Flahaut Jessica
email: jessica.flahaut@univ-lorraine.fr

Janice L Bishop

Simone Silvestro

Dario Tedesco

Isabelle Daniel

Damien Loizeau

Dr Jessica Flahaut

The Italian Solfatara as an analog for Mars fumarolic alteration

Keywords: Mars analog, hydrothermalism, vents, fumaroles, alteration patterns, Solfatara, VNIR spectroscopy, Raman spectroscopy, XRD

The first definitive evidence for continental vents on Mars is the in situ detection of amorphous silica-rich outcrops by the Mars Exploration Rover Spirit. These outcrops have been tentatively interpreted as the result of either acid sulfate leaching in fumarolic environments or direct precipitation from hot springs. Such environments represent prime targets for upcoming astrobiology missions, but remain difficult to identify with certainty, especially from orbit. In order to contribute to the identification of fumaroles and hot spring deposits on Mars, we surveyed their characteristics at the analog site of the Solfatara volcanic crater in central Italy.

Several techniques of mineral identification (VNIR spectroscopy, Raman spectroscopy, XRD) were used both in the field, and in the laboratory on selected samples. The faulted crater walls showed evidence of acid leaching and alteration into the advanced argillic-alunitic facies, with colorful deposits containing alunite, jarosite and/or hematite. Sublimates containing a variety of Al and Fe hydroxyl-sulfates were observed around the active fumarole vents at 90°C. One vent at 160°C was characterized by different sublimates enriched in As and Hb sulfide species. Amorphous silica and alunite assemblages that are diagnostic of silicic alteration were also observed at the Fangaia mud pots inside the crater. A wide range of minerals was identified at the 665 m diameter Solfatara crater that is diagnostic of acid-steam heated alteration of a trachytic, porous bedrock. Importantly, this mineral diversity was captured at each site investigated with at least one of the techniques used, which lends confidence for the recognition of similar environments with the next generation Mars rovers.

Introduction

Hydrothermal systems have often been proposed as a possible cradle for early life (e.g., [START_REF] Shock | Hydrothermal systems as environments for the emergence of life[END_REF][START_REF] Nisbet | The habitat and nature of early life[END_REF] and therefore represent a prime astrobiology target. Hydrothermal environments have long been presumed to exist on Mars based on orbital observations, terrestrial analogs, and martian meteorites (e.g., [START_REF] Farmer | Hydrothermal systems on Mars : an assessment of present evidence[END_REF][START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF][START_REF] Carr | Geologic history of Mars[END_REF]. Evidence for warm and wet environments include the orbital detections by recent VNIR spectral-imagers of a wide range of hydrated minerals [START_REF] Bibring | Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data[END_REF][START_REF] Murchie | A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter[END_REF][START_REF] Carter | Hydrous minerals on Mars as seen by the CRISM and OMEGA imaging spectrometers: Updated global view[END_REF]. In particular, the detection of chlorite, prehnite, epidote, serpentine and carbonates evidences aqueous alteration of Mars basaltic crust at high temperatures (e.g., [START_REF] Murchie | A synthesis of Martian aqueous mineralogy after 1 Mars year of observations from the Mars Reconnaissance Orbiter[END_REF][START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF][START_REF] Carter | Hydrous minerals on Mars as seen by the CRISM and OMEGA imaging spectrometers: Updated global view[END_REF]. Alteration by hydrothermal circulation in the cooling crust and impact-generated hydrothermal alteration have both been proposed as a plausible formation mechanism (e.g., [START_REF] Abramov | Impact-induced hydrothermal activity on early Mars[END_REF][START_REF] Carter | Detection of hydrated silicates in crustal outcrops in the northern plains of Mars[END_REF][START_REF] Carter | Hydrous minerals on Mars as seen by the CRISM and OMEGA imaging spectrometers: Updated global view[END_REF][START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF][START_REF] Ehlmann | Subsurface water and clay mineral formation during the early history of Mars[END_REF][START_REF] Carr | Geologic history of Mars[END_REF]. Hydrothermalism is also invoked as a likely source for alteration in localized surface environments (fumarolic fields or hot springs), leading to the formation of the clays, sulfates and silica-rich deposits identified in Valles Marineris Chasma [START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF][START_REF] Thollot | Most Mars minerals in a nutshell: Various alteration phases formed in a single environment in Noctis Labyrinthus[END_REF], the silica detections in the Nili Patera caldera [START_REF] Skok | Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars[END_REF] or the serpentine and Mg-carbonate detections in Nili Fossae [START_REF] Viviano | Implications for early hydrothermal environments on Mars through the spectral evidence for carbonation and chloritization reactions in the Nili Fossae region[END_REF]. However, the first definitive evidence for volcanic hydrothermal activity (continental vents) on Mars is the in situ detection of amorphous silica-rich outcrops (>90% wt opal-A) by the Mars Exploration Rover (MER) Spirit Miniature Thermal Emission Spectrometer (Mini-TES) instrument [START_REF] Squyres | Detection of silica-rich deposits on Mars[END_REF]. The detection of abundant opaline silica at Home Plate, combined with high Ti content in local soils, was interpreted as an evidence for the dissolution of basaltic soils by low pH fluids [START_REF] Squyres | Detection of silica-rich deposits on Mars[END_REF]. Iron and phosphate-rich soils detected nearby, at Paso Robles, were also interpreted as acid-sulfate alteration products in a hydrothermal (possibly fumarolic) environment of primary phosphate-rich materials [START_REF] Hausrath | Acid sulfate alteration of fluorapatite, basaltic glass and olivine by hydrothermal vapors and fluids: Implications for fumarolic activity and secondary phosphate phases in sulfate-rich Paso Robles soil at Gusev Crater, Mars[END_REF]. The nodular aspect of the Home Plate outcrops led to an alternative suggestion that the Gusev silica could originate from the direct precipitation of sinters as observed in terrestrial hot springs [START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF][START_REF] Ruff | Evidence for a Noachian-aged ephemeral lake in Gusev crater[END_REF][START_REF] Ruff | Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile[END_REF]. The objective of this study is to characterize in detail the fumarolic environments (and alteration patterns) at the Solfatara as a Mars analog site (Campi Flegrei, Italy), and discuss the possibilities and limits of identifying them on Mars. This specific analog site is used to show how trachytic rocks at the volcanic crater were hydrothermally altered into sulfate and silica-rich assemblages. Nevertheless, a variety of alteration assemblages can be produced via fumarolic alteration depending on several factors (e.g., composition of the bedrock, composition of the fluids). Therefore, the Solfatara site may be analogous to a some martian hydrothermal environments, which are likely to be more diverse.

Geologic setting

The Solfatara volcano is the most active site of the Campi Flegrei (CF) caldera, a quaternary volcanic complex located north of the city of Naples, in the Campanian plain graben (Figure 1).

Straddling between the Tyrrhenian Sea to the west and the Apennines to the east, the Campanian plain is thought to originate from the stretching and thinning of the continental crust in an extensional back-arc context. The Campanian plain region is characterized by a general tensional NE-SW and NW-SE tectonic regime, and upwards migrations of magma that resulted in large ignimbrite eruptions starting from 290,000 years ago (e.g., [START_REF] Torrente | Extensional tectonics in the Campania Volcanic Zone (eastern Tyrrhenian Sea, Italy): new insights into the relationship between faulting and ignimbrite eruptions[END_REF]. The largest of the Campanian plain volcano, Campi Flegrei (CF), is a 12km wide, partly submerged depression, inherited from two major collapses during the Campanian Ignimbrite eruption 37,000 years ago and the Neapolitan Yellow Tuff eruption 12,000 years ago (e.g., [START_REF] Rosi | The Phlegrean Fields: structural evolution, volcanic history and eruptive mechanism[END_REF]Piochi et al., 2014 and references therein). The area was later affected by numerous large explosive eruptions that resulted in the formation of more than 20 craters including the 665 m wide, 4200 year old Solfatara volcanic crater (e.g., [START_REF] Armienti | The Phlegraean Fields: magma evolution within a shallow chamber[END_REF][START_REF] Rosi | Phlegrean Fields: Petrography, Quad[END_REF][START_REF] Vito | Volcanism and deformation since 12,000 years at the Campi Flegrei caldera (Italy)[END_REF][START_REF] Pappalardo | Evidence for multi-stage magmatic evolution during the past 60 kyr at Campi Flegrei (Italy) deduced from Sr, Nd and Pb isotope data[END_REF][START_REF] Piochi | The volcanic and geothermally active Campi Flegrei caldera: An integrated multidisciplinary image of its buried structure[END_REF][START_REF] Isaia | Stratigraphy, structure, and volcano-tectonic evolution of Solfatara maardiatreme (Campi Flegrei, Italy)[END_REF]. The CF volcanic complex of potassic alkaline affinity is filled with km-thick pyroclastic deposits, overlapping Miocene flysch facies terrains and Triassic-Paleocene carbonatic outcrops (e.g., [START_REF] Cameli | Geothermal research in western Campania, southern Italy: geological and geophysical results[END_REF]. Previous analyses at the CF Solfatara reported the presence of incoherent products and ignimbrites of trachyte to trachyandesite composition, in addition to localized alteration (e.g., [START_REF] Rosi | The Phlegrean Fields: structural evolution, volcanic history and eruptive mechanism[END_REF].

Current volcanic activity in the CF is expressed through fumarole emissions, thermal pools and passive soil degassing that are most active within the Solfatara crater. According to geophysical measurements, the CF area is underlain by a magma reservoir located at 5 km depth and a Moho interface at 7 -25 km depth [START_REF] Ferrucci | Seismic detection of a major Moho upheaval beneath the Campanian volcanic area, Naples, southern Italy[END_REF][START_REF] Brandmayr | The lithosphere in Italy: structure and seismicity[END_REF]. Combined with recent eruptive activity, this shallow mantle occurrence results in a local high heat flux of up to 200 mW.m -2 (Piochi et al., 2014, and references therein). Shallow intrusions and degassing of magma drive unrest at the Phlegraean fields, and bradyseismic events are frequent in the Pozzuoli area, where the Solfatara crater is located [START_REF] Tedesco | Chemical and isotopic gas emissions at Campi Flegrei: Evidence for an aborted period of unrest[END_REF][START_REF] Caliro | The origin of the fumaroles of La Solfatara (Campi Flegrei, south Italy)[END_REF][START_REF] Piochi | The volcanic and geothermally active Campi Flegrei caldera: An integrated multidisciplinary image of its buried structure[END_REF].

Currently the Solfatara area is one of the most active fumarolic manifestations worldwide, with a diffuse degassing CO2 rate > 1500 ton.day -1 (Caliro et al., 2007, 2014 andreference therein). Gas emissions at the Solfatara include H2O and CO2 with H2S, N2, H2, CH4, He, Ar, and CO as a minor species (e.g., [START_REF] Chiodini | CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy[END_REF][START_REF] Caliro | The origin of the fumaroles of La Solfatara (Campi Flegrei, south Italy)[END_REF]. Fumaroles are distributed along the crater NE and SE faulted rims, whereas mud pots occupy its central region (Figure 1). Stable isotope analyses of the water suggested that the Solfatara fumarolic discharge results from a mixture of CO2-rich fluids degassed from a magma body and the vapor generated at 360°C from hydrothermal fluids [START_REF] Caliro | The origin of the fumaroles of La Solfatara (Campi Flegrei, south Italy)[END_REF]. Underground temperatures between 200 and 300°C were estimated from the mixed rising plume, using geochemical modeling (e.g., [START_REF] Tedesco | Chemical and isotopic gas emissions at Campi Flegrei: Evidence for an aborted period of unrest[END_REF]Sabroux, 1987, Tedesco et al., 1988). Furthermore, S isotopic compositions of H2S from fumaroles argue for a typical magmatic origin [START_REF] Allard | Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei caldera[END_REF], while epithermal minerals support the formation of sulfur species by acidic interaction of H2S with the host rock via oxidation to H2SO4, in a steam-heated environment (e.g., [START_REF] Valentino | Chemical and isotopic compositions of minerals and waters from the Campi Flegrei volcanic system, Naples, Italy[END_REF][START_REF] Piochi | Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry[END_REF].

Sample collection and analyses

Alteration patterns at and around the mud pots (locally named Fangaia) and fumaroles were studied with portable Visible Near InfraRed (VNIR) and Raman spectrometers at 7 sites within the crater during a field campaign in September, 2015 (Figure 1c). Rock samples were collected for laboratory characterization by X-Ray Diffraction (XRD) and chemical analyses (Tables 1234; Figures 1c and2; Flahaut et al., 2017a).

VNIR reflectance spectra were measured onsite with an Analytical Spectral Devices (ASDinc) field spectrometer. The ASDinc Fieldspec 4 Hi-Res instrument collects spectra in the 350 -2500 nm domain (equivalent to 28000 -4000 cm -1 ) with a spectral resolution of 3 to 8 nm. All spectra were acquired in the field with the contact probe (to limit the atmospheric contribution), converted from radiance to reflectance using the ASDinc software and compared with the VNIR spectra of reference minerals from the Bishop spectral library, as well as external sources such as the USGS, RELAB, JPL and CRISM spectral libraries (e.g., [START_REF] Clark | USGS Digital Spectral Library splib06a[END_REF][START_REF] Murchie | Compact reconnaissance Imaging Spectrometer for Mars (CRISM) on Mars Reconnaissance Orbiter (MRO)[END_REF][START_REF] Kokaly | USGS Spectral Library Version 7[END_REF].

Raman spectra were also measured onsite with a portable Raport instrument from the LGL-TPE (Lyon, France). Using a 532 nm -30 mW laser source, the instrument has a sampling resolution of 4-6 cm -1 between 100 cm -1 and 4000 cm -1 . However, given the large amount of amorphous material within the crater, few diagnostic signatures were observed in situ. Additional Raman spectra were collected on specific samples or grains at the LGL-TPE laboratory using an Xplora Raman instrument coupled to an Olympus BX41 microscope and a at 532 nm laser source. Spectra were compared with the reference Raman spectra of minerals in the RUFF libraries and WURM database [START_REF] Downs | The American Mineralogist Crystal Structure Database[END_REF][START_REF] Caracas | The WURM project-a freely available web-based repository of computed physical data for minerals[END_REF].

Semi-quantitative X-Ray diffraction was performed using a Bruker D8 Advance diffractometer equipped with a Cu X-ray source at the Centre de Diffractométrie of the University Lyon 1, France. Samples were dried at 70°C for 48 hours, ground in an agate mortar, and dry sieved to <100 µm prior to measurement. A possible caveat of this preparation in the laboratory is that the phases detected are those observed in ambient laboratory conditions and may not reflect the hydration/structural state of these phases in situ. Random powder measurements were made over a 2-theta range of 3° to 70º. Oriented measurements were also performed on selected samples to enhance the signal originating from the (001) plane of potential clays, but none were detected.

XRD patterns were analyzed and interpreted using the Bruker DIFFRAC. SUITE EVA software, and the fits were performed by comparing the d-spacing values to those in minerals of the International Center for Diffraction Data database and the Crystallography Open Database [START_REF] Kabekkodu | New Powder Diffraction File (PDF-4) in relational database format: advantages and data-mining capabilities[END_REF]Grazulis et al., 2009). The identity and relative mineral abundances were derived from the analyses, using corundum as an external standard. Mineral abundances are expressed as weight percent (wt. %) of mineral species with ~ 5 % accuracy. The degree of crystallinity, DOC, is also calculated from the total areas under the defined crystalline and amorphous components (Table 3).

In addition, major element analyses by ICP-OES, selected trace element analyses by ICP-MS, and S analyses were subcontracted for a selection of rock powder samples (amorphous material of the mud pots and fresher samples from crater walls) at the SARM facility in Nancy, France (Tables 2 and4).

Results

Nature of the bedrock in the crater walls

The local bedrock at Solfatara is comprised of incoherent ignimbrites (pyroclastic fallout, pyroclastic density currents, coarse to fine ash beds) as exposed in the crater walls (e.g., at site L3, Figure 2d), originating from the third and latest subaerial Campanian eruptive period ~ 4500 years ago [START_REF] Isaia | Stratigraphy, structure, and volcano-tectonic evolution of Solfatara maardiatreme (Campi Flegrei, Italy)[END_REF]. With a Si content > 80 wt% and total alkali content of ~0.2-0.3 wt% (Table 2), samples L5R4, L3R1 and L6R1 from the relatively unaltered crater walls plot well in the field of rhyolite in a TAS (Total Alkali vs Silica) diagram. However, it is difficult to find pristine bedrock samples; high Si and Ti concentrations and elevated chemical index of alteration (CIA) indices in these apparently "fresh" ignimbrites suggest that they have been exposed to several episodes of leaching (Table 2). From a mineralogical point of view, all of samples L5R4, L3R1 and L6R1 have a more porous / ashy texture, and are mostly dominated by amorphous Sirich material (> 90 wt%) with minor amounts of anatase (TiO2) and calcite (CaCO3) (Table 3).

More rocky wall samples such as L6R3 and L6R5 are composed of > 85 wt% of the alkali feldspar sanidine (KAlSi3O8) and correspond to trachytic compositions. Native sulfur was not detected in the bedrock samples by XRD, but bulk chemical analyses reveal S contents between 0.3 and 1.4 wt%.

The bedrock material does not show specific Raman or VNIR spectral signature; weak hydration bands are observed in some outcrops (Figure 3a, black spectrum; Flahaut et al., 2017a). The rather symmetric shape of the 1.9 µm band, coupled with the lack of specific Raman signatures, suggests the presence of a hydrated amorphous sulfate, although hydrated silica could also be a possible interpretation (Figure 3a). The rounded spectral features near 1.42, 1.91, and 2.23 m observed for sample L6R1 are strikingly similar to those in opal spectra (C1JB874, McKeown et al., 2011).

Yellow to orange coatings cover the walls locally, which suggests that alteration by sulfur-rich fumarolic gases occurred as they reached the surface and condensed over the porous ignimbrites (sites L2, L3, L6; Figure 2c,d,h,k). Limited outcrops also show dark red (e.g., L6R5, L6R10), grey or turquoise-green colors (e.g., L5R4, L6R6) (Flahaut et al., 2017a). Colorful coatings were abundant along the NE and SE faulted crater walls, where the fluid circulation might be more intense. The paucity of vegetation within the crater defines an area of diffuse degassing much larger than the fumarolic vents and faulted areas. At site L6, a whitish zone delineates a valley cutting the crater rim and the weathered colored wall, suggesting recent erosion and dissolution by rainfall and surficial water runoff (Figure 2k). As sites L2 and L3 are similar to L6 but with a limited extent and a greater vegetation cover, samples from these sites were not analyzed by XRD and ICP-OES.

VNIR spectra of samples from sites L2, L3, L6R1 to R10 are consistent with alunite KAl3(SO4)2(OH)6, jarosite KFe3 3+ (SO4)2(OH)6 and hematite Fe2O3 (Figure 3a). Alunite is identified by features near 1.48, 1.76, and 2.16, 2.31 m and jarosite by features near 0.92, 1.47, 1.85, and 2.26 m (Bishop and Murad, 2005). The VNIR spectrum of hematite includes a shoulder near 0.65 and a band near 0.85-0.88 m that both vary in shape with grain size, but no bands due to H2O or OH (e.g. [START_REF] Morris | Spectral and other physicochemical properties of submicron powders of hematite (a-Fe2O3), maghemite (g-Fe2O3), magnetite (Fe3O4), goethite (a-FeOOH), and lepidocrocite (g-FeOOH)[END_REF][START_REF] Morris | Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to Martian spectral data[END_REF][START_REF] Bishop | Spectroscopic and geochemical analyses of ferrihydrite from springs in Iceland and applications to Mars[END_REF]. XRD analyses detect minor amount of jarosite, alunite, goldichite KFe 3+ (SO4)2•4H2O, alunogen Al2(SO4)3(H2O)12.5H2O and carminite Mg7(SO4)5(OH)4.H2O (Figure 3), but no hematite. This may indicate that the hematite observed by VNIR spectroscopy is poorly crystalline. Raman spectra are consistent with the presence of sanidine remnants, anatase, alunite, jarosite and amorphous material (Figure 4a).

Sediments from the mud pots

CO2 bubbles are continuously released from low temperature mud pools, or mud pots, located at the center of the crater (Site L1, fig. 1c,fig 2a,b). All three bubbling mud pots have a similar pH of ~1.5 and temperature of ~33°C (Flahaut et al., 2017a).

The shallow pools are filled with greyish, muddy deposits (L1R4). XRD analyses reveal the presence of ca. 50 % amorphous material consistent with opaline silica in agreement with the shape and position of the amorphous hump, and with the chemical analyses. Other components include ~ 25 wt% native sulfur and ~ 25 wt% alunite. Major element analyses confirmed the presence of a high abundance of volatile elements including sulfur (Table 4).

The pool ramparts are covered with silica coatings (L1R3Z1), whereas the muddy soil around the pool forms polygonal terrains during the dry season (L1R1), sometimes covered with a yellow coating (L1R2). XRD analyses show that the surrounding rocks contain less amorphous material than the pool material, and that the yellow deposits of sample L1R2 are primarily composed of alunogen + pickeringite (MgAl2(SO4)4•22H2O), with minor K-alum and alunite. Clay minerals were not detected in the XRD patterns. All VNIR spectra are dominated by broad hydration features, consistent with amorphous silica (Figure 3b). The spectral bands are broadened to 1.41-1.48 and 2.16-2.21 m and weak bands are present near 1.76 and 2.31 m, all of which are consistent with a minor content of alunite. Raman spectra are consistent with the presence of native sulfur, sulfates and opal-A (broad bands at 310 and 780 cm -1 ) (Figure 4b).

Deposits from the fumarolic vents

Material of various colors (white, yellow, orange and brown) and textures (fibers, needles, bubbles…) with high crystalline contents (> 85% of the total phases) were collected from within the vents at sites L5 and L6 at 90 °C (Fig. 2g, j; Flahaut et al., 2017a). VNIR analyses reveal the presence of hydrated materials, likely including Al and Fe-rich sulfates (multiple absorptions between 0.86-0.93 and 2.18-2.32 µm are observed; Figure 3c). Most spectra contain a strong water absorption feature that dominates the spectra at wavelengths > 1.8 µm, which may indicate the presence of highly hydrated sulfates such as alunogen or K-alum. However, no unique spectral matches could be found in the reference libraries. This is likely also because of the sensitivity of such hydrated phases to environmental conditions and their ability to adsorb water near steam vents that is saturating their VNIR spectra. Samples R13 and R18 (site L6) show an absorption band at 0.77 µm that is, coupled with features at 0.55, 1.48 and 1.97 µm, consistent with coquimbite Fe2 3+ (SO4)3•9H2O. Orange-colored samples R15 and R19 have a diagnostic 0.87 µm iron absorption band similar to that of hematite, copiapite or ferricopiapite spectra. XRD analyses revealed the presence of a variety of Al, K and Fe-sulfates (mostly alunogen, K-alum KAl(SO4)2•12H2O, alunite, mercallite KHSO4, goldichite, jarosite, coquimbite, plus traces of yavapaiite KFe(SO4)2, millosevichite Al2(SO4)3, tamarugite NaAl(SO4)2•6H2O, mixed with minor Fe-oxides (e.g., ferryhydrite <2%)). Unfortunately, many of these minerals do not have matches in the existing VNIR spectral libraries. These minerals are deposited in and around the vent within ~1 m. These observations suggest that our samples correspond to fumarolic incrustations and/or sublimated minerals; the former produced by gas cooling and condensation, while the latter formed directly through vapor deposition processes (e.g., [START_REF] Africano | Acid alteration in the fumarolic environment of Usu volcano, Hokkaido, Japan[END_REF][START_REF] Aguilera | Hydrothermal alteration, fumarolic deposits and fluids from Lastarria Volcanic Complex: A multidisciplinary study[END_REF].

A distinct variety of minerals was observed at the higher temperature vent at site L4 (gas emissions at 160°C, Figure 2e,f; Table 4). Realgar (AsS), Cinnabar (HgS) and Orpiment (As2S3) were identified thanks to their respective burgundy, red and yellow colors. These colorful minerals are only present as thin surficial coatings (sublimates), while the bulk of these rocks appear to be comprised of the same wall-rock as elsewhere. Spectra of these colorful minerals contain visible-region electronic absorptions corresponding to their colors, and they should not exhibit NIR spectral features since their structures do not include either H2O, OH, or SO4 (e.g., [START_REF] Cloutis | Spectral Reflectance (350-2500 nm) Properties of Historic Artists' Pigments. II. Red-Orange-Yellow Chromates, Jarosites, Organics, Lead(-Tin) Oxides, Sulphides, Nitrites and Antimonates[END_REF]. Cinnabar visible spectra are characterized by a steep absorption edge at 590 nm and no diagnostic absorption features longward of this edge to 2500 nm. Realgar and orpiment spectra are similar to that of cinnabar, but with absorption edges near shorter wavelengths, consistent with their more orange to yellow colors. Weak absorptions in the USGS library spectra of these minerals are attributed to adsorbed water. Strong VNIR OH and H2O bands similar to those of silica sinters from the Yellowstone hot springs were observed in spectra of the L4 white samples, as well as bands near 2.03 and 2.16 µm in the L4 yellow samples, which may indicate the presence of amorphous As2S3, and hydrated amorphous FeAs species (Figure 3d; Bishop et al., 2004a;[START_REF] Henke | Arsenic: environmental chemistry, health threats and waste treatment[END_REF]. Raman spectra also lack references for comparison but are consistent with As-bearing species (Figure 4c). XRD or ICP-OES / ICP-MS analyses were not be performed due to the potential toxicity of the samples.

Discussion

Mineralogical detections and alteration

A large variety of minerals have been detected in the Solfatara crater samples, in agreement with previous studies (e.g., [START_REF] Sgavetti | Two geologic systems providing terrestrial analogues for the exploration of sulfate deposits on Mars: Initial spectral characterization[END_REF][START_REF] Valentino | Chemical and isotopic compositions of minerals and waters from the Campi Flegrei volcanic system, Naples, Italy[END_REF]. Remnants of sanidine in some wall samples are consistent with the presumed, original trachytic composition of the rock. Less competent ash layers are consistent with a high proportion of amorphous material. Calcite detections in the wall samples at L3 and L5 could originate from the underlying sedimentary rocks, or have precipitated as a secondary mineral. Elevated Si and Ti concentrations suggest leaching processes and alteration within the entire crater.

Silicic alteration around mud pots in the central part of the crater is characterized by the complete destruction of the original rock/ primary mineral assemblage as evidenced in the XRD patterns of site L1 samples. The product of this alteration is a porous rock consisting exclusively of residual amorphous silica associated with sulfur and alunite. Secondary minerals such as K-alum, alunogen and pickeringite are detected in the dry, surrounding deposits, a few meters away from the pools (sample L1R2). No phyllosilicates were detected in the L1 (active) mud pots; the low pH likely hindering the formation of kaolinite [START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF]. Muddy gray material in the pools is consistent with amorphous silica. However, small absorption bands in the VNIR spectra of L7 (dry, ancient mud pot towards the periphery of the crater) suggest the presence of minor Al-clays such as kaolinite or montmorillonite (figure 3b). Alunite formation from Kfeldspar hydrolysis has been confirmed by previous SEM and isotopic analyses [START_REF] Piochi | Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry[END_REF][START_REF] Mayer | Hydrothermal alteration of surficial rocks at Solfatara (Campi Flegrei): Petrophysical properties and implications for phreatic eruption processes[END_REF]. Alunite dominance in the mud pot samples L1R3 and L1R4 could also result from the hydrolysis of the K-alum of sample L1R2 in the low pH, wet pool environment [START_REF] Valentino | Chemical and isotopic compositions of minerals and waters from the Campi Flegrei volcanic system, Naples, Italy[END_REF].

Along the crater rim (sites L2, L3, L6), alteration shifts towards the so-called "advanced argillic (or alunitic) facies". The original rocks are replaced by minerals of the alunite group, natroalunite, alunite and jarosite with a prevalence of alunite. These observations are consistent with the sequential zoning observed at several volcanoes, and could be explained by progressive neutralization of the fluid away from the degassing vents (e.g., [START_REF] Fulignati | Presence of native gold and tellurium in the active highsulfidation hydrothermal system of the La Fossa volcano (Vulcano, Italy)[END_REF]. The spatial distribution of alunite and jarosite at sites L2, L3 and L6 could be related to the relative availability of Fe and Al within the bedrock. At a temperature close to 100°C (most gas emissions at Solfatara), alunite and jarosite share a stability boundary at low pH, while at higher pH alunite is presumably associated with hematite [START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF]. We observed both mineral assemblages in some rocks of site L6. VNIR spectra of the L6 alunite-bearing rock (L6R4) share common absorption features with K-alunite references at 1.43 and 1.48 m (rather than 1.44 and 1.49 m for Na-alunite), indicating that this sample contains K-alunite (Figure 5a).

The shape of the 2.22/2.26 µm doublet feature in the jarosite spectra from the Solfatara outcrops are also more consistent with the K-rich endmember, in agreement with the original bedrock composition (Figure 5b). The spectrum of sample L2R1 includes VNIR features consistent with the presence of both alunite and jarosite.

Jarosite is expected to form at low pH ~2-4 (e.g. [START_REF] Bigham | Influence of pH on mineral speciation in a bioreactor simulating acid mine drainage[END_REF][START_REF] Elwood Madden | Jarosite as an indicator of waterlimited chemical weathering on Mars[END_REF], which would be inconsistent with the preservation of K-feldspar. Still, both are observed in the same area, suggesting that alteration is concentrated in layers that are more porous, or fractured and is spatially highly variable. The assemblage silica + alunite ± jarosite is diagnostic of advanced clay alteration as produced by acidic fluids in a steam-heated environment, in the upper portion of hydrothermal systems [START_REF] Rye | A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems[END_REF] such as solfataras (e.g., [START_REF] Bishop | Characterization of alteration products in tephra from Haleakala, Maui: A visible-infrared spectroscopy, Mössbauer spectroscopy, XRD[END_REF].

Previous isotopic measurements of 34 S in sulfate minerals and native sulfur and H2S in the fumarolic gases tend to confirm that the sulfates originate from the oxidation of H2S. While it seems clear that sulfur and sulfates are mostly derived from the H2S-gas [START_REF] Allard | Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei caldera[END_REF], the debate is still vivid regarding the high versus low sulfidation grade of the CF system, and especially in the Solfatara area, given that the observed mineralization could be derived from either one or the other or both processes (e.g., [START_REF] Rye | A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems[END_REF][START_REF] Piochi | Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry[END_REF]. Recent petrological and isotopic surveys concluded that the CF is consistent with a low sulfidation system that may transition towards a high sulfidation system in the most active centers (e.g., Solfatara) [START_REF] Piochi | Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry[END_REF].

A wide variety of sulfate minerals is also detected in the direct vicinity of active fumarole vents.

Crystalline Al and Fe hydroxysulfates are detected in areas where the bedrock lacks indication of primary sulfides, suggesting the influence of fumarole deposits and consistent with a steamheated environment. Sublimates include alunite, alunogen, alum K, mercallite, goldichite, coquimbite, ferrihydrite, millosevichite and tamarugite, which are highly soluble minerals not often preserved. At the highest temperature vent (site L4), sublimates form coatings bearing various arsenic and mercury species, generally found in epithermal systems. Orpiment and other arsenic sulfide minerals are likely derived from the oxidation of realgar.

Coordination of measurement techniques

VNIR, Raman and XRD analyses were carried out to determine the mineralogy of the samples.

The results of these techniques are generally in good agreement (Table 5), but VNIR and Raman surveys can be swayed by coatings (e.g., hematite at site L6, arsenic bearing species) as their sensitivity is limited to the first few top µm of the surface. VNIR and Raman techniques are advantageous in that they are fast, non-destructive, and do not require sample preparation. XRD analyses are however useful in gaining insights into minerals that are easily masked or have no signatures in the VNIR (e.g., feldspars, quartz), and mandatory for deriving quantitative information. The XRD technique is also the sole one to provide access to the full mineral assemblage in the case of complex mixtures, whilst VNIR and Raman measurements often reveal only a few phases.

Most sulfates could easily be identified with all of these individual techniques, although Raman and VNIR reference libraries do not necessarily contain all of the minerals identified by XRD. Some anhydrous sulfates such as yavapaiite do not exhibit spectral features in the NIR (e.g. [START_REF] Lane | Mid-infrared emission spectroscopy and visible/near-infrared reflectance spectroscopy of Fe-sulfate minerals[END_REF]. Some sulfate species (e.g., tamarugite) could only be recognized thanks to the vast, historical XRD standard database. Similarly, the existing VNIR and Raman databases lacked exact matches for all the arsenic sulfides and iron-bearing species that seemed to be derived from, or associated with realgar. Amorphous silica detection is not as straightforward as sulfate detection, but its presence could be inferred in the VNIR data, that are in general more efficient at characterizing hydrated amorphous components than Raman and XRD. Alternatively, the presence of amorphous silica can be inferred from the combined chemical analyses and XRD results. A suite of complementary techniques, such as the one described above, presents the most effective procedure to derive the most comprehensive information on hydrothermal samples in the field.

Both VNIR and Raman spectrometers will be carried onboard the next generation Mars rovers (e.g., MicrOmega on ExoMars [START_REF] Bibring | The MicrOmega Investigation Onboard ExoMars[END_REF], ISEM on ExoMars [START_REF] Korablev | Infrared spectrometer for ExoMars: a mast-mounted instrument for the Rover[END_REF], RLS on ExoMars [START_REF] Rull | The Raman laser spectrometer for the ExoMars Rover mission to Mars[END_REF], SuperCam on Mars2020 [START_REF] Wiens | The SuperCam remote sensing instrument suite for Mars[END_REF], SHERLOC on Mars2020 (e.g., [START_REF] Beegle | SHERLOC: scanning habitable environments with Raman and luminescence for organics and chemicals, an investigation for[END_REF]). A key advantage of both techniques is their efficiency, as they can easily be deployed in situ, without requiring any sample preparation. A possible caveat will be the potential difference in the scale of analysis between the various instruments, at the grain scale for Raman, typically larger for VNIR spectrometers (with the exception of MicrOmega), as noted in previous analog studies (e.g., Flahaut et al., 2017b). Raman spectroscopy will, however, enable the possibility to identify minerals such as feldspar, anhydrous silicates or sulfates, and organic molecules, that are difficult to investigate in the VNIR domain (e.g. [START_REF] Bishop | A spectroscopy and isotope study of sediments from the Antarctic Dry Valleys as analogs for potential paleolakes on Mars[END_REF]Bishop and Murad, 2004;[START_REF] Lopez-Reyes | Analysis of the scientific capabilities of the ExoMars Raman Laser Spectrometer instrument[END_REF][START_REF] Sobron | Natural Fe-bearing oxides and sulfates from the Rio Tinto Mars analogue -Critical assessment of VNIR reflectance spectroscopy, laser Raman spectroscopy, and XRD as mineral identification tools[END_REF][START_REF] Wiens | The SuperCam remote sensing instrument suite for Mars[END_REF]Flahaut et al., 2017b). VNIR spectroscopy will obviously present great advantages to infer the presence of hydrated components, those containing Fe, OH, CO3, SO4, ClO4, PO4 etc., and distinguish between minerals of the same family (e.g., sulfates, see figure 4c). Together, these instruments will have a better chance to identify the key minerals at the selected landing sites. In the absence of XRD however, assessing the full petrologic assemblage of the outcrops or rock samples will only be possible through a combination of observations, e.g., elemental abundances from SuperCam, rock textures from ExoMars Close-UP Imager (CLUPI)... (e.g., [START_REF] Wiens | The SuperCam remote sensing instrument suite for Mars[END_REF][START_REF] Josset | The Close-Up Imager onboard the ESA ExoMars Rover: objectives, description, operations, and science validation activities[END_REF].

Implications

VNIR spectroscopy is currently used on Mars as a remote sensing technique with instruments onboard spacecraft that map the planet's surface composition from a distance (e.g., the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument onboard Mars Express [START_REF] Bibring | OMEGA: Observatoire pour la minéralogie, l'eau, les glaces et l'activité[END_REF], the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument onboard Mars Reconnaissance Orbiter [START_REF] Murchie | Compact reconnaissance Imaging Spectrometer for Mars (CRISM) on Mars Reconnaissance Orbiter (MRO)[END_REF]). Those instruments have the capacity to detect iron and hydrated minerals such as those described at the Solfatara analog site and should therefore be able to identify and map areas of silicic alteration (opaline silica + alunite, as seen in the Solfatara mud pots) and the sequential zoning into argillicalunitic alteration (jarosite-alunite-hematite in the crater walls). Presumably, fumarolic sublimates and condensates (e.g., Al/Fe hydroxyl-sulfates) are also detectable with VNIR spectroscopy techniques, but the main issue of Mars orbital instruments is their limited spatial resolution (18 m/pixel at best from CRISM). Mars VNIR hyperspectral imagers on lander/roverbased missions would have a greater chance of identifying active or former vents than orbital instruments that might only detect extended areas of alteration, e.g. large fumarolic fields or geothermal areas. A number of hydrated species, including amorphous hydrated silica and related materials (e.g., [START_REF] Bishop | Phyllosilicate diversity and past aqueous activity revealed at Mawrth Vallis[END_REF][START_REF] Milliken | Opaline silica in young deposits on Mars[END_REF], Mustard et al., 2008;[START_REF] Skok | Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars[END_REF][START_REF] Mckeown | Interpretation of reflectance spectra of clay mineral-silica mixtures: Implications for Martian clay mineralogy at Mawrth Vallis[END_REF][START_REF] Wray | Columbus crater and other possible groundwater-fed paleolakes of Terra Sirenum, Mars[END_REF][START_REF] Weitz | Gypsum, opal, and fluvial channels within a trough of Noctis Labyrinthus, Mars: Implications for aqueous activity during the Late Hesperian to Early Amazonian[END_REF][START_REF] Weitz | Fresh exposures of hydrous Febearing amorphous silicates on Mars[END_REF][START_REF] Sun | Distinct Geologic Settings of Opal-A and More Crystalline Hydrated Silica on Mars[END_REF] and sporadic jarosite and alunite-rich outcrops [START_REF] Swayze | Discovery of the acid-sulfate mineral alunite in Terra Sirenum, Mars, using MRO CRISM: Possible evidence for acid-saline lacustrine deposits?[END_REF][START_REF] Ehlmann | Identification of hydrated silicate minerals on Mars using MRO-CRISM: Geologic context near Nili Fossae and implications for aqueous alteration[END_REF][START_REF] Ehlmann | Discovery of alunite in Cross crater, Terra Sirenum, Mars: Evidence for acidic, sulfurous waters[END_REF][START_REF] Klingelhöfer | Jarosite and hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer[END_REF][START_REF] Morris | Mössbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity's journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits[END_REF][START_REF] Farrand | Discovery of jarosite within the Mawrth Vallis region of Mars: Implications for the geologic history of the region[END_REF][START_REF] Farrand | Detection of copiapite in the northern Mawrth Vallis region of Mars: Evidence of acid sulfate alteration[END_REF][START_REF] Weitz | Diverse mineralogies in two troughs of Noctis Labyrinthus[END_REF][START_REF] Thollot | Most Mars minerals in a nutshell: Various alteration phases formed in a single environment in Noctis Labyrinthus[END_REF][START_REF] Michalski | Multiple working hypotheses for the formation of compositional stratigraphy on Mars: Insights from the Mawrth Vallis region[END_REF][START_REF] Bishop | Evidence of Salty Residues in Layered Outcrops at Mawrth Vallis and Implications for Evaporative Environments on Early Mars[END_REF], which could be of hydrothermal origin, have been reported previously on Mars from orbital observations (Figure 6). Ongoing efforts to understand the context of formation of those minerals, catalog their occurrences, and recognize key alteration facies/assemblages are supported by analog studies.

More limited in situ observations already demonstrated the occurrence of hydrothermal processes on Mars, although previous rovers were not equipped with VNIR or Raman spectrometers.

Amorphous silica was detected by the MER rover Spirit, at Home Plate, thanks to its mini-TES thermal IR spectrometer. APXS chemistry indicated elevated Ti and P contents in nearby soils that were interpreted as evidence for acid leaching; this process may have left behind some alteration minerals. The presence of sulfates may be inferred in the TES wavelength range, although hydrated minerals are most easily identified in the VNIR domain (e.g., [START_REF] Bibring | Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data[END_REF][START_REF] Ehlmann | Mineralogy and chemistry of altered Icelandic basalts: Application to clay mineral detection and understanding aqueous environments on Mars[END_REF] and are expected to be more readily observed by future rovers such as ExoMars that include VNIR spectrometers. The presence of a VNIR or Raman spectrometer at Gusev crater could help to further assess the presence of alteration minerals and possibly of organics in areas where hydrothermal silica was detected by Spirit [START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF][START_REF] Ruff | Silica deposits on Mars with features resembling hot spring biosignatures at El Tatio in Chile[END_REF]. Sending an XRD instrument to Gusev crater could bring additional information on the nature and relative amount of crystalline phases in these unique silica-rich materials near Home Plate [START_REF] Ruff | Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars[END_REF]. The mini-TES instrument range was however well-suited for capturing the mafic rock diversity in Gusev crater (e.g., [START_REF] Christensen | Initial results from the Mini-TES experiment in Gusev Crater from the Spirit Rover[END_REF]. A significant proportion of amorphous material has been detected at another Mars location, in the soils of Gale Crater, by the CheMin XRD instrument on the Mars Science Laboratory rover. The nature and origin of this amorphous material could not be determined with certainty, but at least part of it seems to be hydrated [START_REF] Bish | X-ray diffraction results from Mars Science Laboratory: Mineralogy of Rocknest at Gale crater[END_REF][START_REF] Dehouck | Constraints on abundance, composition, and nature of X-ray amorphous components of soils and rocks at Gale crater, Mars[END_REF][START_REF] Achilles | Mineralogy of an active eolian sediment from the Namib dune, Gale crater, Mars[END_REF][START_REF] Ehlmann | Chemistry, mineralogy, and grain properties at Namib and High dunes, Bagnold dune field, Gale crater, Mars: A synthesis of Curiosity rover observations[END_REF][START_REF] Yen | Multiple stages of aqueous alteration along fractures in mudstone and sandstone strata in Gale Crater, Mars[END_REF][START_REF] Rampe | Sand mineralogy within the Bagnold Dunes, Gale crater, as observed in situ and from orbit[END_REF][START_REF] Gabriel | Water abundance of dunes in Gale crater, Mars from active neutron experiments and implications for amorphous phases[END_REF]. As demonstrated above, VNIR spectroscopy was efficient as detected hydrated amorphous components such as opal, even in the pumice / ash component of the bedrock, and could assist in characterizing of the alteration phases identified by CheMin at Gale crater (e.g., [START_REF] Bish | X-ray diffraction results from Mars Science Laboratory: Mineralogy of Rocknest at Gale crater[END_REF] if sent on future missions.

In the present study, we have shown an example of how trachytic rocks at the Solfatara crater were hydrothermally altered into sulfate and silica-rich assemblages. At the Solfatara site, steamheated fumarolic alteration due to the presence of acidic sulfate-rich fluids resulted in an increased rock porosity and permeability, and the crater is currently primarily composed of incoherent rocks. The most abundant sulfates are Al, K, and Fe-rich in agreement with the original bedrock composition. Unlike the Solfatara site, the martian crust is more basaltic than trachytic (e.g., Mc Sween et al., 2003), even though trachytes and trachy-andesites have been found locally (e.g., [START_REF] Sautter | In situ evidence for continental crust on early Mars[END_REF][START_REF] Cousin | Classification of igneous rocks analyzed by ChemCam at Gale crater[END_REF]. Different alteration assemblages may be produced based on a number of factors that control hydrothermal alteration such as the rock type, but also the temperature, pressure, permeability, fluid composition and event duration (e.g., [START_REF] Browne | Hydrothermal alteration in active geothermal fields[END_REF]. Previous Mars analog studies of andesitic basalt alteration in Nicaragua reported the presence of mixed phyllosilicates, oxides/hydroxides and sulfates (gypsum + alunite) in the active acidic volcanic systems of Cerro Negro, Momotombo, and Telica [START_REF] Hynek | Assessment of environmental controls on acid-sulfate alteration at active volcanoes in Nicaragua: Applications to relic hydrothermal systems on Mars[END_REF][START_REF] Marcucci | Visible-nearinfrared reflectance spectroscopy of volcanic acid-sulfate alteration in Nicaragua: Analogs for early Mars[END_REF]. Acid-fog weathered basalts at the top of the Hawaian Kilauea volcano were found to be dominated by jarosite/natrojarosite sulfates, iron oxides and amorphous silica (e.g., [START_REF] Schiffman | Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars[END_REF][START_REF] Seelos | Silica in a Mars analog environment: Ka'u Desert, Kilauea Volcano, Hawaii[END_REF]. Icelandic expeditions have found abundant Al and Fe/Mg smectites, hematite and hydrous silica, formed from the alteration of basalts by S-poor fluids [START_REF] Ehlmann | Mineralogy and chemistry of altered Icelandic basalts: Application to clay mineral detection and understanding aqueous environments on Mars[END_REF]. All of these locations have presumed basaltic composition but show various assemblages due to varying external factors. Therefore, it is key to catalog in detail alteration patterns in these various environments and understand the geochemical pathways involved in those transformations on Earth, in order to shed light on past Mars environments.

A crucial aspect of hydrothermal systems is that on Earth, continental vents are known to be favorable to the development of life and the preservation of biosignatures. Therefore, they represent prime targets for the search of Life on Mars (e.g., [START_REF] Damer | First and Last Outposts: The Origin and Preservation of Life in Terrestrial and Martian Hydrothermal Field[END_REF][START_REF] Campbell | Ubiquity, Diversity, and Preservation of Microbial Fabrics in Siliceous Hot Spring Deposits: Relevance to the Columbia Hills[END_REF]. The Solfatara crater is home to several thermophilic, thermo-tolerant and acidophilic Bacteria, algae (e.g., Cyanidium caldarium) and Archea, such as Sulfolobus solfataricus (named after our study area, the Italian Solfatara), Acidianus, Pyrobaculum and Metallosphaera sp (e.g., DeRosa et al., 1974DeRosa et al., , 1975;;[START_REF] Zillig | The Sulfolobus-"Caldariella" group: taxonomy on the basis of the structure of DNA-dependent RNA polymerases[END_REF]Huber et al., 2000a, b). Most of these microorganisms are chemolithotrophs and rely on S, Fe oxides and metal sulfides for energy supply (e.g., Huber et al., 2000a, b). Finding such a biodiversity adapted to this extreme environment is promising for investigation of potential life elsewhere in the Solar System, and especially on Mars. (Bishop et al., 2004b[START_REF] Bishop | Evidence of Salty Residues in Layered Outcrops at Mawrth Vallis and Implications for Evaporative Environments on Early Mars[END_REF][START_REF] Bishop | The visible and infrared spectral properties of jarosite and alunite[END_REF] and the [START_REF] Viviano-Beck | Revised CRISM spectral parameters and summary products based on the currently detected mineral diversity on Mars[END_REF] type spectra marked by V. Bishop, 2018;[START_REF] Bishop | Evidence of Salty Residues in Layered Outcrops at Mawrth Vallis and Implications for Evaporative Environments on Early Mars[END_REF][START_REF] Sessa | Discovery of Alunite in Candidate ExoMars Landing Site, Mawrth Vallis: Evidence for Localized Evaporative[END_REF]. CRISM observation FRT00003BFB is displayed in false color (default RGB, R=2.5361 µm, G=1.3358 µm, B= 0.7749 µm) over CTX image P03_002074_2027_XI_22N018W. c) Alunite is detected together with kaolinite along the walls of the 65 km diameter Cross Crater [START_REF] Bishop | Evidence of Salty Residues in Layered Outcrops at Mawrth Vallis and Implications for Evaporative Environments on Early Mars[END_REF] P15_006945_1494_XN_30S158W. This RGB composite with the summary parameters R=BD2200 [START_REF] Thollot | Most Mars minerals in a nutshell: Various alteration phases formed in a single environment in Noctis Labyrinthus[END_REF], G=Kaol-index [START_REF] Thollot | Most Mars minerals in a nutshell: Various alteration phases formed in a single environment in Noctis Labyrinthus[END_REF], B= SINDEX2 [START_REF] Viviano-Beck | Revised CRISM spectral parameters and summary products based on the currently detected mineral diversity on Mars[END_REF] was chosen to highlight putative alunite detections in purple, and kaolinite detections in yellow (mixed detections appear in white). d) CRISM VNIR spectra of selected Mars mineral detections are compared with library references (Mars spectra: CRISM alunite and jarosite spectra from [START_REF] Viviano-Beck | Revised CRISM spectral parameters and summary products based on the currently detected mineral diversity on Mars[END_REF], 2015) and [START_REF] Bishop | Evidence of Salty Residues in Layered Outcrops at Mawrth Vallis and Implications for Evaporative Environments on Early Mars[END_REF], Silica spectra from CRISM FRT00010628 (average of 3x3 pixels). Reference spectra: Alunite GDS83, Jarosite GDS99 and Opal TM8896 from the USGS spectral library).

Figure captions

Tables Table 1: List of sampling sites and collected samples.

Figure 1 :

 1 Figure 1: a) Location of the study area in Italy. b) View of the Campanian plain and the three

Figure 2 :

 2 Figure2: Field impressions at the different sampling sites (See locations on figure1).

Figure 3 :

 3 Figure 3: VNIR reflectance spectra of selected samples that represent the types of spectral

Figure 4 :

 4 Figure 4: Raman spectra of selected samples that represent the types of spectral features

Figure 5 :

 5 Figure 5: VNIR field spectra of alunite and jarosite in rock coatings, which are occurring within

Figure 6 :

 6 Figure 6: Mars examples of alunite, jarosite and silica detections. a) Silica-rich deposits (magenta

  /brown deposits outside the pool. Polygons on a 20 cm scale R2: yellowish/brown deposits outside the pool R3: hard cemented deposits on the pool ramparts (Z1= at surface, Z2 = at a 5cm depth) R4: Light gray, wet sediments from the pool ignimbrite / ashy layer from the crater wall R2: white ignimbrite / pumice from the crater wall R3: yellowish ignimbrite / ashy layer from the crater wall rock lying on the vent (white, yellow, orange, dark and burgundy coatings) R2: colorful rock lying on the vent (yellow and orange coatings) material (sulfur needles) within the vent R3: friable/ altered, white/beige rocks around the vent R4: friable/ altered rocks with subtle turquoise tones around the vent L6 40.8295 14.1407 fumarolic vent R1: White pyroclastic deposits in the crater wall at 90°C + weathered wall R2: Beige pyroclastic deposits in the crater wall R3: Orange/Brown coatings on the crater wall R4: Yellow coatings on the crater wall R5: Dark red coatings on the crater wall R6: Sea green/turquoise coatings on the crater wall R7: Biology -green layer of algae, possibly Cyanidium caldarium R8: White coatings R9: Yellow sulfur crystals R10: Rock with colorful coatings R11: Yellow sulfur crystals from within a vent R12: White fibrous deposits from within a vent R13: White nodular material from within a vent R14: Orange nodular material from within a vent R15: Orange/brown puffy material from within/around a vent R16: White puffy material from within/around a vent R17: White/beige nodular material around the vent R18: White nodular material around the vent R19: Orange/brown ridged/puffy material around the vent R20: White/orange deposits from within a second vent R21: Orange/dark brown deposits from within a second vent Bulk composition of selected crater wall samples that appeared relatively unaltered. All values are given in wt%; D. L.= Detection Limit. Major oxides have been renormalized to 100% to exclude the loss on ignition. This Ignition loss (as measured by ICP-OES, including S) and the S content (as measured separately, by a C-S analyzer) are given for information. The Chemical Index of Alteration (CIA) is calculated using the equation of Nesbitt and Young (1982): CIA = (Al2O3) x 100 x (Al2O3 + CaO + Na2O +K2O) -1 . CIA values fall within the same range (60-80) of Mayer et al. (2016) who noticed that the Solfatara sample density decreases (porosity increases) with increasing degree of alteration.

  Figure 1

Acknowledgements

CTX and CRISM images of Mars were processed using the MarsSI web application (https://emars.univ-lyon1.fr/MarsSI/). Fieldwork was funded by a N.W.O. VENI grant to J. Flahaut. J. Flahaut was funded by two CNES ExoMars / LIS APR for sample analyses and by a Lorraine Université d'Excellence Future Leader grant at the time of writing / for publication fees. D. Loizeau received support from the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement n° 280168. Support from

the NASA Astrobiology Institute and NASA's SSW program to J. Bishop is appreciated. The authors are very grateful to Dr. Giorgio Angarano, the Solfatara general manager for his strong support and help using and visiting each site of the crater. The authors also wish to thank the Lyon Raport team (G. Montagnac, R. Caracas, H. Cardon) for their help and instrument loan -the Raman facility in Lyon is supported by the Institut national des sciences de l'univers (CNRS).

The authors would like to thank the CDHL at Université Lyon 1, and in particular Ruben Vera for his help with XRD. The authors are also grateful to the e-Mars Lyon team, especially Cathy Quantin-Nataf, and Patrick Thollot, for fruitful discussions. This is CRPG contribution N°2688.

Table 3: Summary of XRD detections for selected samples. For the DOC (degree of crystallinity) values, the -, +, ++ and +++ symbols indicate the following ranges of values, respectively : <30%, >30%, >50%, >70%. For mineral abundances within the crystalline phases, the -, +, ++ and +++ symbols indicate the following ranges of values, respectively : present but <10%, >10%, >40%, >70%. Gray lines indicate samples with a lower amount of crystalline phases (<60%).


Wojciech Tabis

Nityan L Nair

Marie-Eve Boulanger

Francis Laliberté

Sinead Griffin

Sanyum Channa

Anaelle Legros

Sahim Benhabib

Cyril Proust

Jeffrey Neaton

Anaëlle Legros

Louis Taillefer

James G Analytis

Transport signatures of surface states in a Weyl semimetal: evidence of field driven Fermi arc interferometry

A signature property of Weyl semimetals is the existence of topologically protected surface states -arcs in momentum space that connect Weyl points in the bulk. However, the presence of bulks states makes detection of surface contributions to the transport challenging. Here we present a magnetoresistance study of high-quality samples of the prototypical Weyl semimetal, TaAs. By measuring the Shubnikov de Haas effect, we reveal the presence of a two-dimensional cyclotron orbit. This orbit is quantitatively consistent with the interference of coherent quasiparticles traversing two distinct Fermi arcs on the [001] crystallographic surface. The observation of this effect suggests that high magnetic fields can be used to study not only the transport properties of Fermi arcs, but also the interference of their quantum mechanical wavefunctions.

Weyl semimetals host bulk carriers that behave not as massive electrons, but as massless chiral fermions. The topological nature of these charge carriers leads to many new emergent phenomena, including the presence of surface states known as Fermi arcs [1]. Although these are a necessary consequence of the existence of Weyl points, direct measurements of their transport properties remain rare because they are often masked by highly mobile bulk states. In fact, the transport contributions of Fermi arcs have only been measured in ultra-thin samples of the Dirac semimetal Cd3As2 [2][3][4]. Recently, TaAs has been predicted and confirmed to be a Weyl semimetal [5,6]. ARPES measurements have shown the presence of Fermi arc states on the surfaces, as well as non-topological states originating from the surface termination [7][8][9][10].

Transport measurements have observed pronounced quantum oscillations originating from trivial and topological bulk (3D) Fermi surfaces [11]. In this work, we report the observation of a new set of Shubnikov de Haas (SdH) oscillations in high-quality crystals of TaAs. These oscillations can be attributed to states on the surfaces of TaAs and represent the first evidence of coherent electron motion along the Fermi arcs of a Weyl semimetal.

High-quality single crystals of TaAs were grown using standard vapor transport techniques and cut and polished into hall bars with thicknesses ranging from 14μm to 272μm, with the main face normal to the [001] crystallographic axis. Figure 1 shows the basic results from one such device.

At low temperature the resistivity reaches a value of 1 µWcm with a residual resistivity ratio (RRR) of 56, indicating the high crystal quality of the samples. Applying a magnetic field normal to the hall bar reveals the large, non-saturating magnetoresistance typical of these materials and pronounced SdH oscillations in both the longitudinal and hall resistivity starting from fields below 1T. Fourier analysis of these oscillations shows two principal frequencies: 7.3T and 19.9T. These values are in close agreement with frequencies previously reported for the bulk W1 and H1 pockets [11]. Upon subtraction of a high-temperature (10K) background, an additional set of quantum oscillations can be observed that onset at ~8T and continue to high field. These oscillations are found to have a frequency of 285T and to our knowledge have not been previously reported.

The dependence of the 285T oscillation on field angle shows clear indications of a twodimensional Fermi surface. Figure 2 shows the derivative of 𝜌 "# as the field is tilted into the plane of the hall bar. The angle-dependence of the low frequency oscillations mentioned above (inset Figure 2b) can be attributed to the bulk Fermi surfaces of TaAs, and is in quantitative agreement with previous studies [11]. As the field is tilted into the plane of the device (away from the [001] crystallographic direction), the 285T frequency is observed to increase and the onset of the oscillations moves to higher field. The frequency can be well fit by a 1/cos (𝜃) dependence, the hallmark of a two-dimensional Fermi surface. The two-dimensional nature of this surface has been confirmed in multiple devices and along multiple rotation directions (see supplement) and is not expected from the bulk band structure. The possibility that the additional oscillations may be caused by an impurity phase has been considered and can be ruled out by high-resolution x-ray diffraction measurements and careful consideration of potential impurity phases. See supplement for details.

In order to isolate whether the surface is playing a significant role in the transport, we studied the thickness dependence of a single device. A hall bar device was contacted and systematically polished to thicknesses of 272µm, 130µm, 43µm, and 14µm. After normalizing for geometric factors, the magnetoresistance exhibits a strong thickness dependence in both the resistivity and Hall channels, shown in Figure 3.

The complicated thickness dependence can be quantitatively understood using a parallel channel model incorporating both bulk and surface contributions to the resistance. The conductivity tensor of each Fermi pocket (bulk and surface, electron and hole) is assumed to take the semiclassical form, given by

𝜎 = 𝑛|𝑒|𝜇 1 + (𝜇𝐵) 5 6 1 ±𝜇𝐵 ∓𝜇𝐵 1 9
where 𝑛 is the carrier density, 𝑒 is the electron charge, 𝜇 is the carrier mobility, 𝐵 is the applied magnetic field, and ± is for holes and electrons. We assume a partially compensated bulk consistent with what has been measured in TaAs by quantum oscillations and predicted by density functional theory (DFT, see supplement for details) [11,12]. The difference in scaling between the surface and bulk, stemming from their dimensionality, leads to a systematic change in the net carrier concentration as the sample is thinned, causing the Hall resistivity to decrease with sample thickness. This, in turn, changes the proportion of the current carried by the surface, leading to the complicated thickness dependence of the transverse resistivity. The calculated resistivities (insets, Figure 3) closely reproduce the features of the measured data.

Interestingly, the model accurately captures a curious feature in the bulk SdH signal: a phase inversion of the oscillations for thicknesses below 130μm appearing in the ρxx channel but not the ρxy channel. The oscillation around 7T, for example, appears as a peak in the transverse resistivities of the 272µm and 130µm devices, but a trough in the 43µm and 14µm devices. In contrast, the same oscillation appears as a peak in the Hall resistivity for all thicknesses. This feature is well reproduced by the model. We note that a similar inversion of the transverse magnetoresistivity oscillations with respect to the hall signal has been observed in elemental bismuth and antimony and is understood as a competition between diagonal and off-diagonal terms of the conductivity tensor [13,14]. In TaAs, the larger influence by the surface in thinner devices changes the strength of the hall conductance relative to the transverse conductance, leading to a similar phase inversion.

The temperature dependence of the SdH oscillations is shown in Figure 4a. The amplitude as a function of temperature can be fit to the Lifshitz-Kosevich form to extract the cyclotron effective mass for the different frequencies (inset). The 7.3T bulk frequency shows an effective mass of 0.066me, in close agreement with the predicted value of 0.065me [11]. The 285T frequency, on the other hand, shows a much larger effective mass of 0.5me. This large mass can be attributed to a unique surface orbit involving the interference of two Fermi arcs, as discussed below.

High-field measurements were also performed. Interestingly, the frequency of the high-field oscillations appears to show a splitting (Figure 4b). Below 14T, the oscillations appear to be periodic in inverse field with a frequency of approximately 285T. Above 14T, however, additional peaks emerge and a beat pattern can be observed. A Fourier transform (inset) shows that two distinct frequencies, 287T and 274T, can be resolved.

Our thickness dependent measurements show that there is a large surface contribution to the total conductivity. The surface carrier densities obtained from Density Functional Theory calculations, (~2 × 10 >? cm -2 for each electrons and holes, see supplement) are consistent with those determined from our simulations of the thickness dependence (~6 × 10 >? cm -2 ). We note that the two-dimensional quantum oscillatory frequency we observe corresponds to ~7 × 10 >5 cm -2 , accounting for only a small fraction of the total. Both trivial and topological (Fermi arc) surface states are present in this material. Most likely, the trivial states are not sufficiently mobile to allow coherent cyclotron orbits and associated SdH oscillations, although they contribute to the overall transport. In addition, as we argue below, the 285T frequency likely arises from the difference of two Fermi surfaces, and therefore will always appear smaller than the true Fermi surface.

ARPES experiments and DFT calculations can help elucidate the origin of the surface oscillations [20, see supplement]. Figure 5 shows a schematic of the electronic surface structure of the [001] As-terminated surface of TaAs. Although "Weyl orbits" involving individual Fermi arcs, as observed in Cd3As2, would be a natural explanation of the oscillations that we observe, the expected frequencies would be much higher than observed [2]. Moreover, coherent orbits from Fermi arcs require the traversal of the bulk and since our samples are significantly thicker than the quantum mean free path (λ~0.1μm, see supplement) it seems highly unlikely that coherent orbits of this sort can form [16].

However, one orbit that matches our observations corresponds to the cyclotron path connecting two different Fermi arcs, as illustrated in Figure 5b. Such an orbit involves an interband transition from one arc to another, a process known as magnetic breakdown that is commonly observed in materials with nearly degenerate bands near the Fermi energy [17,[START_REF] Shoenberg | Magnetic Oscillations in Metals[END_REF]. What is especially unusual about this orbit is that it is semiclassically forbidden; it involves electrons traveling opposite to the Lorentz force along one of the arcs. Such orbits are known to appear by Stark interference, the interference of two coherent cyclotron trajectories, and have been observed in magnesium and certain organic superconductors [22,23]. The frequency expected from these orbits is given by the usual Onsager relation, 𝐹 = ℏD E 5FG , where 𝐴 I is the k-space area enclosed by the orbit (see supplement). Estimating 𝐴 I from ARPES measurements and DFT calculations predicts a frequency of 277T, in close agreement with the observed 285T [15]. This is also consistent with previous DFT calculations of the Fermi surface reported in [21,22].

Additionally, from different energy cuts, the cyclotron effective mass can be estimated by 𝑚 * = ℏ L 5F MD E MN , and is found to be approximately 0.4me, in reasonable agreement to the observed effective mass of 0.5me. Given that no other surface or bulk states of TaAs, let alone known impurity phases have masses close to this value, this agreement is a strong verification that the observed frequency arises from the quantum interference of the Fermi arc states.

The observation of two distinct frequencies at high field is also consistent with the Fermi arc interference orbit. The [001] surface of TaAs breaks the four-fold rotational symmetry of the crystal. As a result, the surface band structure does not have to be the same in the X P and Y P directions. In fact, the Fermi arcs connecting the W2 Weyl nodes are found to differ slightly in both DFT calculations and ARPES measurements [15]. This difference leads to an approximately 7% change in the area enclosed by the two quantum interference orbits, in remarkable agreement with the 6% splitting of the observed SdH frequencies.

Surface signatures of the Fermi arcs in Weyl and Dirac semimetals have proven extraordinarily difficult to detect, let alone evidence of coherent orbits among them. The only system in which such orbits have been observed is Cd3As2, where the samples lend themselves to microfabrication techniques, allowing device thicknesses to approach the bulk mean free path. In TaAs, observation of these arcs is much more complicated; not only are there many more arcs that intersect in complex ways, but the material chemistry prevents the same micro-fabrication techniques from being used without significantly altering the surface [23]. In the present work, we have studied mechanically polished samples and reveal a new surface state that sustains electron coherence along a cyclotron orbit. Moreover, using DFT calculations we have shown that this orbit agrees with a cyclotron path involving the quantum interference of two Fermi arcs.

These observations demonstrate that there are exciting possibilities not only to reveal the transport behavior of the Fermi arcs in high magnetic fields, but to utilize field-driven interferometry as a means to study and employ their topological properties.

Figure 1 :

 1 Figure 1: a) The resistivity as a function of temperature, showing metallic behavior and a residual resistivity ratio of 56. Inset: The crystal structure of TaAs. Ta atoms are shown in red and As in blue. b) The transverse magnetoresistivity (upper) and hall resistivity (lower) of a polished 33μm hall bar device at 1.8K with field applied along the [001] direction, normal to the device plane. SdH oscillations from the bulk Fermi surface are easily distinguishable. c) The resistivities from (b) with a 10K background subtracted. In addition to the bulk SdH oscillations, a high frequency oscillation can be observed to onset around 8T with a frequency of 285T.

Figure 2 :

 2 Figure 2: a) The derivative of the hall resistivity plotted as a function of inverse magnetic field at different field angles. As the field is rotated into the plane of the hall bar, the additional oscillation increases in frequency and moves to higher field. b) The angle dependence of the oscillatory frequency shows the 1/cos (𝜃) dependence characteristic of a two-dimensional Fermi surface. Inset: The angle-dependence of the low-frequency oscillations is in good agreement with what has been measured in bulk samples of TaAs.

Figure 3 :

 3 Figure 3:Transverse (a) and hall (b) resistivities show a non-trivial thickness dependence. Resistivities were calculated using bulk geometrical factors. For a purely bulk conductivity, this would cause the curves to collapse. In addition, the SdH oscillations in the transverse resistivity appear to show a phase inversion between the 14μm/43μm devices and 130μm/272μm devices. Insets: The thickness dependence can be well-modeling with a simple parallel channel conductance model incorporating both surface and bulk contributions to device resistance. This model captures the overall shape and ordering of the resistivity curves and the phase inversion of the SdH oscillations.

Figure 4 :

 4 Figure 4: a) Temperature dependence of the SdH oscillations with field along the [001] direction and a 10K background subtraction. Inset: The temperature dependence of the oscillatory amplitudes can be fit to the Lifshitz-Kosevich form to extract the cyclotron effective mass. The low-frequency bulk oscillations show an effective mass of 0.066me and the high-frequency 285T oscillations show a much larger effective mass of 0.5me. b) The high-frequency SdH oscillations show additional structure at high magnetic fields. Inset: A Fourier transform shows two frequencies emerging above 14T.

Figure 5 :

 5 Figure 5: a) DFT calculation of the [001] Fermi surface of TaAs, showing both trivial and Fermi arc surface states. b) Two Fermi arcs connect the projections of the W2 Weyl nodes. The interference orbit is highlighted in orange.

Acknowledgements

The authors would like to thank Zhijun Wang and Andrei Bernevig for useful discussions. This work was supported by the National Science Foundation under Grant No. 1607753 and the Gordon and Betty Moore Foundation's EPiQS Initiative through Grant No. GBMF4374. N.L.N. was supported by the NSF Graduate Research Fellowships Program under Grant No. DGE 1106400. The authors would like to thank Camelia Stan for her help in performing x-ray diffraction measurements on beamline 12.3.2 at the Advanced Light Source. The Advanced Light Source is a DOE Office of Science User Facility supported under Contract No. DE-AC02-05CH11231.