels for panels A through D and 2x2 pixels for the panel E. Finally, the right-most column represents the relative reflectance plots, created by normalising the reflectance to the filter centred at 535 nm. Figure credit: Deshapriya et al., 2018.

FIGURE 3 . 30 :

 330 FIGURE 3.30: Normal albedos calculated for the exposed bright features, grouped according to their respective types as represented by the colours used in their mapping earlier.

FIGURE 3 . 31 :

 331 FIGURE 3.31: Spectral modelling of CO 2 ice exposures in Anhur region of the comet. (A) VIRTIS-M reflectance (black curves), and spectral simulations (red curves) carried out with dark refractory material (DM) and H 2 O ice in areal mixing, DM and CO 2 ice in intimate mixing, and areal mixing. The spectral intervals marked in gray correspond to missing data due to instrumental sorting filters. (B) Imaginary part, 'k' of refractive indices of H 2 O ice (blue curve) and CO 2 ice (black curve). Vertical lines mark the 'k' local maxima corresponding to the strongest absorption bands at : 1.05, 1.25, 1.65, 2.05, 3.10 µm for H 2 O ice (blue lines) and at 1.97, 2.01, 2.07, 2.70, 2.78 µm for CO 2 ice (black lines), also shown in (A).Figure credit: Filacchione et al., 2016b

  FIGURE 3.31: Spectral modelling of CO 2 ice exposures in Anhur region of the comet. (A) VIRTIS-M reflectance (black curves), and spectral simulations (red curves) carried out with dark refractory material (DM) and H 2 O ice in areal mixing, DM and CO 2 ice in intimate mixing, and areal mixing. The spectral intervals marked in gray correspond to missing data due to instrumental sorting filters. (B) Imaginary part, 'k' of refractive indices of H 2 O ice (blue curve) and CO 2 ice (black curve). Vertical lines mark the 'k' local maxima corresponding to the strongest absorption bands at : 1.05, 1.25, 1.65, 2.05, 3.10 µm for H 2 O ice (blue lines) and at 1.97, 2.01, 2.07, 2.70, 2.78 µm for CO 2 ice (black lines), also shown in (A).Figure credit: Filacchione et al., 2016b

FIGURE 5 . 1 :

 51 FIGURE 5.1: The 'Cometary Zoo' referring to molecules identified by the ROSINA mass-spectrometer aboard Rosetta and presented in an analogy to animals in a zoo. Figure credit : ESA & ROSINA team.

Fig. 1 .

 1 Fig. 1. Top left: narrow angle camera monochromatic image (F22 filter centred at 649.2 nm) revealing a part of the Khonsu region on the comet where bluish material and exposed bright patches are present in the form of a cluster. Right: an RGB image produced with the STIFF (Bertin 2012) program using the NAC filters centred at wavelengths 882.1, 649.2, and 480.7 nm for red, green and blue colours. This image also depicts 3 regions of interest chosen for the spectrophotometric analysis. Bottom left: spectrophotometric plots of the aforementioned regions of interest are shown. Right: a zoom onto the 3 regions of interest on the RGB image is represented. The red box refers to an exposed bright patch (Deshapriya et al. 2016) containing H 2 O ice, whose reflectance has been sampled (1 × 2 pixels per filter) with scrutiny to avoid saturated pixels and the blue box corresponds to H 2 O ice mixed with dust and material from the nucleus (2 × 2 pixels), whereas the black box (2 × 2 pixels) represents the typical nucleus. The dimensions of the boxes are deliberately exaggerated in order to point out their respective locations on the terrain. The red and green features that sometimes appear adjacent to the shadows and near the frame edges in the RGB images are artefacts arising from the imperfect co-registration/receding shadows of the images.

Fig. 2 .

 2 Fig. 2. Map of bright features observed on 67P. The detections of H 2 O ice are indicated by red, green, blue, and yellow colours, which correspond to the feature types 1-4, respectively; the black dot indicates the unique detection of CO 2 ice (Filacchione et al. 2016b) where H 2 O ice was detected few weeks later(Fornasier et al. 2016). This map is the cylindrical projection of a merged shape model resulting from the shape models SHAP4S(Preusker et al. 2015) and SHAP5(Jorda et al. 2016) for the northern hemisphere and southern hemisphere, respectively.

Fig. 3 .

 3 Fig. 3. Temporal variations of 3 isolated bright patches on smooth terrains in Bes, Imhotep, and Hapi regions belonging to the type 1 of exposed bright features. Each panel corresponds to the morphological evolution of a selected patch during the course of the OSIRIS observations.

Fig. 5 .

 5 Fig. 5. Trigger and morphological evolution of a bright patch resting on a boulder in Bes region. The area measurements are approximative because of varying observational geometries.

Fig. 6 .

 6 Fig. 6. Temporal variation of a cluster of bright patches in the Khepry region prior to the perihelion passage. Several individual patches of the cluster are numbered 1 through 5 to follow their stability over time. Despite varying spatial resolutions and observing geometries of the images, it is clear that the numbered patches have remained stable for about 3 months from end of August to end of November 2014. The latest of this sequence dating to end of March 2015 reveals no surviving patches and suggests their sublimation.

Fig. 7 .

 7 Fig. 7. Panels A-D: four types of the studied bright features included in the catalogue and their spectrophotometric plots. Panel E illustrates the frost and its spectrophotometric characteristics. Leftmost column shows a full-frame NAC (F22 filter centred at 649.2 nm) image of the corresponding observations whereas the following 3 columns show a zoomed-in view of the bright feature; the corrected reflectance against the observed wavelength plot for the sampling area (1 × 2 pixels for A through D and 2 × 2 pixels for E) of the feature is indicated by the red arrow and a relative reflectance plot of the latter. The red dots correspond to the interested feature pointed by the arrow, whereas the black dots represent the cometary nucleus sampled from the vicinity of the interested feature.

Figure 1 .

 1 Figure 1. Left: image of the whereabouts of the Khonsu region on the bilobate nucleus with respect to boundaries of the visible regions overlaid with their names annotated (El-Maarry et al. 2016), right: a zoomed view on the Khonsu region.

  observed therein. The Khonsu region started to be observed by the OSIRIS imaging instrument from 2015 March onwards through the perihelion passage of the comet in 2015 August and beyond. Khonsu displays both smooth and rough terrain with a large quantity of boulders and irregular outcrops. It has pronounced steep boundaries with neighbouring Atum and Apis regions, marked by numerous boulders that might be what remains following a formation, basically through cliff collapse (El-Maarry et al. 2016). Towards the west, Khonsu leads gradually to the smooth areas of Imhotep region and touches Bes region to the south (we refer to El-Maarry et al. 2015 for the nomenclature and definitions of the northern regions of the comet 67P/C-

  ) and2(b) where the definitions are made with boxes of 7×7 pixels and 18×18 pixels, respectively:(a) part of the 'pancake' feature, (b) dark smooth terrain, (c) light smooth terrain, (d) rugged terrain.

Figure 4 .

 4 Figure 4. Spectrophotometric analysis of the bluish material seen in Fig. 3(d).

Figure 5 .

 5 Figure 5. Cumulative SFD of boulders ≥7 m per km 2 over the Khonsu Region. Vertical error bars indicate the root of the cumulative number of counting boulders, divided by the area computed from the 3D shape model of the comet. The continuous line is a fitted regression line to the data, and the power-law index of the SFD is -3.1 +0.2/-0.3 computed in the range of 7-40 m. The bin size is 1 m and the red vertical line indicates the lower limit for the computation of the SFD power-law index.

Figure 7 .

 7 Figure 7. The top panel shows the temporal variation of the bright spot 1 during its observation and each of the temporal observation has its corresponding spectrophotometric plots below it. The observations are named according to the time they were made and corresponding dimensions of the bright patches are given as well as the pixel sampling criteria when available. The central and bottom panels give the spectrophotometric plots for the bright patch featured above them. The sequence 1c refers to the same observation of white spot ID 3 analysed and associated with H 2 O ice byBarucci et al. (2016). As for the sub-figures a and b, we had to avoid the saturated centre pixels and hence sampling was done along the perimeter of the feature. Therefore, we remark that the absolute reflectances given in sub-figures a and b are underestimated and are to be considered as a lower limit.

Figure 8 .

 8 Figure 8. Temporal evolution of the bright patch 2 as seen from OSIRIS NAC and WAC images.

Figure 9 .

 9 Figure 9. Left: the would-be location of the bright patch 2 before the perihelion passage. Right: location of the bright patch 2 on the 'Boomerang' feature after the perihelion passage. Few arrow identifiers are given where the red and green arrows point to some boulders, whereas the yellow arrow points to a terrace. The outburst that has potentially triggered the transition from a to b took place on the 2015 August 1, just about a fortnight before the perihelion passage at -12 • , 196 • coordinates.

Figure 1 .

 1 Figure 1. Image from 2015 May 2 UT 07:53 showing the morphological regions visible at that time and in particular the location of the Anhur and Bes regions.

Figure 2 .

 2 Figure 2. The geomorphological maps of Anhur and Geb regions on 2015 May 2 (top panel), of Anhur and Bes regions on 2016 January 30 (middle panel), and of Anhur and Geb regions in 2016 February 10 (bottom panel). The yellow arrows indicate the flat terrace where the two bright patches were found, and the red arrows the two 150 m high scarps bounding this terrace.

Figure 5 .

 5 Figure 5. Composite RGB image (using filters centred at 882, 649 and 480 nm) showing the appearance of the bright patches in the Anhur-Bes regions on 2015 April 27, UT 18:17 and the surface evolution in the 2015 April 12-May 7 time frame.

Figure 6 .

 6 Figure 6. Composite RGB map (using filters centred at 882, 649 and 480 nm) from 2015 March 25 02:36 images (left), and spectrophotometry of five selected regions of interest. The reflectance is given at phase angle 74 • . The Sun is towards the top.

Figure 7 .

 7 Figure 7. Composite RGB map (using filters centred at 882, 649 and 480 nm) from 2015 April 12 17:15 images (left), and spectrophotometry of five selected regions of interest. The reflectance is given at phase angle 81 • . The Sun is towards the top.

Figure 8 .

 8 Figure 8. Composite RGB map (using filters centred at 882, 649 and 480 nm) from 2015 June 27 17:48 images (left), and spectrophotometry of five selected regions of interest. The reflectance is given at phase angle 89 • . The Sun is towards the top.

Figure 11 .

 11 Figure 11. RGB maps showing jets and activity originated from the Anhur region. On the top: the 2015 August 12 outburst 17:20 (left), and 17:50 (right); middle panel: 2015 August 30 (UT 12:21) jets departing from the Anhur region (left); jets seen on 2016 January 27 (UT 15:20) images; bottom panel: images from 2016 January 27, showing localized jets (left: UT 16:27; right: UT 17:27, the shadow of the plume is visible at its bottom).

Figure 12 .

 12 Figure 12. Pit on Anhur. The pit is about 100 m across, and we can find smooth deposits inside the pit. (c) and (d) show the activity of this pit from images acquired on 2015 December 7 and 2015 June 5, respectively. (b) is the zoomed image of (a).

5. 1

 1 Morphological changes: a new scarp Interestingly, we see important morphological changes on the surface near the Anhur-Bes boundary. A new 10 m high scarp has been formed near the original site of the Bes bright spot (patch B), as shown in Fig. 13 where we compare images taken from 2015 May to 2016 January 30. We are not sure about the precise time of the formation of the scarp because the Rosetta spacecraft was moved into larger orbits from 2015 June to the end of 2015 due to perihelion passage with the consequent increased cometary activity. A careful inspection of the images in the OSIRIS archive indicates that the scarp was not yet formed on 2015 August 1, but already present on 2015 December 10 images (Fig.

Figure 13 .

 13 Figure 13. Surface changes near the bright spots. (a) and (b) are the images taken on 2015 May 2, (c) the image acquired on 2015 December 10, where the new scarp became visible and (d) is an image acquired on 2016 January 30, showing the scarp at a higher spatial resolution. The white arrows point out the boulders A and B that are the same boulders and the yellow arrow indicates the newly formed scarp.

  ), on November 18, 2016 http://science.sciencemag.org/ Downloaded from First release: 17 November 2016 www.sciencemag.org (Page numbers not final at time of first release) 3

Fig. 3 .

 3 Fig.3. Spectrum of Dark Terrain unit that corresponds to the average spectrum of the comet's surface.

Spot 4 .

 4 The OSIRIS NAC observations of 22 November 2014 reveal a cluster of bright features located at the Atum region margin close to its boundaries with the Khonsu and Anubis regions. The bright features seem to be freshly exposed brittle Article number, page 8 of 13page.[START_REF] El-Maarry | Regional Surface Morphology of Comet 67P from Rosetta/OSIRIS Images: The Southern Hemisphere[END_REF]

Fig. 6 .

 6 Fig. 6. Comparisons of OSIRIS NAC images of the cluster of bright spot 6, observed two months apart, which shows the stability of the bright features with time.

H 2 O

 2 ice-rich areas using data acquired by the OSIRIS and VIRTIS instruments. Comparing high spatial resolution VIS images with extended IR range spectra enables us to study the morphological, thermal and compositional properties of these areas at the same time.-The analysis of the spectral properties observed by VIRTIS-M indicates that, on these areas, the H 2 O ice abundance is between 0.1 and 7.2%, mixed in areal or/and in intimate modalities with the dark terrain. -The ice is distributed on the two lobes of 67P/C-G in locations which remain in shadow for longer. -The detected bright spots are mostly on consolidated dust free material surfaces, mostly concentrated in equatorial latitudes. -The mass release of H 2 O at the location of the eight ice-rich spots has been estimated. -Some spots are stable for several months and others show temporal changes connected to diurnal and seasonal variations. Stability of the spots is corroborated by the temperature retrieved at the surface. The behavior of ice on these locations is in very good agreement with theoretical expectations. -Six of the detected H 2 O ice spots are located in approximately the same position of the previously detected cometary jets.

1

  Average monthly walking distances from March 2017 to September 2018, reflecting the daily dose of walking involved with the commute to and fro between home in Paris 18 and LESIA in Meudon. . . . . . . . . . . . . . . . . . . viii 1.1 Radial and vertical mixing of material in the protoplanetary disk due to the conversation of angular momentum. Localised convective cells are also identifiable above and below the accretion disk. Figure credit: Nuth and Johnson, 2012. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2

  Figure credit: Preusker et al., 2017. . . . . . . . . . . . . . . . . . . . . . 2.10 Spectrophotometric analysis of 3 selected regions of interest in the image sequence dated 2015-05-02 15:09 UTC. Top panel: To the left is a full frame NAC image (filter F22, centred at 649.2 nm) of a sequence taken with 11 filters. The areas of interest are marked in distinct colours and to the right are their zoomed views, where squares are used to illustrate the 5x5 sampling of pixels used for the spectrophotometric analysis. Bottom panel: Spectrophotometric plots of the 3 regions of interest. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Spectrum of H 2 O frost spanning from UV wavelengths to IR wavelengths. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis. Figure adapted from Hapke et al., 1981. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Characteristic spectra of (1)H 2 O , (2)CO 2 , (3)NH 3 and (4)SO 2 frosts. The red vertical lines indicate the widest portion of the spectrum that can be used by OSIRIS NAC for the spectrophotometric analysis. Figure adapted from Wagner, Hapke, and Wells, 1987. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 Contextual images and spectral slope maps, derived from OSIRIS NAC data in August 2014. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.14 Contextual images and spectral slope maps, derived from OSIRIS NAC data in June and July 2015. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 Contextual images and spectral slope maps, derived from OSIRIS NAC data in July 2016. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.

  Figure credit: Deshapriya et al., 2018. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.28 A schema describing the diurnal cycle of H 2 O on the comet 67P. As the surface gets irradiated during the day by the Sun, a heat wave is triggered and it propagates to the interior layer of the comet. This results in the sublimation of volatile H 2 O ice and their escape as vapour through the porous matrices of surface layers. During the night, due to the low thermal inertia of the surface layers, some of the H Finally, the right-most column represents the relative reflectance plots, created by normalising the reflectance to the filter centred at 535 nm.Figure credit: Deshapriya et al., 2018. . . . . . . . . . . . . 3.30 Normal albedos calculated for the exposed bright features, grouped according to their respective types as represented by the colours used in their mapping earlier. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxv 3.31 Spectral modelling of CO 2 ice exposures in Anhur region of the comet. (A) VIRTIS-M reflectance (black curves), and spectral simulations (red curves) carried out with dark refractory material (DM) and H 2 O ice in areal mixing, DM and CO 2 ice in intimate mixing, and areal mixing. The spectral intervals marked in gray correspond to missing data due to instrumental sorting filters. (B) Imaginary part, 'k' of refractive indices of H 2 O ice (blue curve) and CO 2 ice (black curve). Vertical lines mark the 'k' local maxima corresponding to the strongest absorption bands at : 1.05, 1.25, 1.65, 2.05, 3.10 µm for H 2 O ice (blue lines) and at 1.97, 2.01, 2.07, 2.70, 2.78 µm for CO 2 ice (black lines), also shown in (A). Figure credit: Filacchione et al., 2016b . . . . . . . . . . . . . . . 128 5.1 The 'Cometary Zoo' referring to molecules identified by the ROSINA massspectrometer aboard Rosetta and presented in an analogy to animals in a zoo. Figure credit : ESA & ROSINA team. . . . . . . . . . . . . . . . . . . . . . . . . 138 Basic characteristics of OSIRIS NAC and WAC. . . . . . . . . . . . . . . . . . . 45 2.2 NAC and WAC filters.λ c and ∆λ correspond to the centre wavelength of the filter and the bandpass of the filter, respectively. . . . . . . . . . . . . . . . . . 46 2.3 Standard Bessell filters and their bandpasses. . . . . . . . . . . . . . . . . . . . 54 3.1 VIRTIS-M observations of exposed bright features, initially detected by OSIRIS and suggested as candidates of exposed H The three data cube parameters refer to the number of spectral bands, and two spatial dimensions x and y of a given data cube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.4 Parameters retrieved by the spectral modelling for each of the VIRTIS-M positive detections for H 2 O ice. The relative error on abundance and grain size is 40%. The column name 'Additional slope' refers to a free parameter to obtain the best fit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.5 Observational circumstances for the data cubes used in pre and post-perihelion Comparison of the phase-reddening effect for individual terrains of the analysis for both pre-perihelion and post-perihelion periods. In bold face are mean values when all the three regions showing positive phase-reddening effect are taken as a whole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Spectral slopes values obtained for cometary nuclei. . . . . . . . . . . . . . . . 3.9 Observational circumstances for the bright features observed in the Khonsu region. OSIRIS NAC observations are used, unless specified otherwise. Attention should be given to the interpretation of the diameter estimates of the features, as their observed dimensions depend upon the observational geometries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Five types of exposed bright features appearing in the catalogue and their respective counts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 The catalogue of exposed bright features on comet 67P. Column description :identification number of the exposed bright feature, first detection time, the type of the feature(1)(2)(3)(4), area of the feature, lifetime of the feature, phase angle at the time of the first detection, spatial resolution, latitude, longitude, observing instrument and remarks which refer to other mentions of the same feature by different publications. The remarks column should be read with the following legend.

	xxvii
	List of Tables
	1.1 A summary of the space missions to comets that have been hitherto launched. 26
	2.1

2

O vapour is trapped in these surface layers and re-condenses to become H 2 O ice, which will be the first to sublimate the following morning as solar photons resume to irradiate the surface. This cyclic process continues as long as solar irradiation is capable of sublimating the H 2 O ice, thus directly depending on the heliocentric distance of the comet. As such this cycle is active as long as the comet is inside the current snowline of the solar system. Figure credit: De Sanctis et al., 2015. . . . . . . . . . . . . 3.29 Four types of studied exposed bright features are presented in the panels A through D, while the panel E refers to frost features. Left most column shows a full frame NAC observation (imaged using F22 filter centred at 649.2 nm), presenting the context for the location of the selected bright feature. The following column offers a zoomed-in view of the bright feature, where a red arrow points to its location. Next column shows the corrected reflectance (using Lommel-Seeliger disk function) plots with the wavelength, where the red dots correspond to the exposed bright feature and the black dots corresponds to the typical nucleus, sampled from the vicinity of the feature in question. It is noted that the sampling areas correspond to 1x2 pixels for panels A through D and 2x2 pixels for the panel E. 2 O ice features, given their spectrophotometric behaviour. MTP refers to Medium Term Plan time intervals corresponding to a period about a month. Positive identifications of H 2 O ice band at 2 µm are given inside brackets. . . . . . . . . . . . . . . . . . . . . . . 80 3.2 OSIRIS Observational circumstances for the eight bright features whose presence of H 2 O ice absorption band at 2µm was detected, as summarised in Table

3

.1. The time in UTC refers to the starting time of the observational sequence, followed by the order of the filters used. An approximate estimate of the diameter of the feature is given in the column of ∼∅, whereas α, ∆, R refer to the phase angle, spacecraft-comet surface distance and spatial resolution of the observations. Finally, latitude (Lat) and longitude (Long) of the each bright feature containing H 2 O ice are reported in the last two columns. . . . 80 3.3 Observational circumstances of VIRTIS-M data corresponding to the positive detections of H 2 O ice. Among the multiple detections of the same feature, the detection that corresponds to the best signal-to-noise ratio is presented for all the 8 exposed bright features of H 2 O ice. A variety of parameters such as phase angle (α), incident angle (i), emission angle (e), spacecraft-comet surface distance (∆), local solar time (LST) and retrieved surface temperature (T sur f ) in Kelvin is given for each detection. spectrophotometric analysis in the Khonsu region. . . . . . . . . . . . . . . . . 95 3.6 Observations that were used for the study of spectral slopes in Khonsu region with several additional data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 xxviii 3.7 B: Barucci et al., 2016, Da: Davidsson et al. (in prep.), De: Deshapriya et al., 2016, Fi(a): Filacchione et al., 2016a, Fi(b): Filacchione

TABLE 1 .

 1

		1: A summary of the space missions to comets that have been hith-
			erto launched.
	Launch	Cometary mission	Comet

TABLE 2 .

 2 1: Basic characteristics of OSIRIS NAC and WAC.

	Aspect	NAC	WAC
	Optical design	3-mirror off-axis	2-mirror off-axis
	Detector	2048 x 2048 CCD 2048 x 2048 CCD
	Angular resolution (µrad px -1 )	18.6	101
	Focal length (mm)	717.4	140 (sag) / 131 (tan)
	Mass (kg)	13.2	9.48
	Field-of-view ( o )	2.20 x 2.22	11.35 x 12.11
	F-number	8	5.6
	Spatial resolution at 100 km (m px -1 )	1.86	10.1
	Typical filter bandpass (nm)	40	5
	Wavelength range (nm)	12	14
	Estimated detection threshold (m V )	21-22	18
	Filters	11	14
	Functional wavelength range (nm)	260 -1000	240 -700

TABLE 2 .

 2 2: NAC and WAC filters. λ c and ∆λ correspond to the centre wavelength of the filter and the bandpass of the filter, respectively.

	Filter	Filter λ c	∆λ	Objective
	Name	ID	(nm)	(nm)
	NAC
	FFP-UV	-	250-850	-	UV focussing plate for use of filters in wheel 2
	FFP-Vis	-	250-1000 -	Vis focussing plate for use of filters in wheel 2
	FFP-IR	-	300-1000 -	IR focussing plate for use of filters in wheel 1
	NFP-Vis	-	300-1000 -	Vis focussing plate for near-nucleus imaging
	Far-UV	F15	269.3	53.6	Surface spectral reflectance
	Near-UV	F16	360.0	51.1	Surface spectral reflectance
	Blue	F24	480.7	74.9	Surface spectral reflectance
	Green	F23	535.7	62.4.	Surface spectral reflectance
	Neutral	F8X	640.0	520.	Neutral density filter
	Orange	F22	649.2	84.5	Surface spectral reflectance
	Hydra	F27	701.2	22.1	Water of hydration band
	Red	F28	743.7	64.1	Surface spectral reflectance
	Ortho	F51	805.3	40.5	Orthopyroxene
	Near-IR	F41	882.1	65.9	Surface spectral reflectance
	Fe 2 O 3	F61	931.9	34.9	Iron-bearing minerals
	IR	F71	989.3	38.2	IR surface reflectance
	WAC
	Empty	-	-	-	Empty position to allow the use of filter wheel 2
	Empty	-	-	-	Empty position to allow the use of filter wheel 2
	UV245	F31	246.2	14.1	Continuum surface spectral reflectance
	CS	F41	259.0	5.6	CS gas emission
	UV295	F51	295.9	10.9	Continuum for OH
	OH-WAC F61	309.7	4.1	OH emission from the vicinity of the nucleus
	UV325	F71	325.8	10.7	Continuum for OH surface spectral reflectance
	NH	F81	335.9	4.1	NH gas emission
	UV375	F13	375.6	9.8	Continuum for CN surface spectral reflectance
	CN	F14	388.4	5.2	CN gas emission
	Green	F21	537.2	63.2	Dust continuum cross-correlation with NAC
	NH 2	F15	572.1	11.5	NH 2 gas emission
	Na	F16	590.7	4.7	Sodium gas emission
	VIS610	F18	612.6	9.8	Continuum for OI surface spectral reflectance
	OI	F17	631.6	4.0	O( 1 D) gas emission for dissociation of H 2 O
	R	F12	629.8	156.8 Broadband filter for nucleus and asteroid detection (NAC
					redundancy)

TABLE 2 .

 2 3: Standard Bessell filters and their bandpasses.

	nm)

Bessell filter Centre wavelength λ c (nm) Bandpass ∆λ (

TABLE 3 .

 3 1: VIRTIS-M observations of exposed bright features, initially detected by OSIRIS and suggested as candidates of exposed H 2 O ice features, given their spectrophotometric behaviour. MTP refers to Medium Term Plan time intervals corresponding to a period about a month. Positive identifications of H 2 O ice band at 2 µm are given inside brackets.

	OSIRIS	# MTP	MTP	MTP	MTP	MTP MTP MTP	MTP MTP Total
	ID		007	008	009	010	011	012	013	014	015
	nl1-5b	1 20[0]	0	0	0	0	0	15[2] 139[0] 89[0]	2
	nl2-7b	-	0	0	0	40[0]	0	16[0] 15[0]	8[0]	8[0]	0
	nl3-12b	-149[0]	0	36[0]	144[0] 64[0]	0	32[0]	77[0] 87[0]	0
	nl4-13	2	0	0	0	0	0	0	4[0]	106[9] 49[0]	9
	nl5-16	-142[0]	0	0	32[0]	26[0]	0	45[0] 121[0] 70[0]	0
	nl6-17a	3	0	0	0	0	0	0	15[0] 116[9] 64[0]	12
	nl7-19	4	0	0	195[32]	0	0	0	29[0]	52[0] 39[0]	32
	nl8-20	5 40[0]	0	0	300[86]	0	0	38[0] 190[0] 87[0]	86
	nl9-22	-225[0]	0	140[0]	86[0]	0	0	21[0]	77[0] 59[0]	0
	nl10-23b -	0	0	0	0	0	27[0] 184[0] 58[0] 27[0]	0
	nl11-14b 6 102[3] 711[0]	72[0]	140[0] 70[0]	0	46[0]	76[0] 65[0]	3
	nl12-24	7 94[10]	0	0	0	0	0	14[0] 122[4] 66[1]	15
	nl13-25	8 326[0]	0	0	345[3] 41[0]	0	43[0]	93[0] 85[0]	3

TABLE 3 .

 3 2: OSIRIS Observational circumstances for the eight bright features whose presence of H 2 O ice absorption band at 2µm was detected, as summarised in Table3.1. The time in UTC refers to the starting time of the observational sequence, followed by the order of the filters used. An approximate estimate of the diameter of the feature is given in the column of ∼∅, whereas α, ∆, R refer to the phase angle, spacecraft-comet surface distance and spatial resolution of the observations. Finally, latitude (Lat) and longitude (Long) of the each bright feature containing H 2 O ice are reported in the last two columns. Since these features are clusters of bright patches, the given values refers to the range of dimensions they could be observed.

	# Time (UTC)	Filters	∼ ∅ Region (m)	α ( o ) (km) (m/px) ∆ R	Lat Long ( o ) ( o )
	1 2015-06-27 F22,F23,F41,F24,F71,F27,	36	Imhotep 89.5 192	3.6	-5.8 189.4
	13:26	F51,F61,F28,F16,F15
	2 2015-06-27 F22,F23,F41,F24,F71,F27,	45	Anhur	89.4 188	3.5	-41.7 63.7
	17:48	F51,F61,F28,F16,F15
	3 2015-04-12 F22,F23,F41,F24,F71,F27,	11	Khonsu 80.5 148	2.7	-23.8 198.3
	21:42	F51,F61,F28,F16,F15
	4 2014-11-22 F22,F23,F24,F27,F28,F51,	10	Atum	92.7 29.5	0.54	-20.7 227.4
	04:57	F61
	5 2014-11-22 F22,F23,F24,F27,F28,F51, 6.5 Imhotep 92.8 29.5	0.54	-22.0 182.8
	06:32	F61
	6 2014-09-19 F22,F16,F23,F24,F41	2-5 * Khepry 70.5 26.5	0.49	4.2	71.7
	09:19
	7 2014-09-05 F22,F23,F27,F16,F28,F41, 3-5 * Imhotep 57.2 41.4	0.77	-8.1 188.3
	05:21	F71
	8 2014-09-05 F22,F23,F27,F16,F28,F41,	6	Imhotep 58.4 40.8	0.75	-2.7 174.8
	08:00	F71

*

Table 3 .

 3 2.

TABLE 3 .

 3 3: Observational circumstances of VIRTIS-M data corresponding to the positive detections of H 2 O ice. Among the multiple detections of the same feature, the detection that corresponds to the best signal-to-noise ratio is presented for all the 8 exposed bright features of H 2 O ice. A variety of parameters such as phase angle (α), incident angle (i), emission angle (e), spacecraft-comet surface distance (∆), local solar time (LST) and retrieved surface temperature (T sur f ) in Kelvin is given for each detection. The three data cube parameters refer to the number of spectral bands, and two spatial dimensions x and y of a given data cube.

	# VIRTIS-M data cube Time	α	i	e	∆	LST T sur f
	parameters	(UTC)	( o )	( o )	( o )	(km)	(h)	(K)
	432,256,158	2015-02-25 21:30 53.02 66.94 41.92 81.51 12.23 203
	432,256,70	2015-03-25 12:23 73.49 67.08 45.83 88.23 15.20 197
	432,256,70	2015-03-25 06:19 74.47 53.03 31.27 94.06 12.43 218
	432,256,86	2014-11-01 11:31 103.07 60.11 43.28 32.39 11.04 168
	432,256,80	2014-12-14 12:59 91.77 43.41 52.15 19.44 15.44 188
	432,256,80	2014-12-04 08:24 91.07 57.34 70.64 23.49 14.81 179
	432,256,109	2014-09-14 23:09 66.89 78.22 30.26 28.16 11.03 163
	432,256,74	2014-12-14 13:30 92.66 48.30 54.87 19.92 15.59 158

TABLE 3 .

 3 4: Parameters retrieved by the spectral modelling for each of the VIRTIS-M positive detections for H 2 O ice. The relative error on abundance and grain size is 40%. The column name 'Additional slope' refers to a free parameter to obtain the best fit. Hapke modelling, which yielded the abundance of end-members for each of the positive VIRTIS detections and grain sizes of the H 2 O ice that fit the model. The column Mixing modalities & Alternative(A)/Simultaneous(S) mode refers to the type of mixing modality used for the modelling and whether the mixing can be of multiple modes, which is interpreted as alternative (A) or a unique mode, meaning that both types of mixing modalities are involved simultaneously (S) to obtain the final parameters. The obtained results indicate two simultaneous cases, for the features 4 and 6, which have a wide range of H 2 O ice grain sizes from micro-metric to milli-metric sizes, which corroborate the findings of Filacchione et al., 2016a. As such, the presence of larger millimetre-sized H 2 O grains on the cometary nuclei becomes more probable, as opposed to the only population of micrometresized H 2 O grains known prior to Rosetta mission from the observations of the cometary nuclei 9P/Tempel (Sunshine et al., 2006) and 103P/Hartley (A'Hearn et al., 2011) respectively by Deep Impact and EPOXI missions.

	#	Mixing modalities & Alternative(A)/Simultaneous(S) mode abundance(%) H 2 O ice	H 2 O ice grain Additional slope Goodness-of-fit size (µm) (%µm -1 ) χ 2
	1	areal (A)	1.1	350	5.2	4.19
		intimate	1.2	450		4.16
	2	areal (A)	0.1	40	no	1.18
		intimate	0.5	200		1.04
	3	areal (A)	1.3	750	-2.8	2.78
		intimate	1.8	1300		3.30
	4	areal (S)	3.2	4500	no	0.45
		intimate	4.0	30
	5	areal (A)	1.7	400	-4.0	1.47
		intimate	2.0	800		2.15
	6	areal S	1.9	6500	-1.8	0.28
		intimate	1.0	250
	7	areal A	1.5	10	-4.0	0.90
		intimate	1.8	15		0.86
	8	areal A	0.3	900	no	0.56
		intimate	0.7	2200		0.55

TABLE 3 .

 3 5: Observational circumstances for the data cubes used in pre and post-perihelion spectrophotometric analysis in the Khonsu region.

	Observation	02 May 2015	10 February 2016
	Time UTC	15:09h	11:53h
	Filters used	11	11
	Phase angle	60 o	65 o
	Rosetta altitude	123.28 km	48.0 km
	Spatial resolution	2.29 m/pixel	0.89 m/pixel
	Heliocentric distance	1.72 AU (in-bound) 2.33 AU (out-bound)
	Spectrophotometric sampling	7x7 pixels	18x18 pixels

TABLE 3 .

 3 6: Observations that were used for the study of spectral slopes in Khonsu region with several additional data.

	Observation	Filters Phase angle Heliocentric distance T = Perihelion passage
	(UTC)		( o )	AU
	2015-03-25 05:36	4	73	2.02	T -20 weeks
	2015-04-12 21:42	11	80	1.88	T -18 weeks
	2015-05-02 15:09	11	60	1.72	T -15 weeks
	2015-05-07 17:43	4	61	1.69	T -14 weeks
	2015-06-05 05:43	11	87	1.49	T -10 weeks
	2015-06-27 11:27	11	90	1.37	T -7 weeks
	2016-01-09 07:29	11	90	2.08	T + 21 weeks
	2016-01-27 10:53	9	63	2.223	T + 24 weeks
	2016-01-28 01:48	9	62	2.228	T + 24 weeks
	2016-02-10 11:53	11	65	2.33	T + 26 weeks
	2016-04-11 07:18	11	70	2.77	T + 35 weeks
	2016-07-02 07:57	5	86	3.31	T + 46 weeks
	2016-07-09 08:53	7	90	3.36	T + 47 weeks

TABLE 3 .

 3 8: Spectral slopes values obtained for cometary nuclei.

	Nucleus	Spectral slope Evaluation Phase	Source	Phase
		%/(100 nm)		angle		reddening
	9P/Tempel	12.5±1	309-950 nm	63 o	Li et al., 2007	No
	(Deep Impact)
		16±3		117 o
	103P/Hartley	7.6±3.6	400-850 nm	85 o	Li et al., 2013	No
	(DIXI/EPOXI)
		8.6±10.6		93 o
	67P/C-G	11	535-882 nm	1.3 o	Fornasier et al., 2015 *
	(OSIRIS/Rosetta)					0.104±0.003
		16		54 o		% /(100 nm)/ o
		16.93±0.224	535-882 nm 62.04 o	This work †
		17.11±0.219		62.90 o		0.1083±0.0161
		17.77±0.313		65.20 o		% /(100 nm)/ o
		17.99±0.427		69.56 o
		18.77±0.889		86.11 o
		18.93±0.766		89.83 o
		21.15±1.14		89.89 o

* Using the data from August 2014 for the averaged nucleus.

TABLE 3 .

 3 9: Observational circumstances for the bright features observed in the Khonsu region. OSIRIS NAC observations are used, unless specified otherwise. Attention should be given to the interpretation of the diameter estimates of the features, as their observed dimensions depend upon the observational geometries.

	Index Observation (UTC)	Filters s/c altitude phase angle Resolution ∼∅ (km) ( o ) (m/pixel) (m)
	K1a *	2015-01-29 11:33	-	27.73	84.7	2.35	7.0
	K1b	2015-03-10 18:40	3	83.94	53.74	1.56	8.5
	K1c	2015-03-25 05:36	4	96.88	73.78	1.80	18.0
	K1d	2015-04-12 21:42	11	149.60	80.46	2.78	8.0
	K1e	2015-04-15 23:47	3	145.28	69.79	2.70	8.1
	K1f	2015-05-02 13:42	11	123.20	60.62	2.29	7.0
	K1g	2015-05-07 17:43	4	141.46	61.44	2.63	5.2
	K2a	2016-01-23 23:45	3	72.45	61.21	1.34	8.0
	K2b	2016-01-28 02:48	9	69.16	62.57	1.23	10.0
	K2c †	2016-03-02 06:35	1	24.41	92.09	2.46	9.8
	K2d † 2016-05-03 01:00	1	18.83	90.72	1.90	5.8
	K2d † 2016-06-24 20:28	1	19.02	87.14	1.92	3.8
	K2e *	2016-07-29 12:51	-	13.61	107.01	1.16	3.5
	K3a	2016-02-10 11:53	11	48.00	65.21	0.89	5.4
	K4a	2016-03-23 07:59	1	40.36	107.80	0.75	1-2 ‡
	K4b	2016-04-23 11:44	3	28.21	115.1	0.52	1-2 ‡
	K4c	2016-05-07 01:38	7	10.42	91.6	0.19	1-2 ‡
	K4d	2016-07-16 19:22	7	8.32	94.28	0.15	1-2 ‡

* This is a Rosetta NAVCAM (navigation camera) observation.

TABLE 3 .

 3 10: Five types of exposed bright features appearing in the catalogue and their respective counts

	Feature type	Count
	1. Isolated patches on smooth terrains	14
	2. Isolated patches close to irregular structures	18
	3. Patches resting on boulders	13
	4. Clusters of patches	12
	5. Frost	-
	Total	57

  the column Obs. refer to NAVCAM and VIRTIS respectively. Source:[START_REF] Deshapriya | [END_REF]

	Table 3.11 -Continued from previous page
	ID Detection	Type	A			Life Region	Phase Res.	Lat Long Obs. Remarks
	(UT)			(m 2 ) (days)	( • )	m/px	( • )	( • )
	14 2014-08-26 01:42		4		-			88 Khepry	38.1	0.94	4.2	71.7 NAC B Spot 6, P Fig. 7b
	15 2014-09-02 02:42		4		-			29 Ash	38.5	0.94	34.7 137.2 NAC P Fig. 1c
	16 2014-09-05 05:21		4		-			146 Imhotep	57.2	0.77	-8.1 188.3 NAC B Spot 7
	17 2014-09-05 06:45		2		8.6		0 Ash	57.8	0.76	22.9 122.9 NAC
	18 2014-09-15 05:42		3		4.2		0 Bastet	67.35	0.51	-1.1	5.2 NAC
	19 2014-09-15 20:32		3		13.8		69 Maftet	73.4	0.50	-15.1 312.5 NAC P Fig. 4c
	20 2015-01-04 02:22		3		29.0		58 Khonsu	95.3	0.53	-23.8 198.3 N	B Spot 3, De Patch 1
	21 2015-03-10 21:55		1		35.8		33 Bes	55.1	1.6	-30.2 260.8 NAC
	22 2015-03-21		1		-			42 Anhur	-	-	-54.5	66.0 V	Fi(b)
	23 2015-03-25 10:42		4		-			16 Khepry	73.4	1.67	-0.2	75.8 NAC
	24 2015-04-27 15:58		1	277.3	10 Bes	73.13	2.43	-54.6	75.6 NAC Fo16
	25 2015-05-02 05:53		2		42.2		0 Bes	61.57	2.29	-84.6 218.2 NAC
	26 2015-05-07 22:06		2	132.1	0 Bes	61.01	2.57	-86.7 243.6 NAC
	27 2015-06-04 23:16		2	104.1	37 Anhur	87.42	3.86	-41.7	63.7 NAC B Spot 2
	28 2015-06-05 07:03		2	220.0	22 Imhotep	87.2	3.83	-5.8 189.4 NAC B Spot 1
	29 2015-06-13 15:20		1		13.6		21 Imhotep	87.8	3.69	10.6 170.8 NAC
	30 2015-06-24 07:59		1		76.9		25 Hapi	89.7	3.24	-8.0 260.4 NAC Da
	31 2015-06-27 08:33		1	204.2	29 Serqet	89.7	3.69	7.7 312.5 NAC
	32 2015-06-27 15:08		1		51.3		22 Imhotep	89.5	3.58	-10.7 135.5 NAC
	33 2015-07-04 08:58		1		98.6		0 Khepry	89.9	3.31	-12.7	91.3 NAC
	34 2015-07-04 13:41		1		74.9		15 Hatmehit	89.8	3.27	9.1 346.9 NAC
	35 2015-07-11 13:41		1	139.2	0 Imhotep	89.6	2.95	3.9 131.1 NAC
	36 2015-07-11 13:41		1		78.3		0 Imhotep	89.6	2.95	10.4 156.7 NAC
	37 2015-07-11 13:41		2		52.2		0 Imhotep	89.6	2.95	12.6 178.8 NAC
	38 2015-07-15 15:58		1		36.7		11 Imhotep	89.5	3.03	-0.8 185.3 NAC
	39 2015-07-26 12:11		1		903		14 Imhotep	89.4	3.88	-29.1 110.3 NAC
	40 2016-01-23 23:45		3		58.0		188 Khonsu	61.2	1.35	-13.3 196.4 NAC De Patch 2
	41 2016-01-28 03:48		2		27.2		0 Imhotep	62.0	1.23	20.1 139.9 NAC
	42 2016-02-10 11:53		2		12.7		0 Khonsu	65.2	0.89	-18.7 196.4 NAC De Patch 3
	43 2016-03-23 07:59		4		-			115 Khonsu	91.6	0.19	-24.2 200.4 NAC De Patch 4
	44 2016-03-23 11:21		3		70.0		178 Bes	106.0	1.08	-31.8	86.3 NAC
	45 2016-04-29 04:42		4		-			50 Aten	92.1	0.31	19.5 104.2 NAC
	46 2016-04-30 10:41		4		-			0 Serqet	90.5	0.30	7.7 311.8 NAC
	ID Detection 47 2016-05-15 19:09		Type 3	0.8	A	Life Region 83 Wosret	Phase Res. 90.3 0.14	Lat Long Obs. Remarks -7.7 357.5 NAC
	(UT) 48 2016-05-19 15:46		2		(m 2 ) (days) 0.5 66 Imhotep	( • ) 101.8	m/px 0.11 -28.7 166.4 NAC ( • ) ( • )
	2014-08-05 23:19 49 2016-06-14 03:37	3	3	50.6 7.2	123 Wosret 0 Khonsu	48.5 57.3	2.25 0.49	-7.8 356.3 NAC -15.2 199.5 NAC
	2014-08-06 00:19 50 2016-07-02 07:57	2	3	48.8 10.6	129 Ma'at 7 Khonsu	49.8 86.1	2.19 0.71	14.2 356.7 NAC P Fig. 4d -20.2 197.7 NAC
	2014-08-06 02:43 51 2016-07-09 02:42	3	4	1.26	-	44 Anuket 0 Babi	51.0 89.6	2.17 0.30	-1.4 304.7 NAC P Fig. 1d 6.3 70.1 NAC
	2014-08-14 08:43 52 2014-10-03 23:22	1	2	12.4 0.3	36 Aker 97 Ash	35.7 92.9	1.76 0.32	-3.7 34.1 196.3 NAC P Fig. 3b 61.4 NAC
	2014-08-14 08:43 53 2014-10-04 19:16	2	3	31.0 1.0	154 Atum 0 Bastet	35.7 92.8	1.76 0.31	-26.7 232.3 NAC P Fig. 4b 2.3 5.9 NAC P Fig. 3d
	2014-08-16 14:59 54 2014-10-05 23:54	3	4	3.1	-	98 Imhotep 0 Maftet	38.9 92.3	1.84 0.31	13.4 116.5 NAC Fi(a) BAP 1 -11.2 313.0 NAC P Fig. 4a
	2014-08-16 15:59 55 2014-10-05 23:54	2	4	1.8	-	98 Imhotep 50 Maftet	39.4 92.3	1.86 0.31	-2.7 180.6 NAC Fi(a) BAP 2 -10.5 324.7 NAC P Fig. 3a
	2014-08-22 16:42 56 2016-01-28 05:33	2	2	49.9 13.4	12 Ash 0 Anhur	34.0 61.9	1.09 1.22	21.1 200.9 NAC -44.7 33.1 NAC Fo17
	2014-08-22 16:42 57 2016-09-02 00:30	4	2	-	22.5	1 Ash 23 Atum	34.03 107.5	1.09 1.23	36.4 136.8 NAC -22.4 225.2 WAC
	2014-08-23 07:44		2		13.0	91 Imhotep	40.1	1.09	-22.0 182.8 NAC B Spot 5, O Fig.9, P F
	2014-08-23 12:42		2		74.0	10 Geb	47.2	1.12	-48.3 295.8 NAC
	2014-08-25 19:12		4			-	133 Atum	37.5	0.93	-20.7 227.4 NAC B Spot 4
	2014-08-25 23:42		3		21.5	36 Imhotep	37.7	0.93	-2.4 174.8 NAC B Spot 8

TABLE

Table 1 .

 1 Observational parameters of the surface feature comparison

	ID	Time	Altitude (km)	S/R (m/pixel)	Heliocentric distance (au)	Phase angle	Pixel box dimensions
	1	2015-05-02_15h09 UTC	123.28	2.29	1.72	60. • 48	7×7
	2	2016-02-10_11h53 UTC	48.00	0.89	2.33	65. • 20	18×18

Table 4 .

 4 Temporal variation of the size of the bright patches 1 and 2 with their observational conditions. The column S/R refers to the spatial resolution.

	ID	Observation time	Approximate	S/R	Phase ( • ) Filters	Filter wavelengths (nm)
			diameter (m) (m pixel -1 )
	1a 2015-03-10_18h40	8.5	1.56	53.8	3	[480.7,649.2,882.1]
	1b 2015-03-25_05h36	18	1.77	73.8	4	[480.7,535.7,649.2,882.1]
	1c 2015-04-12_21h42	8.0	2.75	80.5

Table 4 .

 4 Parameters retrieved by modeling for each bright area. The relative error on abundance and grain size is 40%, as estimated in Raponi 2014.Maarry et al. 2015) in a rocky-like area. The bright patches of varying individual sizes span an area of about 25 meters in diameter. The largest patch appears to be about 10 m in size in this observation. Further analysis, including earlier images, reveals that this feature was observed as early as 2 September 2014. In the meantime the latest appearance of this feature on OSIRIS NAC observations was on 23 November 2014.

	Spot 1 2 3 4 5 6 7 8	VIRTIS file sample, line I1_00383518966 s: 184, l: 24 I1_00385906923 s: 173, l: 42 I1_00385885107 s: 120, l: 35 I1_00373462192 s: 93, l: 35 I1_00377182711 s: 61, l: 69 I1_00376302211 s: 16, l: 38 I1_00369356914 s: 239, l: 66 I1_00377184571 s: 6, l: 7	Mixing modalities Alternative/Simultaneous abundance(%) H 2 O ice areal A 1.1 intimate 1.2 areal A 0.1 intimate 0.5 areal A 1.3 intimate 1.8 areal S 3.2 intimate 4.0 areal A 1.7 intimate 2.0 areal S 1.9 intimate 1.0 areal A 1.5 intimate 1.8 areal A 0.3 intimate 0.7	H 2 O ice grain Additional slope Goodness size (µm) χ 2 (%µm -1 ) 350 5.2 4.19 450 4.16 40 no 1.18 200 1.04 750 -2.8 2.78 1300 3.30 4500 no 0.45 30 400 -4.0 1.47 800 2.15 6500 -1.8 0.28 250 10 -4.0 0.90 15 0.86 900 no 0.56 2200 0.55
	materials (El-

table . H 2

 .2

	O ice abundance(%)	H 2 O ice grain Additional slope Goodness size (µm) (%µm -1 ) χ 2
	1.3 0 2.6 1.3 1.3	750 750 750 75 7500	-2.8 -4 0 -2.5 -3.9	2.78 31.2 15.0 9.4 6.8

Table 6 .

 6 Mass release rate of H 2 O from the surface of 67P/C-G at the location of H 2 O ice-rich spots. In the last column an indication of the observed lifetime of the spots is summarised.

	Spot 1 2 3 4 5 6 7 8	T [K] Q H 2 O [kg m -2 s -1 ] 203 3.357 × 10 -4 197 1.336 × 10 -4 218 2.485 × 10 -3 168 0.662 × 10 -6 188 3.003 × 10 -5 179 0.625 × 10 -5 163 2.156 × 10 -7 158 0.701 × 10 -7	OSIRIS observations size receded from 57m to 36m in 3 weeks observed for 5 weeks size receded from 18m to 8m in 3 weeks, disappear in the following 3 weeks cluster stable for 11 weeks stable for 13 weeks cluster stable for 13 weeks cluster stable for 3 weeks stable for 15 weeks

  2 O [kg] the mass of one molecule of H 2 O, k B the Boltzmann constant, T [K] the temperature of each spot, and P H 2 O the saturation vapor pressure, which can be written as

The OCs are the most common type of meteorites, accounting for more than 80% of the meteorite falls.

The reader is referred to the subsection 1.2.3 for a dedicated description of this small body population.

These resolutions are available at https://www.iau.org/static/resolutions/Resolution_GA26-5-6. pdf

The small bodies classified as NEOs are primarily asteroids and relatively fewer comets with perihelion distances (q) less than about 1.3 AU, thus bringing them to the vicinity of the Earth's orbit. NEOs can be further divided into several categories: Amor, Apollo, Aten, Atira asteroids and Potentially Hazardous Asteroids (PHAs) depending on their orbital parameters.

The reader is referred toEl-Maarry et al., 2015; El-Maarry et al., 2016 for a complete description of the nomenclature used in naming different regions in the northern and southern latitudes of the comets, which encompass 26 geologically defined regions.

sourced from ESA technical documents

OsiTrap and OsiCalliope will be detailed in the following section.

at the time of writing in March 2018.

Planetary Data System : An international standard of storing astronomical data as per the criteria specified at https://pds.nasa.gov/ and OSIRIS data adhere to this criteria.

There are three mirrors in NAC and two in WAC.

Parsec is a distance measurement unit used with cosmological distances and 1 parsec is equal to 206264.8 AU or 3.26 light years.

Angle subtended at the solar system object by the illumination source and the observer.

An IAU-adopted method to fit data to a phase function, here H is for the absolute magnitude and G is for the slope parameter of the phase curve in question.

An astronomical imaging and data visualization tool.

An interactive display tool for astronomical images written in IDL (Interactive Data Language) programming language.

Boresight refers to the field-of-view of the remote sensing instrument. It refers to the shape of the field-ofview, boundary vectors that define its field-of-view and the coordinate frame in which the vectors are defined.

[START_REF]Materials and methods are available as supplementary materials on Science Online[END_REF] Accessible at http://spiceypy.readthedocs.io/en/master/index.html.

SUMFILES referred to here, have been produced by Bob Gaskell from a Digital Elevation Model (DEM) in the process of creating a shape model. Using SUMFILES, it is possible to render synthetic images of observations given the pointing of an instrument, independent of SPICE data.

shapeViewer is a dedicated 3D model visualiser for geomorphological analysis of small bodies. It uses SPICE kernels and a shape model for the visualisation and is available at http://www.comet-toolbox.com/ shapeViewer.html.

Atmospheric seeing is a measurement of the effect of atmospheric turbulence.

The concept of spectral slope analysis will be introduced in the following sub-section.

In the context of the comet 67P smooth terrains are supposed to contain a lot of dust with the absence of surface geological irregularities like boulders, cliffs, etc. which are characteristic to the 26 defined regions of the comet.

(A) The OSIRIS NAC full-frame context images acquired with the F22 filter centred at 649.2 nm in August 2014, when the comet was at about 3.6 AU from the Sun.(B) The spectral slope maps evaluated between 882 nm and 535 nm, corresponding to the above observations.

(A) The OSIRIS NAC full-frame context images showing some local topography of the Khonsu region on the southern latitudes, acquired with the F22 filter centred at 649.2 nm in July (∼ 3.3 AU) 2016, when the comet was outbound, getting away from the perihelion of its orbit at 1.24 AU from the Sun. (B) The spectral slope maps evaluated between 882 nm and 535 nm, corresponding to the above observations of the Khonsu region. Local heterogeneities of the spectral slopes are identifiable.FIGURE 2.15: Contextual images and spectral slope maps, derived from OSIRIS NAC data in July 2016.

Given the differences of the spatial resolutions of both instruments, there is a factor of more than 13 between the corresponding pixel sizes.

MTP stands for Medium Term Plan in the Rosetta mission planning jargon, referring to time intervals of about a month in order to structure the mission and data.

(A) Optical design of the VIRTIS-M spectro-imager (B) Optical design of the VIRTIS-H spectrometer (C) Assembled VIRTIS instrument prior to the integration into the spacecraft. VIRTIS-M and VIRTIS-H can be recognised by their respective square and cylindrically shaped baffle.

Rugosity is a measure of the surface roughness, that takes into consideration the small-scale variations of amplitude in the height of a surface.

The perihelion passage of the comet took place on 13 August 2015 at 1.24 AU from the Sun.

(A) Four selected terrains in the Khonsu region on 02 May 2015. (B) Corresponding spectrophotometry of the above terrains.

(A) Four selected terrains in the Khonsu region on 10 February 2016. (B) Corresponding spectrophotometry of the above terrains.

(A) Spectral slopes for the pre-perihelion period.(B) Spectral slopes for the post-perihelion period.

NAVCAM images were used to better constrain the period during which the displacement would have taken place.

Sintering is the process of forming a consolidated/less porous mass of material by heat or pressure without melting it.

A process by which energetic photons break bonds between molecules, resulting in new molecules.

Radiolysis involves photons with more energies than the photolysis or electrons, ions with high kinetic energies transferring energy into ice to ionise the original molecules. This is observed in the icy moons in the outer Solar system and in the Saturn's rings.

J. D. P.Deshapriya et al.: Exposed bright features on the comet 67P/Churyumov-Gerasimenko: distribution and evolution

A36, page 7 of 14

A36, page 8 of 14 Appendix B. Authored and major co-authored publications

C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society at Observatoire de Paris -Bibliotheque on December 20, 2016 http://mnras.oxfordjournals.org/ Downloaded from

MNRAS 462, S274-S286 (2016) at Observatoire de Paris -Bibliotheque on December 20, 2016 http://mnras.oxfordjournals.org/ Downloaded from

MNRAS 469, S93-S107 (2017) Downloaded from https://academic.oup.com/mnras/article-abstract/469/Suppl_2/S93/3852302 by Observatoire De Paris -Bibliotheque user on 20 June 2018

This paper has been typeset from a T E X/L A T E X file prepared by the author.MNRAS 469, S93-S107 (2017) Downloaded from https://academic.oup.com/mnras/article-abstract/469/Suppl_2/S93/3852302 by Observatoire De Paris -Bibliotheque user on 20 June 2018

The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 m 2 ) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.on November 18, 2016 http://science.sciencemag.org/ Downloaded from

June

2016; accepted 27 October 2016 Published online 17 November 2016 10.1126/science.aag2671 on November 18, 2016 http://science.sciencemag.org/ Downloaded from

Astronomy & Astrophysics manuscript no. 28764_MB_checked

Article number, page 5 of 13page.13

Article number, page 11 of 13page.13

Acknowledgements. OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, CISAS-Università di Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. Rosetta is an ESA mission with contributions from its member states and NASA. All Rosetta NAVCAM images are released under the Creative Commons license ESA/Rosetta/ NAVCAM -CC BY-SA IGO 3.0.

AC K N OW L E D G E M E N T S

OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, CISAS -University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany.

The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB) and the ESA Technical Directorate is gratefully acknowledged. Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. ACKNOWLEDGMENTS OSIRIS was built by a consortium led by the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS, University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. The support of the contributing research institutes of the OSIRIS consortium and of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. M.F.A. acknowledges NASA funding through Jet Propulsion Laboratory contract no. 1267923 and from the Akademie der Wissenschaften zu Göttingen. W.-H.I acknowledges the Ministry of Science and Technology, Taiwan (grant no. NSC 102-2112-M-008) and Macau university of Science and Technology (grant no. FDCT 017/2014/A1). We thank the ESA teams at European Space Astronomy Centre, European Space Operations Centre, and European Space Research and Technology Centre for their work in support of the Rosetta mission. P. H. Hasselmann and D. Perna are acknowledged for helpful discussion. Rosetta/OSIRIS data are available through the ESA's Planetary Science Archive (PSA) http://www.cosmos.esa.int/web/psa/rosetta. Some images used in this paper which are not yet available at PSA can be downloaded from the MPS OSIRIS website at https://planetgate.mps.mpg.de/WebFileShare/Released_Images/Fornasier+Sc ience. Acknowledgements. OSIRIS was built by a consortium of the Max-Planck-Institut für Sonnensystemforschung, Göttingen, Germany, CISAS-University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille, France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the Research and Scientific Support Department of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain, the Universidad Politéchnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik und Kommunikationsnetze der Technischen Universität Braunschweig, Germany. VIRTIS was built by a consortium from Italy, France, and Germany, under the scientific responsibility of IAPS, Istituto di Astrofisica e Planetologia Spaziali of INAF, Rome, which lead also the scientific operations. The VIRTIS instrument development for ESA has been funded and managed by ASI (Italy), with contributions from Observatoire de Meudon (France) financed by CNES and from DLR (Germany). The VIRTIS instrument industrial prime contractor was former Officine Galileo, now Finmeccanica in Campi Bisenzio, Florence, Italy. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged. LESIA, Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France e-mail: antonella.barucci@obspm.fr INAF-IAPS, 00133 Rome, Italy Univ Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris Cedex 13, France Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg, 3 37077 Göttingen, Germany Physikalisches Institut, Sidlerstrasse 5, University of Bern, CH-3012 Bern, Switzerland Dipartimento di Geoscienze, University of Padova, Italy Observatoire des Sciences de l'Univers, Besançon, France Department of Physics and Astronomy "G. Galilei", University of Padova, Vic. Osservatorio 3, 35122 Padova, Italy Laboratoire d'Astrophysique de Marseille UMR 7326, CNRS & Aix Marseille Université, 13388 Marseille Cedex 13, France Centro de Astrobiología, CSIC-INTA, 28850 Torrejón de Ardoz, Madrid, Spain International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland Research and Scientific Support Department, European Space Agency, 2201 Noordwijk, The Netherlands Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden PAS Space Reserch Center, Bartycka 18A, 00716 Warszawa, Poland Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, 38106 Braunschweig, Germany Department for Astronomy, University of Maryland, College Park, MD 20742-2421, USA Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany LATMOS, CNRS/UVSQ/IPSL, 11 Boulevard d'Alembert, 78280 Guyancourt, France INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy Department of Information Engineering -University of Padova, Via Gradenigo 6, 35131 Padova, Italy JPL, 4800 Oak Grove Drive, Pasadena, CA91109, USA Dipartimento di Fisica, Universita del Salento, Italy INAF -Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy Institute for Space Science, National Central University, 32054 Chung-Li, Taiwan ESA/ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain Instituto de Astrofísica de Andalucía -CSIC, 18080 Granada, Spain NASA Ames Research Center, CA 94035, USA UJF-Grenoble 1/CNRS-INSU, France Article number, page 13 of 13page.13

Closest encounter 12.08.1978 Internatioanl Cometary Explorer (ICE) * , NASA 21P/Giacobini-Zinner 11.09.1985 at 7850 km 1P/Halley 28.03.1986 at 28x10 6 km Halley Armada 15.12.1984 Vega 1, USSR/France 1P/Halley 06.03.1986 at 8889 km 21.12.1984 Vega 2, USSR/France 1P/Halley 09.03.1986 at 8030 km 07.01.1985 Sakigake, ISAS-Japan 1 1P/Halley 11.03.1986 at 6.99x10 6 km 18.08.1985 Seisei, ISAS-Japan 1P/Halley 08.03.1986 at 151x10 3 km 02.07.1985 Giotto, ESA 1P/Halley 13.03.2016 at 596 km 26P/Grigg-Skjellerup 10.07.1992 at 200 km 24.10.1998 Deep Space 1, NASA 19P/Borelly 22.09.2001 at 2171 km 07.02.1999 Stardust, NASA 81P/Wild 02.01.2004 at 237 km Stardust-NExT † , NASA 9P/Tempel 15.02.2011 at 181 km 03.07.2002 COmet Nucleus TOUR (CONTOUR), NASA

Appendix A: Catalogue of exposed bright features

has remained with the lowest reflectance in both observations. This observation could be explained in the way that there was increased cometary activity during the perihelion passage (2015 August) that occurred between the two observational periods and that it has lifted off some of the dust sitting on the cometary surface (Fornasier et al. 2016). Fresh material with higher reflectance could have thus been exposed in the smooth terrain during the perihelion passage, as observed in 2016 February. On the other hand, rugged terrain, composed of condensed cometary material, does not seem to have been affected, as it is probably resistant to the same kind of cometary activity that refreshed the surface of smooth terrains. We computed the spectral slopes (see Table 2) for each of the four pre-defined surface features, integrating the signal over the boxes of identical pixel dimensions, following the definition in equation (4), where R882.1 and R535.7 are radiance factors for a given pixel for the corresponding filters (centred at 882.1 and 535.7 nm). We compute the mean spectral slope value from the integrated flux over a given box, excluding the contribution from pixels located in shadows. Generally, spectral slopes are helpful to quantify and compare colour variations of surfaces. For instance, they have been used to explore global colours of the comet (Fornasier et al. 2015), to analyse the selected landing site Agilkia (La Forgia et al. 2015) for the Philae lander, to study the landing site candidate A (Pajola et al. 2015), to characterise the landing site Abydos (Lucchetti et al. 2016) and to carry out a variegation analysis of the active regions of the comet (Oklay et al. 2016). The spectral slope values for each of the above features are presented mainly because of the local heterogeneities of the terrain. A general estimate for entire terrain is given in Table 2 for the sake of the statistics, which should be given special care due to the plethora of surface features in the Khonsu region and we would encourage the use of local individual spectral slope values when comparing.

SpectralSlope[ per cent/100 nm]

With the obtained spectral slope values for the given phase angles, we can associate Khonsu region to the 'average spectral group' among the three spectral slope groups introduced by Fornasier et al. (2015). This average spectral slope group is defined between 14 and 18%/(100 nm) at a phase angle around 50 • , and the Anuket Spectrophotometry of the Khonsu region on 67P S279 Figure 3. RGB images featuring Khonsu region and some of its neighbourhood. For the red, green and blue colours, OSIRIS NAC images with filter wavelengths centred at 882.1, 649.2 and 480.7 nm have been used, respectively. Some images have been cropped to better highlight the details pertaining to Khonsu region. The red and green features that sometimes appear adjacent to the shadows are artefacts arising from the imperfect co-registration/receding shadows of the images.

the used NAC image presented the possibility to derive meaningful statistics for boulders with diameters ≥7 m, the 3 pixels sampling that minimises the likelihood of misidentifications of what we are observing (Nyquist 1928). Since the observations were performed with a phase angle α ∼ 62 • , the presence of elongated shadows on the surface aided with the identification of even smaller boulders (2 pixels diameter, ∼4-5 m) (Pajola et al. 2015(Pajola et al. , 2016a)). We did not include these smaller populations in the cumulative size-frequency distribution (SFD), because they do not represent a complete data set for such small sizes. Before performing the boulder counting, we defined the positive reliefs detectable in multiple images as 'boulders', obtained with different observation geometries with the constant presence of an elongated shadow (if α > 0 • ) whose extension depends on the illumination geometry (Pajola et al. 2015(Pajola et al. , 2016a)). We then manually identified such features on the image using the ARCGIS software as done in Pajola et al. (2016a) and measured their position on the surface of the comet, assuming a spherical shape. We present here a physiographic boulder analysis of the Khonsu region for the boulders extracted from the entire boulder data set of Pajola et al. (2016b). Eventually, to obtain the cumulative boulder SFD per km 2 , we used the corresponding area of 1.705 km 2 computed from the 3D shape model of the comet (Jorda et al. 2016).

The total number of identified boulders ≥7 m is 471 (69 being in the 6 m bin, 49 in the 5 m bin, 27 in the 4 m bin and 6 boulders in the 3 m bin). We consequently plotted the cumulative binned number of boulders versus boulder size in metres (Fig. 5). We obtained the power-law index of -3.1 +0.2/-0.3, by fitting a continuous regression line to the data in the range of 7-40 m. The cumulative number of boulders ≥7 m per km 2 derived on this area is 276, which is 2.8 times the number derived on the entire northern and equatorial regions of the comet (Pajola et al. 2015), while completely comparable to the value obtained on the Southern hemisphere (Pajola et al. 2016b). In order to understand what this index reveals about the possible processes that produced and modified the Spectrophotometry of the Khonsu region on 67P S281 Figure 6. Four bright patches observed in Khonsu region and their spectrophotometric plots. For each bright patch, the leftmost panel shows the NAC reference image used for the analysis, along with a yellow square indicating the surface area used for a zoomed-in view as illustrated in the second panel from left. The third panel from the left shows the reflectance of the bright feature in red for the observed OSIRIS NAC filters, whereas the reflectance values of the typical nucleus extracted from the vicinity of the feature are indicated in black. The images used for this analysis are corrected for the illumination conditions using Lommel-Seeliger disc law. The rightmost panel presents the relative spectral behaviour with normalisation at the green filter centred at 535.7 nm. We remark that the absolute reflectances given in the plots for the bright patches 1, 2 and 4 are underestimated due to the saturation of few centre pixels resulting from high signal. After thorough scrutiny of individual pixels, the sampling for these three patches has been performed along their perimeters to avoid any contamination from saturated pixels. Therefore, the curves in question should be regarded as an indicative lower limit. Table 3. Observational conditions for the four bright spots shown in Fig. 6. The column S/R refers to the spatial resolution and the southern latitudes are given with negative sign.

ID

Observation time Altitude (km) Spectrophotometry of the Khonsu region on 67P

S285

measurement and hence the plot should be considered only as an indicative lower limit. Even at saturation, the signal of bright patches was greater than that of the nucleus by a factor larger than 6. This means that the real factor would have been even larger. We recall that the normal albedo of 67P/C-G to be 6.8 per cent at 649 nm (Fornasier et al. 2015) and assuming a similar phase function between the dark terrain of the comet and the observed bright patches, we could estimate a normal albedo for the bright patches to be >40 per cent. Combining this estimation with the flat behaviour of the spectrophotometric curves, we could conclude that the source of these bright patches is plausibly freshly exposed H 2 O ice.

C O N C L U S I O N S

We demonstrated that the composition of the Khonsu region is largely dominated by the desiccated refractory material with subtle spectrophotometric differences. The studied surface features, except the bright patches, show a red spectral behaviour, similar to what has been seen on the most of the other regions on the comet. General spectrophotometric characteristics of the Khonsu region relate to those of Anuket and Serqet regions, in comparison to the spectral slope work of Fornasier et al. (2015). We also note, in comparison with the work done by Fornasier et al. (2015) and Oklay et al. (2016), the presence of bluish material on Khonsu at different observation periods, relating to the spectrophotometric behaviour of Hapi region in the pre-perihelion period. Mainly dominated by H 2 O ice, this bluish material has been observed more abundantly throughout the comet after the perihelion passage.

Our boulder analysis yields a power-law index of -3.1 +0.2/-0.3 that reflects a boulder formation shaped by varying geological processes for different morphological units. Present morphology of Khonsu region stands true to this by embodying a multitude of heterogeneous surface features. A higher resolution boulder identification analysis reaching minimum boulder dimensions close to 1-2 m is foreseen in the future, in order to extend this interpretation and to have wider statistics and focus on more localised areas inside this highly complex region of the comet.

We detected four bright patches associated with the presence of H 2 O ice and two of them have shown long-term survivability, considering their location and size. We report that the larger bright patches under investigation have a life span of at least a couple of months, whereas the smaller ones last for a few weeks or fewer. We observe variations in the size of the two larger bright patches, with their sizes first increasing and then decreasing because of the diurnal and seasonal insolation conditions. The long-term survivability of the bright patches can be explained by the existing sub-surface ice layers, which could sustain them despite the ongoing sublimation. We also remark the estimation of >40 per cent of normal albedo for some bright patches, suggesting that they were composed of freshly exposed H 2 O ice. The current spectrophotometric study of the bright patches and the terrain could be extended with the available observations and high-resolution images recorded during the later phases of the mission. The Southern hemisphere of the 67P/Churyumov-Gerasimenko comet has become visible from Rosetta only since 2015 March. It was illuminated during the perihelion passage and therefore it contains the regions that experienced the strongest heating and erosion rates, thus exposing the sub-surface most pristine material. In this work we investigate, thanks to the OSIRIS images, the geomorphology, the spectrophotometry and some transient events of two Southern hemisphere regions: Anhur and part of Bes. Bes is dominated by outcropping consolidated terrain covered with fine particle deposits, while Anhur appears strongly eroded with elongated canyon-like structures, scarp retreats, different kinds of deposits and degraded sequences of strata indicating a pervasive layering. We discovered a new 140 m long and 10 m high scarp formed in the Anhur-Bes boundary during/after the perihelion passage, close to the area where exposed CO 2 and H 2 O ices were previously detected. Several jets have been observed originating from these regions, including the strong perihelion outburst, an active pit and a faint optically thick dust plume. We identify several areas with a relatively bluer slope (i.e. a lower spectral slope value) than their surroundings, indicating a surface composition enriched with some water ice. These spectrally bluer areas are observed especially in talus and gravitational accumulation deposits where freshly exposed material had fallen from nearby scarps and cliffs. The investigated regions become spectrally redder beyond 2 au outbound when the dust mantle became thicker, masking the underlying ice-rich layers.

Key words: methods: data analysis -methods: observational -techniques: photometriccomets: individual: 67P/Churyumov-Gerasimenko.

I N T RO D U C T I O N

The Rosetta spacecraft has orbited comet 67P/Churyumov-Gerasimenko (67P) for more than 2 yr providing the unique oppor-E-mail: sonia.fornasier@obspm.fr tunity to continuously investigate its nucleus, activity and evolution from a heliocentric distance of 4 au inbound, through its perihelion passage (1.24 au), then up to 3.5 au outbound. A large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted were hosted on board Rosetta. The Optical, Spectroscopy and Infrared Remote Imaging System (OSIRIS) instrument is the scientific camera system of the The Anhur-Bes regions on 67P comet S95 Table 1. Observing conditions for the NAC images used in this study. The time refers to the start time of the first image in the case of multiple filter observations. Filters: F22 (649.2 nm), F23 (535.7 nm), F24 (480.7 nm), F16 (360.0 nm), F27 (701.2 nm), F28 (743.7 nm), F41 (882.1 nm), F51 (805.3 nm), F61 (931.9 nm), F71 (989.3 nm) and F15 (269.3 nm)

The Anhur-Bes regions on 67P comet S97 of the underlining consolidated material (Fig. 2). In some cases even gravitational accumulation deposits at the scarp feet can be intermixed with altered outcropping material with evident breaks and voids (Fig. 3). This might suggest an in situ degradation more effective than previously supposed. Fine materials, occasionally associated with some isolated boulders (the major ones, larger than 18 m, even resolved within the maps) are scattered throughout the analysed regions and preferentially deposited on flat terrains. The thickness and extension of such deposits could be variable and in some cases they are thin enough to let the underlined consolidated material be easily inferred. The fine material deposits are likely formed by airfalls (Thomas et al. 2015b;Lai et al. 2016). Boulders can be erratic, i. Scarps are often complicated by arch-shaped niches (Fig. 4). They could be simply crown areas of gravitational falls or remnants of former pits.

C O M P O S I T I O N A N D C O L O U R C H A N G E S

Large exposed water-ice-rich patches

Two bright patches of about 1500 m 2 each were revealed in the Anhur and Bes regions (Fig. 5), near their boundary (Fornasier et al. 2016). These areas are 4-6 times brighter than the comet dark terrain and they have a spectral behaviour completely different from that of the nucleus dark terrain. Their spectrophotometry is flat and consistent with a water ice abundance of about 20 per cent (patch B, Fig. 5) and 32 per cent (patch A), according to the linear mixing model presented in Fornasier et al. (2016). This water ice abundance is considerably higher than the few per cent measured locally by the VIRTIS instrument (De Sanctis et al. 2015, Barucci et al. 2016;Filacchione et al. 2016a). The bright patches appeared for the first time on images acquired on 2015 April 27 (Fig. 5), and they were still clearly visible on 2015 April 29 and May 2 images. Patch A in Fig. 5 Subsequent images covering the region of interest were acquired only on 2015 June 5, revealing that the water ice patches had completely sublimated and that the spectrophotometry of those regions previously covered by the bright patches was totally indistinguishable from that of the surrounding comet dark terrain (see fig. 2 in Fornasier et al. 2016 for the spectrophotometric evolution of these surfaces).

The bright water-ice-rich regions were located on a flat terrace bounded by two 150 m high scarps (Fig. 2). The overall structure suggests a strata plane covered by different deposits. In particular, the bright patches were formed on a smooth terrain made up of a thin seam of fine deposits covering the consolidated material, which, indeed, outcrops nearby and displays a polygonal network of fractures. Chaotic and heterogeneous terrains classified as gravitational accumulation or diamictons surround the fine materials. Diamictons in this location are more probably the result of in situ differential erosion on the consolidated material since on high-resolution images these deposits are strictly intermixed with outcropping materials.

In order to estimate the amount of water ice present in the bright patches we performed thermal modelling of the Anhur-Bes region. The optical and thermal properties of the surface are strongly affected by the type of mixing of the refractory material with the ice. Because the type of material mixing on a sub-resolution scale has MNRAS 469, S93-S107 (2017) The Anhur-Bes regions on 67P comet S99 Table 2. Thermal modelling for the region containing the two bright icerich patches (see Fig. 5). A B is the bond albedo, lat s the sub-solar latitude.

The Anhur-Bes regions on 67P comet S101

J E T S A N D AC T I V I T Y

Anhur is one of the most active regions on the cometary surface, and in particular it is the source of the perihelion outburst (Vincent et al. 2016a), the most intense activity event on the 67P comet recorded by Rosetta observations. Fig. 11 shows this spectacular event that we observed on 2015 August 12 at UT 17:20, with intensity fading out in half an hour. Other jets departing from the Anhur region were observed on 2015 August 30, shortly after the perihelion passage, but also several months later, on 2016 January at a heliocentric distance of 2.23 au outbound (Fig. 11). Table 3 gives the position, in longitude and latitude, of the region on the nucleus where the jets originated. Notably on 2016 January 27 images, diffuse jets are seen departing from several sources inside the Anhur region (UT 15:20). The activity progressively decreased. In the following couple of hours, two localized jets, close but originating from different positions, are observed near the Anhur and Aker boundary. In particular, the one seen on the image acquired at 17:27 (Fig. 11B) produced a dust plume casting a shadow on the surface. The plume was about 1.7 times brighter than the surrounding regions. Comparing the reflectance of the terrain before and during the shadow cast by the plume, we measure a decrease of 35 per cent of the reflectance in the shadow. This implies that the plume is optically thick, with an estimated optical depth of ∼0.43.

The Southern hemisphere of the comet has fewer pits compared to the Northern one. The Seth region in the Northern hemisphere is the most enriched of this kind of morphological structure, and some of them have been shown to be active (Vincent et al. 2015). The Anhur region shows the only pit proven to be active in the Southern hemisphere. It is a pit, 100 m across, whose floor is covered by very

The Anhur-Bes regions on 67P comet S103 Table 3. Locations of the jets seen in Fig 11. For the 2016 January 27 UT 15:20 image the approximate positions of the two brightest jets are given. Other fainter jets are visible but their source region on the nucleus is not easy to be clearly identified.

The Anhur-Bes regions on 67P comet S105 The Anhur-Bes regions on 67P comet S107 Dr. E. Howell for her comments and suggestions which helped us to improve this article.

Cite as: S. Fornasier et al., Science 10.1126Science 10. /science.aag2671 (2016)).

REPORTS

First release: 17 November 2016 www.sciencemag.org (Page numbers not final at time of first release) 1

All cometary nuclei observed to date have appeared to be dark, with only a limited amount of water ice detected in small patches (1,2), although water is the dominant volatile observed in their coma.

The Rosetta spacecraft has been orbiting comet 67P/Churyumov-Gerasimenko since August 2014, providing the opportunity to continuously investigate its nucleus. The comet has a distinct bi-lobate shape and a complex morphology (3)(4)(5), with a surface dominated by anhydrous and organic-rich refractory materials (6). Small amounts of water ice have been identified by the VIRTIS (Visible, Infrared and Thermal Imaging System) spectrometer in different regions (7)(8)(9), and several isolated or clustered bright spots were observed in OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) images and interpreted as exposures of dirty water ice (9,10).

Here we report on seasonal and diurnal color variations of the surface of 67P's nucleus from observations with the Narrow Angle Camera (NAC) of the OSIRIS imaging system (11) onboard Rosetta as caused by the evolution of the dust mantle and exposures of water ice.

We monitored the color evolution of the nucleus from 3.6 AU to perihelion (1.24 AU) and beyond by measuring changes in the spectral slope in the 535-882 nm range.

Rosetta's comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature Map of comet 67P/Churyumov-Gerasimenko, resulting from merging a more detailed shape model SHAP4S (Preusker et al. 2015) for the northern hemisphere and shape model SHAP5 (Jorda et al. 2016) for the southern hemisphere. In red the selected bright spots are reported, based on OSIRIS images and a spectro-photometric analysis, considered as good targets to be investigated by an analysis of VIRTIS data, plus the two bright spots analysed by Filacchione et al. (2016a). The numbers (1)(2)(3)(4)(5)(6)(7)(8) represent the spots with positive detection of H 2 O ice by VIRTIS analysis discussed in this paper.

Table 1. Observing conditions for the OSIRIS images as reported in Fig. 2, where the ice spots have been identified. The time (UT) refers to the start time of the first image of each sequence, followed by the number of filters available. The diameter size (d) of the spots along with the location region, phase angle (α), distance between Rosetta spacecraft, and comet surface (∆), spatial resolution (R), latitude (Lat), and longitude (Long) are reported.

where i is the incident angle, e is the emission angle, and α is the phase angle.

Next we proceeded to generate synthetic images to correct for the illumination conditions and observational geometry using the OASIS tool (Jorda et al. 2010) and the shape model SHP5 (Jorda et al. 2016). This yields incident and emission angles for each pixel, which enables us to apply the Lommel-Seeliger disk law

Derived from I/F plots we also produced relative reflectance plots with radiance factors normalized to a given filter (i.e. F23, green filter at 535 nm), enabling us to get an insight into the composition of the areas sampled for this analysis. As the ice displays a flatter spectrum in comparison to the red nature of Article number, page 3 of 13page. 13A&A proofs: manuscript no. 28764_MB_checked computation is SHAP5, derived from OSIRIS images by using a stereophotoclinometry method (Jorda et al. 2016). The coordinates grid is based on the Cheops frame (Preusker et al. 2015). Some examples of geometry parameters computed for VIRTIS-M nucleus observations are given in Filacchione et al. (2016b). The geometry information relative to each pixel is therefore ingested in a database.

Search for the 2 µm absorption feature

Using the method described above, the search of VIRTIS-M pixels located in correspondence with the bright spot coordinates identified on OSIRIS images was performed. As a general rule, we selected all VIRTIS-M pixels within a radius of 2 • in longitude and latitude around the position estimated on the OSIRIS images. The corresponding reflectance spectra are grouped together to form a spectrogram (Filacchione et al., 2016a), one for each MTP, and then further processed by calculating the 2 µm band depth, used as a proxy to determine the presence of H 2 O ice on the surface in the case of values that were larger than a 5% threshold. Water ice reflectance shows diagnostic absorptions at 1.5, 2.0 and 3.0 µm. The decision to use only the 2 µm band as a proxy to identify the presence of water ice on 67P/C-G surface was driven by two different requirements: i) the 1.5 µm band is partially corrupted by the presence of an instrumental order-sorting filter, which makes it difficult to retrieve a correct band shape, particularly for pixels close to sharp illumination transitions and shadows; ii) the intense 3 µm band has a complex shape owing to the overlapping of the water and organic material absorptions, which causes changes in shape, center, and depth, depending on the relative abundances of the two end-members.

Conversely, the 2 µm spectral range is not influenced by similar effects. Moreover, the 2 µm band is well-defined for a wide range of grain sizes, making it a good spectral marker to identify the presence of water ice. As a general rule, the detection of the H 2 O ice on a given place can be limited by unfavorable instrumental signal-to-noise conditions, spatial resolution on ground and oblique illumination/viewing geometry. A summary of the observations showing a positive identification of the 2 µm H 2 O ice-band feature is given in Table 2. Infrared color images of the eight spots are shown in Fig. 4, together with the observed reflectance spectra and best spectral fits, as discussed later in Section 4. In absence of water ice, VIRTIS spectra correspond with the Dark Terrain unit, as reported in Fig. 3, which shows a featureless red slope in the 1-2.6 µm range and an intense organic material absorption band at 3.0 µm.

Temperature of the icy spots

For each spot we identified VIRTIS-M observations showing the eight spot areas imaged with the best illumination and viewing geometries to maximize the S/N ratio. The VIRTIS-M dataset considered in this work is summarized in Table 3.

Surface temperature is derived from VIRTIS-M infrared data by modeling the 4.5-5.1 µm spectral radiance (at the pixel where the spectrum has the deepest content of H 2 O ice) with a Bayesian approach (Tosi et al., 2014). On the surface of the nucleus, the temperature of each point is generally a function of local thermophysical properties (albedo, composition, grain size, roughness, thermal conductivity, volatiles sublimation) and instantaneous illumination conditions (solar incidence angle, or true local solar Article number, page 6 of 13page.13 For each spot, we report the VIRTIS-M observed reflectance (black curve), not corrected for phase angle, and best fit (red curve) as derived from the pixels reported in Table 3 and indicated by red circles on the images. The gaps in the spectral ranges correspond to order-sorting filter-junction wavelengths that can produce unreliable features and, for this reason, are not taken into account in the analysis.

Article number, page 7 of 13page. 13A&A proofs: manuscript no. 28764_MB_checked time). All measurements considered in this work were acquired by VIRTIS-M in the local solar timeframe between late morning to early afternoon, with pixel resolution ranging between 5.0 and 23.5 m/pixel. In these conditions, H 2 O ice-rich spots show temperatures ranging between about 158 and 218 K. Since the comet's surface is not isothermal, because of local roughness, the temperature values retrieved by VIRTIS should be considered representative only of the warmest fraction of the pixel, corresponding to the more illuminated areas. Moreover, the instrumental noise-equivalent temperature is about 150 K, corresponding to the minimum temperature detectable by the instrument. The error associated with the temperatures reported in Table 3 are between ±30 K for the measurement at minimum temperature T=158 K, and ±10 K for the one at maximum T=218 K.