Antoine Simonneau
email: antoine.simonneau@lcc-toulouse.fr

Michel Etienne

Dr Julien Louvel

Enhanced Activation of Coordinated Dinitrogen with p-Block Lewis Acids

Keywords: dinitrogen complexes, Lewis acids, p-block, dinitrogen functionalization, Frustrated Lewis Pairs

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

Introduction

Molecular dinitrogen complexes have been the focus of intensive studies since the mid 1960s when Allen and Senoff disclosed the first coordination compound exhibiting an end-on coordinated N 2 unit (Figure 1, left). [1] Chemists immediately realized the potential of such entities as a platform to study chemical transformations of the abundant, but very stable and inert N 2 molecule. At the advent of this chemistry, the reactivity of these complexes with Brønsted acids was intensively investigated. [2] These studies offered simplified models of the reactions occurring at the active site of the nitrogenases (Figure 1, right), i.e. the successive transfers of protons and electrons to the FeMo cofactor necessary for N 2 reduction. At the same time, these investigations paved the way for the development of catalysts for ammonia synthesis that could potentially operate under ordinary pressure and temperature conditions. [START_REF] Stucke | Recent reviews on nitrogen fixation catalyzed by transition metal complexes: a)[END_REF] The synthesis of organonitrogen compounds also fueled interest in the coordination chemistry community. [4] Having an estimate of the Brønsted or Lewis basicity of the N 2 ligand appeared to be important for the conception of the above-mentioned reactions. The preparation and characterization of Lewis acid (LA) adducts of dinitrogen complexes was therefore envisaged as a valuable source of information, since it would allow one to judge whether the N 2 ligand is accessible to electrophiles in general, and what response the transition metal center would give upon complexation of the Lewis acid in terms of d⟶π* electron back donation. Here, key analytical probes are typically the N-N bond stretching frequency NN in IR spectroscopy, or metrical parameters from X-ray diffraction studies. Recently, some research groups have revisited combinations of end-on dinitrogen phosphine complexes with p-block Lewis acids. This has allowed for the preparation of models of elusive species relevant to catalytic dinitrogen reduction processes, as well as the discovery of unprecedented reactivity patterns, thus giving a new momentum toward the study of this class of coordination compounds.

Early Works

While the nitrogenases had been known for decades when Allen and Senoff made their discovery, knowledge about their mechanism was not as advanced as it is today. [5] As they were already known

to have a bimetallic nature, Chatt and his colleagues at the Nitrogen Fixation Unit of the University of Sussex examined the interaction of some end-on dinitrogen complexes with electrophilic transition metal or main-group compounds in order to get some insights into the mechanism of nitrogen fixing metalloenzymes. [6] They could prepare and characterize a series of heterobimetallic complexes for which they proposed a structure featuring an N 2 ligand bridging two metals in a η 1 :η 1 fashion, as a result of Lewis acid-Lewis base (LB) interaction. In some instances (e.g. with TiF 4 , TiCl 4 , FeCl 3 , CuCl or PF 5 ), these reactions failed to produce a stable N 2 -bridged complex and resulted in oxidation of the starting N 2 complex. [6b,c] The reactions of isoelectronic group 6-8 dinitrogen phosphine complexes with different aluminum compounds (AlMe 3 , AlPh 3 and AlCl 3 ) were investigated and the isolated adducts, when viable, were characterized by 1 H NMR and IR spectroscopy. [6e,f] Although the complexes investigated [Mo(0), W(0), Re(I) and Os(II)] formed stable 1:1 adducts with AlMe 3 without noticeable amounts of alkylated products (Scheme 1, adduct formation between the Mo complex 1 and AlMe 3 ), the reaction with AlPh 3 was of narrower scope, while treatment with AlCl 3 generally resulted in decomposition along with N 2 evolution. [START_REF] Later | the reactivity of cis-[W(N2)2(PMe2Ph)4] towards ECl3 (E = Al or Ga) was shown by Hidai and co-workers to afford dimeric complexes of the type [Cl-WL4=N=N-ECl2]2: a) T[END_REF] Correlation between d electron delocalization from the metal center into the π* molecular orbital (MO) of the N 2 ligand and basicity of its β nitrogen was experimentally established. The weakening of the N-N triple bond observed by IR spectroscopy was attributed to a "push-pull" mechanism where electron density withdrawal on one side of the bridging N 2 ligand triggers enhanced d⟶π* back donation from the transition metal center. [6a,b] Scheme 1. Adduct of a N2 complex with an aluminum Lewis acid.

Adducts with a Side-On, End-On N 2 Complex

The late 1990s and early 2000s witnessed major breakthroughs in the chemistry of molecular dinitrogen complexes: N 2 cleavage by low-coordinate, well-defined Mo complexes; [8] the achievement of catalytic ammonia synthesis, [9] albeit with limited turn-over numbers and turn-over frequencies; and hydrogenation of coordinated N 2 , [10] the latter becoming possible with the discovery of the side-on coordination mode in homobimetallic complexes. [11] Remarkably, all these processes occurred in Mo

Ph 2 P PPh 2 PPh 2 Ph 2 P N N N N Dn NN -94 cm -1 excess AlMe 3 C 6 H 6 trans-[Mo(N 2 )(N 2 AlMe 3 )(dppe) 2 ] 1 2 a b
especially mild conditions. In 2005, a joint study on the interaction of the Ta complex 3), with various p-block LA [AlMe 3 , GaMe 3 (both not shown) and B(C 6 F 5 ) 3 , Scheme 2] [12] by Tuczek, Fryzuk and coworkers sporadically brought back into fashion group 13 LA adducts of N 2 complexes. The goal of this investigation was to better apprehend the reactivity of the N 2 ligand in the rare side-on, end-on coordination mode. [13] LA addition to complex 3 occurred at the β nitrogen to give 4. A bathochromic shift of the NN stretch (-20 cm -1

[{(NPN)Ta} 2 (μ- H) 2 (μ-η 1 :η 2 -N 2 )] (
) in the IR spectrum was evidence of further weakening of the N-N bond in 4. The bonding situation in a model complex, [{PH 3 (NH 2 ) 2 Ta} 2 (μ-H) 2 (μη 1 :η 2 -N 2 -BH 3 )], was investigated by means of DFT calculations. Boron complexation leads to a decrease of the HOMO's energy. [14] Although the overall symmetry is different than that in the end-on mononuclear complexes, the HOMO also corresponds to a bonding interaction of the metal centers' filled dπ orbitals with antibonding π* of the dinitrogen ligand. Increased N 2 polarization can be traced according to Natural Charges (NCs) resulting from a Natural Population Analysis (NPA). The MOs involved in the N-B dative bond are principally a hybrid of the p σ and a π bond of the N 2 fragment and the empty p orbital at boron.

Scheme 2. Adduct of a side-on, end-on (NPN)Ta complex with B(C6F5)3.

Adducts between Iron-N 2 Complexes and Strong Lewis Acids for the Understanding of Catalytic Processes

Very recently, the team of Szymczak investigated the spectroscopic and electrochemical properties as well as electronic structures of adducts between the end-on iron-dinitrogen complex [Fe(depe) 2 (N 2 )] (5, depe = 1,2-bis(diethylphosphino)ethane) and various LAs. The idea was to test the "push-pull hypothesis" by which acidic residues in the active site of the nitrogenase pull electron density from the reduced iron center onto the N 2 ligand (see Figure 1), [5] thus rendering it more reactive towards protonation (Scheme 3). [15] In adducts of the type [(depe) 2 Fe(η 1 :η 1 -N 2 -BR 3 )] (R = 2,6-F 2 C 6 H 3 , 2,4,6-F 3 C 6 H 2 , C 6 F 5 , OC 6 F 5 or F), a good correlation was found between the Lewis acidity of the boranes [16] and the activation of the N 2 ligand, as judged by the NN stretching frequency in IR spectroscopy. Solid-state (X-ray) and electronic (DFT) structures were reported for the B(C 6 F 5 ) 3 adduct 6. The NNB arrangement is bent [137.0(3)°], as a result of p B /π* N2 mixing for the formation of the B-N dative bond. Activation of the N-N bond (elongation by +0.04 Å and NN red shift of 126 cm -1

) upon LA complexation was explained, in terms of orbital considerations, by (i) a greater stabilization of the HOMO, which consists of a combination of d orbitals with the π* of N 2 , and (ii) an increase of its π* character, analogous to 4. NCs calculations clearly indicate enhanced polarization of the N 2 ligand, with an increase of the negative charge at the terminal N. This is counterintuitive, as one would have expected the acceptor to quench its basicity. An important implication of these observations is that transfer of electron density from the metal favors reactivity of electrophiles at the N 2 ligand instead of the reduced iron center. This assumption was clearly verified by the protonation of 6 with HBAr Scheme 3. A Lewis acid adduct of an iron-N2 complex aimed at verifying the "push-pull hypothesis" regarding the active site of the nitrogenase.

Beyond validating the "push-pull hypothesis", characterization of such mono-protonated species offers a snapshot of what may resemble elusive FeN 2 H intermediates in catalytic N 2 reductions. [3b,17] Ashley and coworkers cleverly made use of the more Lewis acidic Me 3 Si + silylium ion as a proton mimic to characterize the first cationic silyldiazenido iron complexes (Scheme 4). [18] Treatment of 5 or

[Fe(dmpe) 2 (N 2 )] (8, dmpe = 1,2-bis(dimethylphosphino)ethane) with Me 3 SiCl/KBAr ) as the consequence of a greater "pull" effect. The X-ray diffraction structures of complexes 9 and 10 show an elongation of the N-N bond of ca. +0.05 Å, while the Fe-N bond displays greater double bond character than in 5 or 8. DFT calculations performed on the silyldiazenido complexes were in line with those run by Szymczak and co-workers (vide supra). According to their diamagnetic character, trigonal bipyramid geometry and d orbital-centered HOMOs, the complexes were best described as being Fe(0), d 8, rather than Fe(II), high spin d 6 . Relevance to the intermediates in catalytic cycles of N 2 reduction to silylamines [START_REF] Yuki | N2 reductions to silylamines catalyzed by Fe complexes: a)[END_REF] can also be invoked here. Indeed, several silyldiazenido complexes have been reported prior to Ashley's work. Such compounds were obtained by substitution of a (pseudo-)halide in R 3 SiX compounds, [START_REF] Betley | Examples of such silyldiazenido and/or disilylhydrazido Fe complexes: a)[END_REF] and occurrence of R 3 Si + cations is rather unlikely.

[21] Scheme 4. A cationic Fe-silyldiazenido complex mimics intermediates of catalytic N2 reduction.

7 Fe R 2 P R 2 P PR 2 PR 2 N N Me 3 SiCl, KBAr F 4 (1:1) DFB -KCl Fe R 2 P R 2 P PR 2 PR 2 N N SiMe 3 + BAr F 4 - R = Et, 5 R = Me, 8 9
Dd Fe-N -0.06 Å Dd N-N +0.05 Å, Dn NN -224 cm -1

Lewis Acid Adducts of N 2 Complexes and the Frustrated Lewis Pair Chemistry

The "push-pull" paradigm for N 2 activation illustrated by the two previous examples can be extended to the recent chemistry of the frustrated Lewis pairs (FLP). [22] This now mature field exploits the simultaneous action of a LA and a Lewis base (LB)  prevented from forming a classical LA-LB adduct by steric interactions  for the activation of a vast array of (small) molecules (see scheme 5, top; activation of CO, isoelectronic to N 2 , by an intramolecular P/B FLP [23] ).

Scheme 5. Comparison of the "conventional" FLP chemistry of isoelectronic CO (top) with the N2 complexes/LA adducts discussed herein, which can be viewed as transition metal FLP templates (bottom).

Although initially based solely on p-block compounds, mixed systems incorporating a metal complex have started to emerge as alternatives to pure main group combinations. [24] The iron dinitrogen complexes/LA combinations of Szymczak and Ashley can be viewed as metal/p-block FLP templates that activate N 2 , the difference from a conventional FLP being that electron density of the Lewis basic component lies in an orbital of different geometry (see Scheme 5, bottom). The involvement of the metal's filled d orbital in the activation of N 2 is here the key to make N 2 amenable to FLP-type chemistry. [START_REF] Jupp | While this article was being written, Stephan and co-workers reported the isolation of the adduct of diazodiphenylmethane with B(C6F5)3, Ph2C=N=N-B(C6F5)3. A prelude to the FLP chemistry of N2 with pure main group system is thus composed[END_REF] In conventional phosphorus/boron systems, the P-centered HOMO involved in the small molecule activation cannot build a constructive overlap with the high-lying N 2 π*. We recently drew an explicit parallel with this type of N 2 activation with FLPs when we reported the Mo and W complexes 12 and 13. [26] Similarly to Szymczak's and Ashley's iron examples, they were prepared from the group 6 dinitrogen complexes 1 and 11, respectively, by treatment with B(C 6 F 5 ) 3 . 12 and 13 share a bent NNB arrangement, with a substantially elongated NN bond. This is reflected by the important bathochromic shift of the NN in both complexes (-226 and -236 cm -1

, respectively, Scheme 6). Although no calculations were performed on these complexes, it can reasonably be assumed that the same effects explain the activation of the N 2 ligand, i.e., enhanced metal-to-ligand charge transfer triggered by LA's interaction with N 2 's π*. [START_REF]Although the Fe complexes 6, 9 and 10 on the one hand and the Mo and W complexes 12 and 13[END_REF] One N 2 ligand is lost upon complexation, as opposed to

Chatt's example of the AlMe 3 adduct with 1.

[5d] The strong charge transfer caused by the borane may impart a substantial trans influence to the N 2 unit, which thus hampers coordination in the second apical site. Compared to the Fe complex 6, the higher activation of the N 2 ligand, as illustrated by NN and N-N bond lengths, correlates with the greater π-donating ability of Mo and W. Similar structural and spectroscopic changes are achieved in the Fe complexes of Ashley and co-workers, as a result of the use of the highly electrophilic silylium cation.

C O The Mo and W complexes 12 and 13 were shown to react with hydroboranes and hydrosilanes (Scheme 7). It was postulated that the size of the [(dppe) 2 M(N 2 )] fragment might facilitate dissociation of the borane, which would allow subsequent B-H and Si-H bond splitting and functionalization of the N 2 ligand with boryl and silyl groups in 14-17. Such reactivity again compares well with that of usual nitrogen/boron (N/B) FLPs (Scheme 7, top). [START_REF] Voss | Formation of a classical (C6F5)3B•NR3 adduct does not preclude a FLP behavior: a)[END_REF][29][30] No borylation or silylation of N 2 occurs in the absence of B(C 6 F 5 ) 3 . Akin to 9 and 10, the N-N bonds in the functionalized complexes are rather long (1.23-1.27 Å for the W complexes), with NN below 1670 cm -1

B(C 6 F 5 ) 2 Mes 2 P B(C 6 F 5 ) 2 Mes 2 P C O M L L L L B R' R' R' M L L L L E R' R' R' N N M L L L L E R' R' R' N N M L L L L E R' R' R' N N or P R R R B R' R' R'
. These products can be seen as adducts of d 6 dinitrogen complexes with the elusive [Et 3 Si] + [HB(C 6 F 5 ) 3 ] -silylium cation, [31] or the unknown [(C 6 F 5 ) 2 B] + [HB(C 6 F 5 ) 3 ] -borinium ion, [32] but we proposed to describe these ionic species 14-17 as boryl-or silyl-diazenido complexes with metal centers in the +2 oxidation state. [START_REF] Komori | Examples of related complexes[END_REF] In the silyldiazenido complexes 16 and 17, the NMR resonances of the 29 Si nucleus are shifted to high field, strongly suggesting that the positive charge is not carried by the Si atom. [START_REF]adducts of Me3Si + with tertiary amines where 29 Si NMR suggests partial localization of the positive charge at Si: a) G. A[END_REF] The actual electronic situation in the group 6 complexes 14-17 remains debatable and calculations are under way to better appreciate it. This should allow detailed comparison with Szymczak's and Ashley's examples.

Figure 1 .

 1 Figure 1. First reported N2 complex (left) and the active site of the nitrogenase showing N2 bound to the FeMo cofactor (right).

F

  5-(F 3 C) 2 C 6 H 3 ) which smoothly occurred at the terminal N to give 7 in excellent yield (Scheme 3, 6 ⟶ 7).

F 4 afforded the ionic compounds 9

 9 and 10 bearing a bent NNSi arrangement [Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ]. IR data were in line with a notable weakening of the N-N bond. As expected with the stronger LA Me 3 Si + , the NN shifted towards lower energies (-224 cm -1

  Scheme 6. Adducts of Mo-and W-N2 complexes with the strong LA B(C6F5)3.

Scheme 7 .Dd

 7 Scheme 7. Conventional nitrogen/boron FLP chemistry (top) compares well with the borylation and silylation of N2 in complexes 12 and 13 (bottom).

These examples of B-H and Si-H bond splitting act as an incentive to extend this N/B FLP-type reactivity. We know from Szymczak's and Ashley's work that d 8 iron dinitrogen complexes form stable adducts with strong Lewis acids. In-depth studies involving complexes of different metals, ligands and LAs are under way. A deeper understanding of such combinations will allow the design of new FLP templates with tailored properties and reactivity. The chemistry of conventional N/B FLPs is very rich, and its extension to M-N≡N/B systems gives a glimpse of new possibilities regarding the functionalization of coordinated N 2 .

Summary and Outlook

Since the pioneering work led by the group of Chatt during the 1970s, progress has been made in the understanding of the nitrogenases, and catalytic N 2 reductions have been achieved. The field of the Frustrated Lewis pairs has also emerged as a strategy of choice for the activation of small molecules. All these advances of modern chemistry will continue to encourage chemists to give a new look to Lewis acid adducts of (end-on) dinitrogen complexes. This body of recent work clearly demonstrates that combinations of Lewis acids with dinitrogen complexes may develop as a complementary approach for N 2 activation and functionalization. A common feature of the examples discussed above is that LA complexation to coordinated N 2 lowers the energy level of the HOMO of the N 2 complex, a combination of metal-centered d orbitals and N 2-π* orbitals. As a result, important charge transfer takes place from the metal to N 2 , increasing its polarization. In depth understanding of the bonding situation should help predict the reactivity of such adducts, and gain insights into mechanistic pathways. A challenge associated with this chemistry may reside in the occurrence of electron transfer processes, [6b,c] as (strong) Lewis acids can also behave as one electron oxidants. [35]


C Sunday

N Murdoch

O Cherrier

S Morales Serrano

C Valeria Nardi

T Janin

I Avila Martinez

Y Gourinat

D Mimoun

A novel facility for reduced-gravity testing: A setup for studying low-velocity collisions into granular surfaces

This work presents an experimental design for studying low-velocity collisions into granular surfaces in low-gravity. In the experiment apparatus, reduced-gravity is simulated by releasing a free-falling projectile into a surface container with a downward acceleration less than that of Earth's gravity. The acceleration of the surface is controlled through the use of an Atwood machine, or a system of pulleys and counterweights. The starting height of the surface container and the initial separation distance between the projectile and surface are variable and chosen to accommodate collision velocities up to 20 cm/s and effective accelerations of ∼0.1 to 1.0 m/s 2 . Accelerometers, placed on the surface container and inside the projectile, provide acceleration data, while high-speed cameras capture the collision and act as secondary data sources. The experiment is built into an existing 5.5 m drop tower frame and requires the custom design of all components, including the projectile, surface sample container, release mechanism, and deceleration system. Data from calibration tests verify the efficiency of the experiment's deceleration system and provide a quantitative understanding of the performance of the Atwood system.

I. INTRODUCTION

On November 12, 2014, the European Space Agency's (ESA's) Rosetta spacecraft became the first mission to successfully deliver a lander to a comet's surface. Though the Philae lander eventually came to rest, its anchoring harpoons failed to fire upon descent, and the lander proceeded to rebound twice over a duration of approximately 2 h before reaching its final destination, roughly 1 km away from the intended landing site. 1,2 These unanticipated events led to significant changes in the operational schedule in order to perform as many scientific measurements as possible, within the limited lifetime of the lander.

The Rosetta events provide one example of how mission planning can be influenced by lander-surface interactions. Hayabusa-2, a Japanese Space Agency (JAXA) asteroid sample-return mission, will be facing similar challenges to Rosetta in the coming years. The Hayabusa-2 spacecraft will arrive at the C-type near-Earth asteroid (162173) Ryugu in mid-2018 and deploy several science payloads to its surface. 3 Among these payloads is a 10 kg lander, the Mobile Asteroid Surface Scout (MASCOT), provided by the German Space Agency (Deutsche Forschungsanstalt fur Luft und Raumfahrt (DLR)) with cooperation from the French Centre National d'Etudes Spatiales (CNES). In addition to housing four instruments for in situ science investigation, MASCOT a) Current address: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91107, USA.

contains a mobility mechanism that will correct its orientation and enable it to hop to various measurement sites. [START_REF] Ho | Lunar and Planetary Institute Science Conference Abstracts[END_REF] A lander similar to MASCOT is also proposed to be part of the European Space Agency's (ESA) Asteroid Impact Mission (AIM). AIM is one part of the Asteroid Impact & Deflection Assessment (AIDA) mission together with NASA's Double Asteroid Redirection Test (DART) mission. AIM and DART complement each other in the validation of the kinetic impact approach to deflect threatening asteroids as well as in the characterization of physical and dynamical properties of the mission target, the Didymos binary asteroid system. [START_REF] Michel | Science case for the Asteroid Impact Mission (AIM): A component of the Asteroid Impact & Deflection Assessment (AIDA) mission[END_REF]6 Based on thermal infrared observations [START_REF] Harris | Proc. Int. Astron. Union[END_REF][START_REF] Campins | [END_REF][9][10] and previous space missions, [11][12][13][14][15][16][17] it is strongly believed that asteroids are covered by loose regolith. [START_REF] Murdoch | Asteroid surface geophysics[END_REF] The asteroids' granular surfaces, in combination with the low surface gravity, make it difficult to predict a lander's collision behavior from existing theoretical models. This is partially due to the fact that granular materials have the ability to display either solid, liquid, or gaseous behavior. Anticipating rebound dynamics is particularly important for landers, like MASCOT, that do not have attitude control or propulsion systems. While an analysis of the Philae mission may assist in MASCOT's operation planning, further experimentation is required to construct and validate landing models.

The objective of this work, derived from the needs of current and future small-body missions, is to present an experimental design for studying low-velocity collisions into granular surfaces in low gravity. Though this setup is designed specifically around a collision study, this work shows how the experiment concept can be re-configured and used as a repeatable and inexpensive method for general reducedgravity testing.

Section II outlines the main considerations behind the design of this experiment while Section III describes the setup itself. Section IV explains the supporting theory used to construct the experiment setup and Section V details its performance. Lastly, Sections VI and VII discuss how this setup will be used for future experimentation and how it can be reconfigured to accommodate other low gravity tests.

II. BACKGROUND ON REDUCED GRAVITY EXPERIMENTATION

Reduced-gravity experimentation has been a continued challenge for the Space industry. Present-day methods of simulating micro-gravity on Earth include parabolic flights, drop tower neutral buoyancy laboratories, magnetic levitation, rotating wall vessels, and off-loading gantries.

The budget and scheduling constraints associated with parabolic flights, large, air-evacuated drop towers, and neutral buoyancy laboratories can make these facilities impractical for frequent use. Furthermore, the majority of these methods are difficult to implement for more complicated, dynamic experiments. In order for neutral buoyancy or magnetic levitation to be effective for granular experiments, each particle must be neutrally buoyant or magnetically levitated. Off-loading weight from only the container or projectile will not suffice because the particles inside of the container will still feel the effects of gravity.

Drop tower facilities and parabolic flights have been extensively used for micro-gravity experiments related to dust and regolith dynamics. [START_REF] Hofmeister | AIP Conf. Proc[END_REF][START_REF] Murdoch | [END_REF][21][22][START_REF] Dove | AGU Fall Meeting Abstracts[END_REF][START_REF] Colwell | European Planetary Science Congress[END_REF] Colwell and Taylor and Colwell studied micro-gravity collisions into granular surfaces over the course of two different payload experiments aboard the Space Transportation System (Space Shuttle). [START_REF] Colwell | [END_REF]26 Colwell et al. completed another series of micro-gravity impact tests over three parabolic flights, with the goal of studying low velocity collisions of centimeter sized particles. [START_REF] Colwell | European Planetary Science Congress[END_REF] Goldman and Umbanhower observe reduced-gravity collisions into granular material by using an Atwood machine to change the effective gravity between a projectile and a surface at their moment of impact. 27 Altshuler et al. also employ an Atwood machine setup in order to study settling into dry granular media. 28 An Atwood-type machine, using balanced weights linked in a loop, has also been used to simulate the landing of Huygens on Titan and for spacecraft impact tests on small bodies. These experiments had an emphasis on penetrometer tests and had impact velocities of 0.9-3 m/s. 29 Other groups use drop tower experiments to study impacts between solid and agglomerate bodies. [30][31][32] By conducting their experiments aboard the Space Shuttle and NASA C-9 airplane, Colwell and Taylor, Colwell, and Colwell et al. are able to observe impact velocities ranging from 1 to 110 cm/s. [START_REF] Colwell | European Planetary Science Congress[END_REF][START_REF] Colwell | [END_REF][26] With drop tower setups, Goldman and Umbanhower and Altshuler et al. observe collisions with higher impact velocities of 40-700 cm/s. 27,28 Altshuler et al. employ a setup similar to the one proposed in this work, but he uses polystyrene beads as a surface simulant. 28 Additional studies are needed that combine the setup from Altshuler et al. with lower collision velocities and granular materials that are more representative of the regolith found on small bodies.

The key challenges to designing an asteroid collision experiment are as follows: (1) Finding a way to simulate reduced gravity conditions on Earth, so that the prevailing forces in micro-gravity collisions can be reflected in the experimental results, and (2) simulating low-velocity collisions in such a way that data can be collected over a sufficiently long time frame. Based on prior success and the opportunity for a customizable setup, the proposed method to achieve this goal is through the use of an Atwood machine. In this approach, a freefalling projectile impacts a surface with a constant downward acceleration, or an acceleration less than that of gravity, so that the effective surface acceleration felt by the grains is very small. For example, if the projectile is in free-fall and the surface is controlled to have a downward acceleration of 9.0 m/s 2 , then the surface experiences an effective acceleration of 0.81 m/s 2 . In reducing the effective surface acceleration of the granular material, the confining pressure and interparticle friction will be reduced 21 and the medium's inter-grain cohesion forces will become more important compared to its weight force. 33 Consequently, the properties of the granular material will become more representative of those on an asteroid's surface. Since both the surface and projectile are falling in the Atwood machine setup, the projectile requires a minimum amount of time to catch the surface before the collision begins. This extended free-fall period provides a solution to the second experimental design challenge and makes it possible to use accelerometers and high-speed cameras for data collection. This type of test would be impossible to perform on a parabolic flight because directional, very low-levels of gravity are required to create low-velocity collisions with the granular surface. Gravity levels on a Zero-G aircraft tend to fluctuate around zero.

Goldman and Umbanhower and Altshuler et al. conduct reduced-gravity testing using the same method as described above. However, the previous experiments are limited in the range of collision velocities and effective accelerations that their setups can attain. Figure 1 compares the velocity and acceleration regimes that are studied by Goldman and Umbanhower, Altshuler et al., and this work. The experiment in this work is built into an existing drop tower structure, which is managed by the Department of Mechanical Structures and Materials at the Institut Supérieur de l'Aéronautique et de l'Espace (ISAE-SUPAERO) in Toulouse, France. With 5 m of available drop height, 25 cm of possible separation between the projectile and surface, and a 160 kg filled surface container, the ISAE-SUPAERO setup can reach lower gravity levels and higher impact velocities than the setups used by Goldman and Umbanhower and Altshuler et al. The regime limits for the ISAE-SUPAERO drop tower are based on theoretical values and are dependent on the properties of the granular material being tested (see Section IV).

III. APPARATUS

The idea of effective acceleration drives the design of this experiment and results in the following key features. First, the granular surface is provided a constant downward acceleration using an Atwood machine, or a system of pulleys and coun- 28 Approximate values for the Goldman and Umbanhower setup are based on ranges of collision velocities and gravity levels provided in Goldman and Umbanhower, Section II. 27 Theoretical values for the ISAE-SUPAERO setup are calculated as described in Section IV. terweights (see Figure 2). Next, the projectile and surface are simultaneously released from rest, where the starting height of the surface container and the initial separation distance between the projectile and surface are variable and selected based on the desired collision velocity and effective acceleration. Finally, at the end of the data collection period, the surface container is decelerated at a rate that does not result in damage to either the surface container or projectile.

Figure 3 shows a technical illustration of the full experiment, with reference to its six primary components: The support structure, the counterweight and pulley system, the surface container, the projectile, the release mechanism, and the deceleration system.

A. Design of the drop tower base structure

The experiment uses ISAE-SUPAERO's existing drop tower, shown in Figure 4 is usually designated for aircraft and material drop-tests 34 but has been re-purposed to accommodate a counterweight and pulley system. This system makes it possible for materials to fall according to a predefined acceleration and is key to performing reduced gravity, rather than micro-gravity, testing.

The tower provides approximately 5.5 × 0.65 × 0.65 m 3 of working space and includes an electronically operated motor and pulley system that can be used to raise the container assembly to the desired height. The tower also features two low-friction guide rails that permit test materials to fall and have an impact normal to the structure's base.

B. Design of the pulley system

Four low-friction pulleys are mounted at the top of the structure's frame, and light-weight cords connect the surface FIG. 4. ISAE-Supaero's existing drop tower structure. The tower features an electronically operated motor and pulley system for raising test materials and two low-friction rails for keeping the test materials vertically aligned.

container to the counterweights. The counterweights are enclosed inside a hollow tube so that their vertical acceleration and stopping motions can be controlled. The counterweight holders alone weigh 400 g each, and mass can be added to each holder at an increment of 100-250 g. The maximum counterweight mass that the current system can support, including the holder mass, is 8.8 kg (4.4 kg on each side). However, the holders can be replaced in order to accommodate larger masses. Figure 3 shows the mounted pulley system, counterweight, and guide tube components of the assembly.

C. Design of the surface container, release mechanism, and projectile

The surface container sub-assembly comprises of three parts: the surface container, the release mechanism, and the projectile. The surface container is sized according to a literature review so that the walls of the container will not influence the rebound dynamics of the collision. 27,35 If the container is too small, then individual grains may interlock instead of moving in relation to one another as they would if unconstrained. The front and back panels of the surface container are made of 10 mm thick Makrolon polycarbonate material, while the two side panels are made of a light-weight 4 mm thick aluminum alloy. The container is fastened and reinforced at its joints using 2017 aluminum alloy members. With the exception of the opening for the release mechanism, the container is closed on all four sides for safety reasons. A narrow beam traverses the center of the container and acts as a support for the electromagnetic release mechanism. An electromagnet is mounted at the end of a supported tube, which can be raised and lowered to change the separation distance between the projectile and the surface. The electronics box for controlling the electromagnet is mounted to the top of the container. Square markings of various orientation are placed on the front face of the container for use during the post-processing of the high-speed camera images. Lastly, two guide pieces fasten to the sides of the container and mate with the rails on the drop tower structure. These guide pieces constrain the container's motion to the vertical direction. Figure 5 shows an illustration of the surface container and the location of its different features. The total dry mass of the container assembly is 80 kg and it is filled to a height of approximately 17 cm with ∼80 kg of sand. Detailed discussion of the surface material used will be provided in Murdoch et al. [START_REF] Murdoch | An experimental study of low velocity impacts into granular material in reduced gravity[END_REF] The release mechanism consists of a permanent electromagnet and a contact switch. The contact switch is integrated FIG. 5. Surface container sub-assembly, showing the 10 cm diameter projectile, the electromagnetic release mechanism, and reference markings for image post-processing. The surface container is 62 cm long, 45 cm wide, and 59 cm high. into the release hook of the surface container such that the switch opens with the container's release. Using an onboard battery system and an electronic control card, the electromagnet is activated after the switch is opened. The activation of the electromagnet cancels the magnetic field of the permanent magnet allowing the projectile to fall. The time between the contact switch opening and the activation of the electromagnet can be adjusted via the onboard electronics to be between ∼40 and 400 ms. For these experiments, the shortest time (40 ms) was used. Testing confirms that the magnetic field generated by the electromagnet does not distort the accelerometers' measurements. A light-emitting diode (LED) on the surface container's electronics box indicates when the contact switch is tripped so that the high speed camera images can be synchronized with the accelerometer data during post-processing.

The experiment's projectile, shown in Figure 6, is fabricated out of 2017 aluminum alloy and is designed to hold two wireless accelerometers (see Section III E). Spherical markings of various radii are placed outside of the sphere and are used for target monitoring during the post-processing of the high-speed camera images.

D. Design of the deceleration system

The surface container is decelerated using two 7 cm thick honeycomb panels. The HexWeb HRH-10 Aramid Fiber panels have a cell size of 4.8 mm and a density of 96 kg/m 3 . Together, the panels can absorb at least 5 kJ of energy or the equivalent energy of the falling surface container in the most extreme test scenario used in these trials (3 m drop height).

E. Data acquisition

In the experimental setup, two different methods are available for capturing the motion of surface container and projectile. First, 3-Space Sensors, sold by YEI Technology, are mounted to the projectile and surface container. These sensors are data-logging devices that contain an Attitude and Heading Reference System (AHRS), an Inertial Measurement Unit (IMU), and a micro-SD card for on-board data storage. The YEI 3-Space Sensors were specifically selected because of their low mass (28 g), low dynamic range (selectable from ±2 g/±4 g/±8 g with a noise density of 99 µg/ √ Hz), and high shock resistance (up to 5000 g). 37 These features allow the sensors to record the impact between the projectile and the sand with high precision and to survive to the final shock at the end of the drop. The sensors can also be integrated into the projectile without impacting its desired design features, such as size and weight. In addition to the accelerometers, an Ultima APX-RS Photron FASTCAM with a Sigma 24-70 mm f/2.8 DG lens is used to capture high-speed images of the projectile collision at 1000 frames per second and a 1024 × 1024 pixel resolution. The camera was placed at a distance of ∼2.7 m from front of the surface container, and a focal length of 24 mm was used for the lens.

IV. THEORY

In order to customize this novel Atwood setup for different experimental trials, several attributes of the system must be calculated. For example, the available ranges of counterweight masses, separation distances between the projectile and container, and starting heights of the container must be identified. Sections IV B-IV D provide a theoretical understanding of the system and the scientific trials that are consequentially available given the setup's physical constraints.

A. Sizing of the Atwood machine counterweights

The "reduced-gravity" element of the experiment is introduced through the use of a counterweight and pulley system, which allows the surface container to have a constant downward acceleration less than that of gravity. If pulley friction and chord elasticity are neglected, then the controlled acceleration is simply a function of mass. The expression for the surface container's acceleration a s is derived by balancing the forces on the container (subscript, s) and counterweights (subscript, cw) and is given by Equation ( 1), where m s is the mass of the surface container and m c w is the total combined mass of all counterweights,

a s = g ( m s -m c w m s + m c w ) . (1)

B. Calculating the required starting height for experimental trials

It is necessary to calculate the starting drop height of each experimental trial so that a prediction can be made as to where the collision takes place along the height of the drop tower. Having this information serves two purposes. First, it helps bound what effective accelerations and collision velocities can be achieved within the physical limits of the drop tower, and second, it provides an estimate for where the high-speed cameras should be positioned in order to record the collision and rebound phases of the test.

The total drop height can be described by four phases experienced by the projectile: a free-fall phase, a collision phase, a rebound phase, and a deceleration phase. The freefall phase of the experiment begins when the surface (subscript, s) and projectile (subscript, p) are released from rest and ends when the two objects make contact for the first time. The collision phase begins at the end of the free-fall phase and ends when the projectile returns to the same position relative to the surface, after experiencing some level of surface deformation. Even though the projectile may never return to its starting position, such as in the case of a purely inelastic collision, the definition of this phase assumes it does in order to size for the most extreme height case. The rebound phase begins at the end of the collision phase and ends shortly before the projectile impacts the surface for a second time. The duration of the rebound phase depends on the collision's coefficient of restitution (COR) or ratio of energy lost during the collision. For the purposes of this calculation, a value for the collision's COR is estimated from a literature review (see Section IV C), and the rebound phase is considered to begin at the end of the collision phase and end when the projectile has reached a maximum theoretical separation from the surface. Using this alternative definition for the rebound phase decreases the total height of the experiment while still providing an adequate window of time to observe the projectile's rebound behavior. Finally, the deceleration phase begins at the end of the rebound phase and ends as soon as the surface container is brought to a complete stop.

The theoretical distance traveled and time elapsed during each phase can be derived from kinematic equations and energy conservation. The total height (H total ) required for each trial is then equal to the sum of the heights for the free-fall, collision, rebound, and deceleration phases (h sd , h sc , h sr , and h sdec , respectively),

H total = h sd + h sc + h sr + h sdec . (2)
The total height as a function of velocity and time is given by Equation (3), where V c is the collision velocity, a s is the acceleration of the surface, g is the acceleration of gravity, V sc0 is the velocity of the surface container at the beginning of the collision phase, t c is the estimated collision time, V sc F is the velocity of the surface container at the end of the collision phase, and t r is the estimated rebound time,

H total = V 2 c 2 ( a s g 2 -2a s g + a 2 s ) + V sc0 t c + 1 2 a s t 2 c + V sc F t r + 1 2 a s t 2 r + h sdec . (3)

C. Estimated parameters

Two unknown parameters are required to calculate the total drop height for each experimental trial: the time that elapses during the collision phase (t c ) and the collision's coefficient of restitution (COR) that will determine the rebound time (t r ).

Different collision models can be used to predict the collision time, but only when certain properties of the medium are known. For example, if the equivalent stiffness k * and damping c * of the sand and projectile under low-gravity conditions were known, then the collision time t c could be estimated using the spring-dashpot collision model shown in Equation ( 4), where ω d is the damped natural frequency and m i is the mass of the impactor, 38

t c = π ω d = 2πm i  4k * m i -c * 2 . ( 4
)
When the Young's modulus E of the surface material is known, t c can be estimated using Hertz collision theory for the elastic collision of two spheres,

t c = 2.87 ( m * 2 RE * 2 V c ) 1 5 , (5)
where E * is the equivalent Young's modulus of the impactor and surface materials, m * is the equivalent mass of the two spheres, and R is the reduced radius of the two spheres, [START_REF] Krijt | [END_REF] 1

E * = 1 -ν 1 2 E 1 + 1 -ν 2 2 E 2 , ( 6
) 1 R = 1 R 1 + 1 R 2 , (7)
1 m * = 1 m 1 + 1 m 2 . ( 8
)
While the Hertz collision model does not accurately represent collisions into granular materials, it can still be used to obtain a lower bound estimate on collision time. For example, using the Hertz collision model, Krijt et al. predict a collision time of 1.4 × 10 -8 s for an 8 m/s head-on collision between icy spheres. [START_REF] Krijt | [END_REF] This is the lower-bound collision-time estimate because the Hertzian model considers solid-to-solid body collisions, as opposed to much more inelastic solid-togranular surface collisions. 27,[START_REF] Krijt | [END_REF] Collision times observed in previous granular impact experiments range from 0.04 s to 0.5 s. 27,28,40 Altshuler et al. observe the longest collision times of 0.2-0.5 s while studying the sink dynamics of a 23 g sphere into polystyrene beads. 28 Goldman and Umbanhower find a collision time of 0.1 s for a 10 g disk impacting glass beads at 60 cm/s, a collision time of 0.08 s for a steel sphere with a 1.91 cm radius impacting glass beads at 2.86 m/s, and a collision time of 0.07 s for a 147 g nylon sphere impacting beads at 47 cm/s. 27 Goldman and Umbanhower conclude that the collision time for a steel sphere colliding with glass beads is independent on impact velocity for sufficiently high impact velocities but that a regime change occurs for low collision velocities ( 1.5 m/s), where collision time actually increases with decreasing impact velocity. 27 A study by Ambroso et al. finds a collision time of approximately 0.04 s for a 5 cm diameter wooden sphere impacting glass beads at 226 cm/s. 40 Since the properties of sand under low gravity conditions are unknown and the collisions are likely to be inelastic, it is difficult to predict the collision time using the Spring-dashpot and Hertz collision models. In order to size the experiment structure, an estimate of 0.1 s is used for the collision time. This estimate is based on the study by Goldman and Umbanhower. Though results from Altshuler et al. show longer collision times, Goldman and Umbanhower use a more comparable experimental setup and provide an estimate that still allows for a conservative collision-height calculation.

The second unknown parameter in the height equation is the collision's coefficient of restitution. Space Shuttle payload experiments by Colwell and Taylor and Colwell show that the COR for impacts at less than 12 cm/s are either unobservable or as low as 0.02. [START_REF] Colwell | [END_REF]26 At the other extreme, analysis of the Hayabusa-1 touchdown data on asteroid Itokawa suggests that the COR is as large as 0.84, 16 although there are many uncertainties associated with this measurement. To make an approximation on the height calculation, an average value for the COR is estimated based on lower and upper bound observations from previous impact experiments. The average value is used instead of the highest value because the beginning of the rebound phase can still be observed even if the COR is underestimated.

The lower estimate is taken as 0.02 based on Colwell, and the upper estimate on COR is taken from studies observing low-velocity collisions between centimeter-sized dust aggregates in drop tower configurations. Beitz et al. find an average COR of 0.35 ± 0.12 for collision velocities of 0.8-37 cm/s between agglomerates made of SiO 2 micrometer-sized dust particles. 30 In another drop tower experiment that extends the work of Beitz et al., Schräpler et al. develop an expression for COR as a function of impact velocity for collision velocities in the range of 1-10 cm/s. 31 This expression, shown in Equation ( 9), acts as the upper-bound for the COR estimate,

COR ma x = 0.11V -0.51 c (m/s) 0.51 . ( 9
)
Based on the studies of Colwell, Bietz et al., and Schräpler et al., the COR of the collision is estimated using Equation (10) for collision velocities >1 cm/s. This estimate is not the expected COR of the collision but simply a reasonable guess that will allow for the sizing of the experiment,

COR mean = 0.02 + 0.11V -0.51 c 2 (m/s) 0.51 . (10)

D. Tower capabilities and constraints

Figure 7 shows how the separation distance between the projectile and surface material and how the required experimental height, from the initial release to the beginning of the deceleration phase, evolves for different effective surface accelerations and collision velocities. The curves on the plot are constructed using Equation (3), and the shading indicates the range of possible height values caused by estimating the coefficient of restitution and collision time in the calculations, as discussed in Section IV C. The plot shows that for a given impact velocity, the required height increases as the effective acceleration decreases. Also, for a given impact velocity, initial separation distance decreases as the effective acceleration increases. As a consequence, the lowest effective acceleration that can be observed, for a given collision velocity in the experiment, is constrained by the drop tower's physical height, and the highest effective acceleration that can be observed is limited to a separation distance that is physically reasonable to set up and measure.

V. RESULTS

The performance of the experimental setup is analyzed based on a number of qualitative and quantitative factors, such as projectile and surface material visibility, surface container deceleration, system friction, and release synchronization. The descent of the surface container is of particular interest because the guide rail, pulley, and counterweight guide tube features of the experiment cause the desired acceleration of the surface container to deviate from its theoretical value.

A. Experiment performance and repeatability

The difference between the surface container's actual and predicted acceleration is analyzed using accelerometer data from drop tests with varied counterweight masses. The top plot in Figure 8 shows the container's vertical acceleration profile for a sand-filled container dropped from a height of 2.20 m FIG. 8. Top: acceleration profile for a 160 kg, sand-filled container dropped from a height of 2.20 m. The effective acceleration of the surface material increases as counterweight mass increases. Bottom: zoomed-in view of the surface container's vertical acceleration over the free-fall period. Due to friction, the container's acceleration fluctuates along the length of the rails, though more so at the top of the rails than the bottom. TABLE I. Comparison of theoretical acceleration (a t h ) and experimental accelerations (a e x ) for a 160 kg, sand-filled surface container dropped from a height of 2.0 m. The container's experimental acceleration is determined by averaging the raw accelerometer data over the container's free-fall period, excluding fluctuations from the container's initial release. The experimental free-fall duration (t e x ) is estimated by taking the elapsed time between the peaks in the accelerometer data that indicate the initial release and then impact with the honeycomb panels. 8. The percent difference between the actual and predicted accelerations range from 0.93% to 4.31%, increasing with increasing counterweight mass. Also as expected, the container's free-fall duration increases as more counterweights are added to the system. The duration of the free-fall phase can be estimated from the experimental trials by taking the elapsed time between the peaks in the accelerometer data that indicate the container's initial release and then its impact with the honeycomb panels. Table I lists the container's estimated free-fall duration for the 2.0 m drop tests.

m c w (kg) t e x (s) a t h (m/s 2 ) a e x (m/st 2 ) % diff
The accelerometer data from these trials are also used to make observations about the combined friction in the guide rails, pulleys, and counterweight guide tubes. The bottom plot in Figure 8 shows a zoomed-in view of the surface container's Y-axis acceleration over the free-fall period. After the container is released, its acceleration fluctuates along the length of the rails, though more so at the top of the rails than the bottom. In addition to friction and cord elasticity, these fluctuations may be caused by deformation in the rails or backand-forth tilting of the surface container as it falls.

The surface container's deceleration profile is used to quantify the container's shock upon impact with the honeycomb and to verify that the container is brought to a controlled stop. Four drop tests were performed at a height of 3 m to verify the functionality of the deceleration system. For these tests, a KISTLER type 8704B500 accelerometer with a range of ±500 g and sensitivity of 10.4 mV/g was fixed on the surface container in order to record the container's deceleration profile. As seen in Figure 9, the surface container experiences a shock of about 80 g at impact. The honeycomb panels successfully prevent the container from becoming damaged after the fall. Accelerometers installed inside of the surface container and projectile are used to analyze the consistency of the electromagnetic release mechanism. Like friction, the conditions of the projectile's release influence the calibration of the container's starting height. If the release of the projectile is not synchronized with the release of the container, then the delay needs to be accounted for so that the projectile's collision with the surface will take place within the camera's fixed viewing window. Though the release of the surface container and projectile is not perfectly synchronous, initial testing reveals that no additional calibration is required to compensate for the difference.

During several of the performance tests, the surface container was partially filled with sand, and high speed cameras were used to capture the container's descent. The images are used to verify that the sand does not lift and obstruct the view of the projectile during the fall. Figure 10 shows a sequence of images for a 2.0 m drop test. For this test, the initial separation distance between the projectile and surface material is ∼3.0 cm. The projectile is clearly visible within the FIG. 10. Images sequence depicting the initial release, free-fall, and collision phases for a 2.0 m drop test. The initial separation distance between the projectile and surface material is ∼3.0 cm. The top row of images is from an AEE MagiCam SD100 camera that was mounted inside of the surface container, and the bottom row of images is still frames from the Ultima APX-RS Photron high-speed camera.

camera frame throughout the release, free-fall, and collision phases of the trial.

These trials are also used to study the packing of the surface material upon impact with the honeycomb. If the sand level changes between the start and finish of the experiment, then a method to re-prepare the sand is required so that the bulk density of the surface material remains consistent between trials. In the calibration tests, it was observed that the sand was no longer level at the end of the deceleration phase. Once the sand was brushed, however, its level measurement was unchanged from its value at the beginning of the trial. One hypothesis is that the sand compresses during its initial shock but becomes disturbed and uncompressed during the oscillations that proceed the shock. The deceleration system naturally regulates the bulk density of the surface material, indicating that no extra treatment of the surface sample is likely to be required between trials.

VI. DISCUSSION

Preliminary verification tests indicate that the experiment setup accomplishes its primary purpose: to provide a surface container with a controllable downward acceleration so that a free-falling projectile may impact a surface in a reducedgravity state. Figure 11 shows the acceleration profiles of the projectile and surface container for a scientific trial with a starting drop height of 2.20 m, an initial separation distance between the projectile and surface of 3 cm, and a counterweight mass of 4.8 kg. The data were smoothed using a low pass filter in MATLAB, with a cutoff frequency of 10 Hz. In this trial, the surface has an effective acceleration of approximately 0.82 m/s 2 . The surface container and projectile are released from rest around the 0.40 second mark. Then, the projectile and surface container are in free-fall until the projectile collides with the surface at 0.75 s. The collision phase lasts for approximately 0.19 s, or nearly twice as long as the duration used in theoretical calculations, and does not result in a detectable rebound. After an estimated 0.67 s of free-fall, the surface container collides with the honeycomb panels and begins to decelerate. A complete analysis of the collision dynamics from this trial, as well as other trials with varying parameters, will be part of a future study.

The combined realities of friction and air drag cause the container's downward acceleration to deviate from its theoretical value. There is also a significant shock that can be observed on the surface sensor (at ∼0.5 s in Figure 11) shortly after the release when the chain, visible in Figure 5, falls onto the box. This is, however, not problematic as long as the chain impact occurs well before the projectile collides with the sand. In the experiment's current state, friction is difficult to correct for because it varies significantly with time. However, to reduce the impacts of friction on the container's acceleration profile, a lubricate can be added and re-applied to the guide rails between trials. The experimental trials can also be configured so that the drop height is skewed towards the bottom of the tower, where the container's acceleration profile tends to fluctuate the least.

Without adding additional counterweight mass to the system, the surface container is naturally slowed to a downward acceleration of approximately 0.1 m/s 2 , marking the lowest possible effective acceleration that the setup can achieve. This acceleration is within the scope of planned trials and is less than that of asteroids (1) Ceres and (4) Vesta, with surface gravities of 0.29 m/s 2 and 0.25 m/s 2 , respectively. 41,42 The largest effective acceleration that has currently been tested (1 m/s 2 ) is comparable to the surface gravity of Saturn's moon Enceladus.

Accelerations lower than 0.1 m/s 2 can be achieved by redesigning the setup for a free-falling container. This would eliminate the constraints that rail friction imposes on the system. In addition, the unique experimental data obtained in these trials may also be valuable to benchmark different numerical simulation approaches (distinct element method, finite element method, etc.). These simulations can then subsequently be used to extrapolate the results to even lower gravity regimes.

Measurements of the surface material's level before and after each trial indicate that the system inherently disturbs the material during the deceleration process, effectively holding the bulk density of the surface material constant between trials. If as few as 2 trials are performed on a given day, then no additional actions are required to prepare the sand between each trial. However, the level of the sand should still be monitored between trials so that it can be treated in the case that packing is observed.

In addition to providing quantitative insight into the setup's performance, calibration trials help configure the setup's data acquisition tools and ground support equipment. These activities include optimization of camera placement, camera lighting, and camera and accelerometer settings.

VII. CONCLUSIONS

The presented design has been used to study low-velocity collisions into granular surfaces. The first round of testing used a 10 cm metallic sphere as a projectile and sand as a surface simulant. The collision velocities and effective accelerations that can be attained in this setup are constrained by the size and physical limitations of the drop tower structure. The system can currently support effective accelerations of ∼0.1 to 1 m/s 2 , though higher values can be reached by simply adding more counterweight mass to the pulley sub-system. Other test parameters are easily adjustable and open-up possibilities for future experimentation. For example, the release mechanism can support changes to the size, mass, and shape of the projectile, while the container can hold a range of surface simulant depths and materials.

Since the surface materials in this setup are subject to the effects of gravity at all times, this setup does not have the same concern for particulate lift as parabolic flight experiments. Therefore, there are no limitations from the perspective of the initial release as to the size or material of granular matter that can be used. However, it should be noted that air resistance may be a limiting factor in grain-size selection. The interstitial air effect is negligible for grains with diameters >0.1 mm. [START_REF] Katsuragi | Physics of Soft Impact and Cratering[END_REF][START_REF] Pak | [END_REF] This is demonstrated specifically for low-speed impacts into a granular material by Katsuragi and Durian. 45 The only constraint to the type of solid material that can be used in the current setup is that it must survive the shock of deceleration.

Outside of collision experiments, this setup can be used to test the general strength and structural properties of granular materials in reduced-gravity environments. For instance, the surface container can be redesigned to incorporate penetrometers and shear tools. The container can also be redesigned to accept entire mobility or sampling mechanisms in order to validate the mechanism's at-rest and in-motion surface interactions. For these tests, the particle size and effective acceleration of the surface material can be altered to mimic the properties of asteroids and comets. The limiting factor in using this setup for other experiments is the short duration of the drop time. If the surface container free-falls (i.e., no counterweights) from the maximum possible drop height of 5 m, only 1 s elapses before the container impacts the deceleration material.

The drop tower structure, the counterweight and pulley system, and the deceleration system are stand-alone components of the experiment in the sense that they do not require any redesign or re-calibration in order to be used for other tests. This aspect of the experiment makes it ideal for frequent, inexpensive, and repeatable experimentation.

FIG. 1 .

 1 FIG. 1. Relative regimes accessible from known Atwood machines. Approximate values for the Asthuler et al. setup are based on data extracted from Asthuler et al., Figures 2 and 3.28 Approximate values for the Goldman and Umbanhower setup are based on ranges of collision velocities and gravity levels provided in Goldman and Umbanhower, Section II.27 Theoretical values for the ISAE-SUPAERO setup are calculated as described in Section IV.

  FIG. 1. Relative regimes accessible from known Atwood machines. Approximate values for the Asthuler et al. setup are based on data extracted from Asthuler et al., Figures 2 and 3.28 Approximate values for the Goldman and Umbanhower setup are based on ranges of collision velocities and gravity levels provided in Goldman and Umbanhower, Section II.27 Theoretical values for the ISAE-SUPAERO setup are calculated as described in Section IV.

  FIG.2. Isometric line drawing illustrating the basic operation of an Atwood machine.

FIG. 6 .

 6 FIG.6. Semi-hollow experiment projectile, with two mounted YEI 3-Space Sensors.

FIG. 7 .

 7 FIG. 7. Top: experimental height for different effective surface accelerations and collision velocities. Shading indicates the range of possible height values caused by estimating the coefficient of restitution and collision time. The required experiment height increases as the desired effective acceleration decreases. Bottom: initial separation distance between the projectile and surface material for different effective surface accelerations and collision velocities. The initial separation distance decreases as the desired effective acceleration increases.

FIG. 9 .

 9 FIG. 9. Deceleration profile of the surface container at the end of a 3 m drop, with normalized acceleration on the y-axis and time on the x-axis. The surface container experiences a shock of about 80 g upon impacting two 10 cm thick honeycomb panels.

FIG. 11 .

 11 FIG. 11. Acceleration profile of the projectile and surface container for a scientific trial with a starting drop height of 2.20 m, an initial separation distance between the projectile and surface of 3.0 cm, and a counterweight mass of 4.8 kg. The surface has an effective acceleration of approximately 0.82 m/s 2 . The collision phase lasts for approximately 0.19 s and does not result in a detectable rebound.

  The data were smoothed using a low pass filter with a cutoff frequency of 10 Hz. As expected, the surface material's effective acceleration decreases with increasing counterweight mass. Based on Equation (1), a 160 kg surface container should have accelerations of 9.71, 9.47, 9.24, and 9.01 m/s 2 for counterweight masses of 0.8, 2.8, 4.8, and 6.8 kg, respectively. The container's acceleration during the experimental trials is determined by averaging the raw accelerometer data during the container's free-fall period, excluding fluctuations from the container's initial release. TableIlists the theoretical and experimental accelerations of the surface container for the different trials depicted in Figure

					in a
	0.8	0.645	9.71	9.62	0.93
	2.8	0.667	9.47	9.16	3.38
	4.8	0.674	9.24	8.87	4.02
	6.8	0.684	9.01	8.63	4.31
	and resisted by counterweight masses of 0.8, 2.8, 4.8, and
	6.8 kg.

ACKNOWLEDGMENTS

This project benefited from some financial support from the Centre National d'Etudes Spatiales (CNES) and was a collaborative effort between several departments at ISAE-SUPAERO. Alexandre Cadu and Anthony Sournac, both from the Département Electronique, Optronique et Signal (DEOS) and Systèmes Spatiaux (SSPA), provided extremely valuable help and advice concerning the installation of the electromagnetic release system. Emmanuel Zenou, from the Département d'Ingénierie des Systèmes Complexes (DISC), helped prepare the experiment in such a way to facilitate the image analyses during the data processing. Daniel Gagneux and Thierry Faure, from the Département Mécanique des Structures et Matériaux (DMSM), completed the experiment's detailed design and fabrication. Lastly, we thank Jens Biele for his helpful comments on this manuscript.


Zhichao Li
email: zhichao.li@teledetection.fr

Thibault Catry
email: thibault.catry@ird.fr

Nadine Dessay
email: nadine.dessay@ird.fr

Emmanuel Roux
email: emmanuel.roux@ird.fr

Etienne Mahé
email: etienne.mahe@ird.fr

Frédérique Seyler
email: frederique.seyler@ird.fr

MULTI-SENSOR DATA FUSION FOR IDENTIFYING MALARIA ENVIRONMENTAL FEATURES

Keywords: Multi-sensor, optical and SAR, classification, remote sensing, malaria

Malaria remains a persistent health problem in the crossborder area between French Guiana and Brazil. Various factors like environmental changes, population movements, etc. significantly influence the malaria transmission. In such context, optical and SAR data were used for producing a precise land cover map, including the resting and breeding sites of malaria vectors and human habitats. This study is a foundation for quantifying the degree of exposure of human populations to malaria vectors and thus for estimating and monitoring the malaria transmission risk.

INTRODUCTION

Malaria mostly covers tropical and subtropical zones, with an estimated 214 million new cases and 438,000 deaths in 2015 [START_REF] Seyler | World Malaria Report 2015[END_REF]. In the Americas, the Amazonian region is the most affected by the disease. Movement of malaria across international borders poses a major obstacle for malaria elimination due to various human activities, population movement, human vulnerability due to limited formal education and few financial resources, inadequate health systems, etc. [START_REF] Wangdi | Cross-border Malaria: A Major Obstacle for Malaria Elimination[END_REF]. Natural and human-made ecosystem changes, such as climatic perturbation and land use and land cover (LULC) changes, can affect significantly the distribution of malaria by making disease vector (anopheles darlingi mosquito) habitats more or less suitable [START_REF] Stefani | Land cover, land use and malaria in the Amazon: a systematic literature review of studies using remotely sensed data[END_REF][START_REF] Li | Mapping a Knowledge-Based Malaria Hazard Index Related to Landscape Using Remote Sensing: Application to the Cross-Border Area between French Guiana and Brazil[END_REF].

Using remote sensing is a pertinent tool that can characterize almost completely and continuously the environment related to malaria transmission on both sides of the international border. Stefani et al. (2013) did a systematic review of literature showing that many studies in the Amazon successfully used optical data for accessing the different land cover types related to malaria and analyzed their relationship with entomological and / or epidemiological data [START_REF] Stefani | Land cover, land use and malaria in the Amazon: a systematic literature review of studies using remotely sensed data[END_REF]. However, the identification of wetland types (vegetated or not, persistent water, temporarily flooded areas, etc.) considered as main resting and/or breeding sites for malaria vectors, is a formidable challenge due to dense vegetation cover in tropical areas. Lband Synthetic aperture radar (SAR) imagery has the capability to penetrate such barrier and is sensible to the variation of soil moisture and water level, which could provide more detailed information about wetlands. Thus, the integration of optical and SAR data is a suitable solution for the production of a precise land cover map related to malaria transmission.

The objective of this study is to identify the different LULC types related to malaria transmission combining optical and SAR data.

STUDY AREA

The Franco-Brazilian cross-border region considered here is composed by the French municipalities of Camopi, Saint-Georges-de-l'Oyapock and Ouanary, which are located on the western bank of the Oyapock River, and the Brazilian municipality of Oiapoque on the eastern bank of the river (Figure 1). Human habitats are mainly located in Saint-Georges (FR), Oiapoque (BR) and the villages of Camopi (FR) (see the red rectangles in Figure1).

DATA AND METHOD

LULC types and dataset

The LULC types related to malaria transmission in study area were visually identified using the review of [START_REF] Stefani | Land cover, land use and malaria in the Amazon: a systematic literature review of studies using remotely sensed data[END_REF], aerial images (BD-ORTHO®IGN, 50 cm, 2006) and one archive PALSAR image (Table 1). Forest and flooded forest are usually identified as resting site of adult vectors [START_REF] Stefani | Land cover, land use and malaria in the Amazon: a systematic literature review of studies using remotely sensed data[END_REF][START_REF] Li | Mapping a Knowledge-Based Malaria Hazard Index Related to Landscape Using Remote Sensing: Application to the Cross-Border Area between French Guiana and Brazil[END_REF]. Free water, flooded forest and swamps can provide potential larval habitats [START_REF] Stefani | Land cover, land use and malaria in the Amazon: a systematic literature review of studies using remotely sensed data[END_REF]. The optical and SAR data used in this study cover the two major areas of human presences: Saint-Georges and Oiapoque.

Pixel class attribution of optical-SAR classifications

Pretreatments of optical and SAR data

One SPOT 5 image was acquired at 1A level in which radiometric calibration and orthorectification have been successively performed by using the ENVI 5.1. For the PALSAR image acquired at level 1.5 in which radiometric calibration, geometrical correction and speckle filtering (Lee filter 3*3) have already been performed by Japanese Space Agency (JAXA).

Multi-sensor classifications

Several spectral indices were calculated from the pretreated SPOT 5 MS image (Table 2). A decision tree-based classification was then carried out for mapping the various LULC types using ENVI 5.1 (Figure 2). The resulting classification was post-processed using the majority analysis (kernel size 3*3) for removing the isolated pixels. Finally, we evaluated the classification by randomly choosing 20 sites in each class and establishing the actual land cover of these sites by photo-interpretations of one Pleiades image dated September 30, 2012 with a 50centimeter spatial resolution.

A textural analysis (GLCM under Sentinel tool box) was performed and a simple thresholding method on the variance of PALSAR signal and a colored composition (Red = VH, Green = VV, Blue = VH/VV) was then used for extracting bare soil, flooded forest, forest, free water and swamps.

Figure 3: Summarized method of optical-SAR classification fusion for mapping the malaria environmental features.

Decision level fusion of classifications

Combining the information from SPOT 5 and PALSAR data using the knowledge-based decision rules: 1) If a pixel belongs to PALSAR free water or SPOT 5 free water, then the pixel is labelled as free water; 2) If a pixel belongs to PALSAR bare soil or SPOT 5 bare soil/built-up/road, then the pixel is labelled as bare soil/built-up/road; 3) If a pixel belongs to one of PALSAR wetlands classes and SPOT 5 vegetation class, then the pixel is labelled as wetland. The overview of method is presented in Figure 3.

RESULTS AND DISCUSSION

Results

Figure 4 showed the resulting SPOT 5 classification. The overall accuracy was found to be 0.90 and the kappa coefficient was 0.84. Comparing with the SPOT 5 classification, this method allowed us to characterize free water, flooded forest and swamps (Figure 5) using an archive 2010 image. Moreover, it is important to remind that we used free archive PALSAR imagery in order to test the contribution of L-band SAR data to the identification of malaria environmental features, before considering the acquisition of expensive new L-band data.

The validation of PALSAR classification is complicated but some insights could be obtained from 2010 archive optical imagery if the cloud cover is not too dense. Recent field surveys during the similar period (similar season) can also be used for the validation since the identified wetlands are not strongly changed with time.

Figure 6 shows the various types of wetlands identified in the study area during a field survey conducted in 2015 where open waters (rivers A and ponds B), and various types of swamps (C and D) more or less densely covered by surface vegetation. L-band SAR (here 12.5 m resolution PALSAR data) is an efficient tool for the detection of large wetlands, which can define main resting, and breeding sites of malaria vectors.

Discussion

This study shows that fusion of land cover layers extracted from optical and SAR high-resolution imagery is an efficient tool for detecting large permanent (or long-lasting) wetlands (potential breeding sites) and various forest types (potential resting and feeding sites). Land cover classification of SPOT 5 image suffers two mains flaws: the impossibility to penetrate the forest canopy and disturbance from persistent cloud cover in tropical areas. In particular, L-band SAR has proven its ability to detect the water under forest, a key element for the identification of breeding sites of malaria vectors and malaria vector control.

Until now, only free archive SAR data were used in order to test the methodology but the acquisition of PALSAR 2 data is planned. Its main advantage is the possibility to access very high spatial resolution. Combining very high resolution optical (SPOT 6-7 / Pleiades) together with the very high resolution SAR permit characterize the urban structures (Figure 7) and minor sizes, non-permanent water bodies such as domestic wetlands within the urban areas, at the direct contact with vulnerable populations. The layers of forest / vegetation and wetland types, are then combined to a layer of urban classification and 2 fieldbased layers: one regarding the entomology of mosquitoes and the malaria epidemiology in the area. These combined layers allow the calculation of landscape indicators and consequently the creation of malaria transmission risk map integration vector-related environmental information (hazard estimation) and human related environmental factors (vulnerability assessment).

CONCLUSION AND PROSPECTS

This study provides a method of identification of environmental features associated with malaria vector resting and breeding sites. Optical and SAR remote sensing have been proved to be complementary for this purpose. This method provides the complete LULC information about malaria transmission, which may be automatically and easily reproduced at temporal scale using a time series of images. The fusion of layers containing spatialized information extracted from optical and SAR data from various sensors and at various resolutions aims at producing indicators and malaria transmission hazard/risk maps to support decision-making processes by local institutions /authorities and public health actors.

Figure 1 :

 1 Figure 1: Map of cross-border region between French Guiana and Brazil.

Table 2 :Figure 2 :

 22 Figure 2: Classification decision tree. Decision tree nodes are in grey.

Figure 4 :

 4 Figure 4: Pixel-based land cover map of SPOT 5 image in the region of Saint-Georges and Oiapoque.

Figure 5 :

 5 Figure 5: Wetlands land cover map from PALSAR image in the region of Saint-Georges and Oiapoque.

Figure 6 :

 6 Figure 6: Various types of wetlands identified in region of Saint-Georges and Oiapoque.

Figure 7 :

 7 Figure 7: Pixel-based land cover map based on aerial images of 2006 and the domestic (observed and potential) wetlands within urban area (Google Earth background) in Oiapoque.These results of land cover classification fusion from optical and SAR data are the basis of a landscape-based model of malaria transmission risk (Figure8)[START_REF] Li | Mapping a Knowledge-Based Malaria Hazard Index Related to Landscape Using Remote Sensing: Application to the Cross-Border Area between French Guiana and Brazil[END_REF].

Figure 8 :

 8 Figure 8: Landscape-based model of malaria transmission risk involving the decision-level fusion of spatialized information layers [4].

Table 1 :

 1 LULC types and data used in this study

	LULC types	Data/Sensors
	Bare soil / built-up / road
	Forest	SPOT 5 (10 m)
	Medium vegetation	Multispectral (MS)
	Herbaceous vegetation
	Slash-and-burn / asphalt
	Free Water

(2012/10/14) (Dry season) Bare soil Flooded forest Forest Free Water Swamps PALSAR (12.5 m) (L-band, 2010/10) (Dry season)

ACKNOWLEDGMENTS

This research was supported by the TéléPal project (CNES-TOSCA 2014 call), the Biodiva program funded by GUYAMAZON action (IRD, MAEDI, Brazilian FAPs), the GAPAM-Sentinela project (Guyamazon call) and the China Scholarship Council.


Zhichao Li
email: zhichao.li@teledetection.fr

Thibault Catry
email: thibault.catry@ird.fr

Nadine Dessay
email: nadine.dessay@ird.fr

Emmanuel Roux
email: emmanuel.roux@ird.fr

Frédérique Seyler
email: frederique.seyler@ird.fr

MAPPING SOIL TYPOLOGIES USING GEOMORPHOLOGIC FEATURES EXTRACTED FROM DEM AND SAR DATA: AN ENVIRONMENTAL FACTOR AFFECTING MALARIA TRANSMISSION IN THE AMAZON

Keywords: Soil typologies mapping, SRTM, ALOS PALSAR, geomorphology, malaria

Soil typologies are characterized by different distribution and circulation of water, associated with specific water chemical properties, and thus potentially affects the distribution and density of malaria vectors in the Amazon. Based on a conceptual model of tropical soil evolution and distribution, the curvature of watershed slopes is a key indicator of soil typology. The average curvature of each subwatershed in our study area was identified using 30 m resolution SRTM data. PALSAR image with a spatial resolution of 12.5 m was used to implement a land cover map, including water and non-water surfaces. The nonwater surface was then integrated with the average curvature of subwatersheds for soil typologies mapping. An indirect approach of soil typologies prediction was proposed, which is complementary with the classical soil classification methods.

INTRODUCTION

Malaria is one of the most common vector-borne diseases in the world, with an estimated 214 million new cases and 438,000 deaths in 2015 [START_REF] Seyler | World Malaria Report 2015[END_REF], which mostly covers tropical and subtropical zones. In the Americas, the Amazonian region is the most affected by the disease. Soil typologies play an important role in malaria transmission. Actually, the distribution and density of malaria vectors is impacted by water physical and chemical properties, which are linked with soil typology.

In the Amazon, sustained high temperature and heavy rainfall significantly affect soil evolution and consequently geomorphologic features. Geomorphologic features (elevation, slope, aspect, curvature, etc.) are then indicators of soil evolution. They are the consequence of movements and redistribution of water and matter in the landscape [START_REF] Mulder | The use of remote sensing in soil and terrain mapping -A review[END_REF]. Moreover, soil typologies mapping is a difficult topic in the Amazon because of dense forest cover and poor accessibility for field data [START_REF] Guiet | Landform and landscape mapping, French Guiana (South Americ)[END_REF]. In such context, using remote sensing is a solution for digital soil typologies mapping. Digital elevation model (DEM), combined with optical and/or Synthetic Aperture Radar (SAR) imagery may be used to characterize geomorphologic features and water distribution [START_REF] Mulder | The use of remote sensing in soil and terrain mapping -A review[END_REF]. SAR data are cloud-penetrating which enables to reduce the information omission caused by clouds.

The objective of this study is to propose a novel method of digital soil mapping based on geomorphologic features determination and remote sensing.

STUDY AREA

This study was carried out in an Amazonian area, the cross-border region between French Guiana and Brazil, corresponding to the Oyapock basin and including the French municipalities of Camopi, Saint-Georges-de-l'Oyapock and Ouanary, and the Brazilian municipality of Oiapoque (Figure 1). The altitude increases from the northeast to the southwest, with the eastern coastal lowlands and the western highlands (Figure 1). The climate is typically tropical, with an average annual temperature of 27°C and humidity ≥ 80%.

DATA AND METHODS

Knowledge-based model of soil evolution

In our study area, ferralsols derived from a granitogneissic basement are dominant. The soil evolution presents three main processes (Figure 2) [START_REF] Melfi | Distribuição dos solos nas paisagens lateríticas da Amazônia. Elaboração de um modelo conceitual ilustrando a evolução hydro-biogeoquímica destas paisagens[END_REF]:

1. Vertical lessivage of ferralsols with the lateral impoverishment of ferrum; 2. Impoverishment and redistribution of ferrum with the blockage of B-Horizon by illuvial clay;

3. Podzolisation hydromorphic: ferralsols is progressively degraded and disappear while clay minerals are attacked. The stages of soil evolution correspond to convex, convex-concave and concave interfluves, respectively. Ferralsols (red) are predominant in convex interfluves. Soils with a little development (Red-yellow) mainly occur in convex-concave interfluves. Hydromorphic soils mainly appear in concave interfluves.

Soil typologies mapping

The Shuttle Radar Topography Mission (SRTM) 30m DEM was used for identifying the curvature, flow direction and the outlet of each subwatershed. The flow direction data and outlets were used to compute a layer of subwatersheds, which was integrated with curvature layer using zonal statistic for determining the average curvature of each subwatershed. Finally, all subwatersheds were classified as convex, concaveconvex and concave, which the three soil types in the soil evolution model. The above data processing was performed using Arcmap 10.2.

Water surfaces and artificial lands do not contain information relevant for soil typologies mapping [START_REF] Mulder | The use of remote sensing in soil and terrain mapping -A review[END_REF]. Moreover, artificial lands accounts for a very small proportion in our study area. One ALOS PALSAR image dated August 7, 2010 with a spatial resolution of 12.5 m (radiometrically calibrated, with the speckle filtered and geometric correction) was used to perform a textural analysis (GLCM under Sentinel tool box). Then, a simple thresholding method on the entropy of SAR signal was used for creating a binary land cover map: water and non-water surfaces. The convex, concave-convex and concave were applied on the nonwater surface for implementing the specialization of soil typologies. The entire method is summarized in Figure 3. Figure 4a shows that the water and non-water surfaces are distinctly visible in PALSAR image. Water surfaces were detected using an appropriate threshold value of entropy (figure 4b). The subwatersheds were presented in figure 5. Three soil typologies located in the three different landscapes in term of curvature (figure 6). Figure 6: Map of soil typologies related to soil evolution.

Discussion

In our study, the knowledge of tropical soil evolution is the cornerstone. The proposed method is suitable for our study area because of the predominance of the dense vegetation cover and the small amount of artificial lands. Other environmental factors or geomorphologic features may be integrated in this study. For example, the effect of vegetation as indicator of soil evolution may be relatively easy to characterize using remote sensing and should be considered in the future. High-resolution satellite imagery could be used to extract artificial lands in other study areas where artificial lands accounts for a larger proportion. The validation of soil classification could be performed, using the existing pedological maps, field data and expert knowledge. Moreover, using the proposed method, the DEM at a higher spatial resolution may be used to refine the map of soil typologies which could be used at a local scale.

CONCLUSION ET PERSPECTIVES

This study proposed a novel method of soil typologies mapping based on the knowledge of tropical soil evolution and the combining of DEM and SAR data.

The approach enables the identification of geomorphologic features and the spatialization of soil types. The proposed approach may be used to access the breeding sites of malaria vectors and their water physical and chemical properties.

Figure 1 :

 1 Figure 1: The elevation map and administrative division of study area.

Figure 2 :

 2 Figure 2: The knowledge-based model of soil evolution [4].

Figure 3 :

 3 Figure 3: Summarized methodology for soil typology mapping using DEM and SAR data

Figure 4

 4 Figure 4: a) ALOS PALSAR imagery; b) the binary land cover (water and non-water surfaces) (subset of Saint-Georges city).

Figure 5 :

 5 Figure 5: Example of subwatersheds in our study area.

ACKNOWLEDGMENTS

This research was supported by the TéléPal project (CNES-TOSCA 2014 call), the Biodiva program funded by GUYAMAZON action (IRD, MAEDI, Brazilian FAPs), the GAPAM-Sentinela project (Guyamazon call) and the China Scholarship Council.