V Ansan
email: veronique.ansan@univ-nantes.fr

Upper crustal stratigraphy beneath the InSight lander on Mars and implications for its formation

Keywords: Stratigraphy, Volcanism, Sedimentary filling, Martian regolith, Martian soil, Elysium Planitia

InSight landed on ~60 m thick, Early Amazonian, degraded volcanic flows overlapping >500m thick, Late Hesperian layered volcanic material

Introduction

The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) is the first mission mainly dedicated to the understanding of the Martian internal structure [START_REF] Banerdt | InSight: a discovery mission to explore the interior of mars Lunar Planet[END_REF][START_REF] Banerdt | Initial results from the InSight mission on Mars[END_REF][START_REF] Smrekar | Pre-mission InSights on the Interior of Mars[END_REF]. Among its science tools are a seismometer (Seismic Experiment for Interior Structure, SEIS) for detecting quakes [START_REF] Lognonné | SEIS: Insight's Seismic Experiment for Internal Structure of Mars[END_REF][START_REF] Lognonné | Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data[END_REF], a heat flow probe (Heat Flow and Physical Properties Package, HP 3 ) designed to take the planet's temperature [START_REF] Spohn | The Heat Flow and Physical Properties Package (HP 3 ) for the InSight Mission[END_REF], including a radiometer (RAD) instrument for measuring the surface brightness temperature [START_REF] Spohn | The Heat Flow and Physical Properties Package (HP 3 ) for the InSight Mission[END_REF], a magnetometer (InSight FluxGate, IFG), and sensors for gauging wind and air pressure [START_REF] Gómez-Elvira | REMS: the Environmental Sensor Suite for the Mars Science Laboratory Rover[END_REF]Bandfield et al., 2019).

Determining the best landing site adequate for instruments safety and scientific success was challenging. The pre-landing area had to be close enough to the equator to ensure power to the lander solar panels, have a thick atmospheric column to deploy the parachute and slow down the landing, be relatively flat, and consist of geological material that is both stable for the seismometer and non-cohesive to facilitate the HP 3 mole penetration [START_REF] Golombek | Selection of the InSight landing site[END_REF].

A 130 km x 27 km landing ellipse was selected in the western Elysium Planitia (Fig. 1), ~500 km north of the Martian dichotomy and ~1,300 km southwest of Elysium Mons, in which the mean elevation is around -2600 m without topographic relief except impact craters and wrinkle ridges. At regional scale, the landing area would have been covered by at least 200 m thick smooth volcanic material, locally showing lobate flow margins and probably consisting of basaltic composition [START_REF] Golombek | Geology and physical properties investigations by the InSight Lander[END_REF][START_REF] Pan | Crust stratigraphy and heterogeneities of the first kilometers at the dichotomy boundary in western Elysium planitia and implications for InSight lander[END_REF]. The subsurface would consist of clastic material covering almost the entire region with an average thickness of 3 m and up to 18 m locally, derived from mapping of fresh, no rocky ejecta craters within the landing ellipse [START_REF] Golombek | Selection of the InSight landing site[END_REF][START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]. Based on 100 meter-scale THEMIS data [START_REF] Christensen | The Thermal Emission Imaging System (THEMIS) for the Mars 2001 Odyssey mission[END_REF], a thin sandy layer [START_REF] Presley | Thermal conductivity measurements of particulate materials, Part I: A review[END_REF][START_REF] Piqueux | Temperature-dependent thermal inertia of homogeneous Martian regolith[END_REF]Christensen, 2011, Golombek et al., 2017) would cover the material of the Early Hesperian transitional unit (eHt), comprised of layered volcanic and/or sedimentary materials [START_REF] Tanaka | Geologic map of Mars[END_REF]. So, all sub-surface property conditions were present for favoring both seismology and heat flow acquisition [START_REF] Golombek | Selection of the InSight landing site[END_REF]2018;[START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF].

On November 26, 2018, the InSight lander [START_REF] Banerdt | Initial results from the InSight mission on Mars[END_REF] touched down at 4.502°N/135.623°E [START_REF] Parker | Localization of the InSight lander. 50th Lunar and Planetary Science[END_REF][START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF], ~25 km westward from expected location (Fig. 1), in a probably highly degraded, ~1 m deep, ~27 m-in-diameter impact crater in Elysium Planitia, called "Homestead hollow" [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF][START_REF] Grant | Modification of homestead hollow at the insight landing site based on the distribution and properties of local deposits[END_REF][START_REF] Grant | Modification of Homestead hollow at the Insight landing site based on the distribution and properties of local deposits[END_REF][START_REF] Warner | Geomorphology and origin of Homestead hollow, the landing location of the Insight lander on Mars: LPSC L[END_REF][START_REF] Warner | An impact crater origin for the InSight landing site at Homestead Hollow, Mars: Implications for near surface stratigraphy, surface Processes, and erosion rates[END_REF]. Although the hollow lacks a sharp rim, its boundary shows a significant increase in rocks of cobble up to boulder size as compared to its relatively smooth interior, composed of a majority of fine clastic material ranging from sandy to dusty clasts mixed with few % of pebbles [START_REF] Hauber | Geology of the Insight Landing Site, Mars: Initial Results[END_REF][START_REF] Grant | Modification of homestead hollow at the insight landing site based on the distribution and properties of local deposits[END_REF]2020;[START_REF] Weitz | Clast sizes and shapes at the InSight landing site[END_REF]Weitz et al., , 2020)). No aeolian bedform is identified within the hollow, although the region is subject to seasonal winds of up to 25 m.s -1 [START_REF] Spiga | Atmospheric Science with InSight[END_REF]Bandfield et al., 2020) at the lander deck height of ~1.2 m above the ground, and dust devil tracks are identified near lander location before and during the first 420 Martian days [START_REF] Perrin | Dust devils' signature around the InSight landing site (Mars): analysis of HiRISE satellite images and SEIS seismic data[END_REF]2020;[START_REF] Rodriguez | Searching for geological surface changes around the InSight landing site (Mars) from HiRISE satellite images[END_REF] from High Resolution Imaging Science Experiment (HiRISE) images [START_REF] Mcewen | Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE)[END_REF].

The InSight landing site is characterized by an average surface magnetic field strength of 2013 ± 53 nT with a direction south-east and upward [START_REF] Johnson | Crustal and Time-Varying Magnetic Fields at the InSight Landing Site[END_REF] which is ten times stronger than predicted by satellite-based models (e.g. [START_REF] Connerney | First results of the MAVEN magnetic field investigation[END_REF][START_REF] Langlais | In situ and remote characterization of the external field temporal variations at Mars[END_REF]. Magnetization sources would be carried in older than 3.9 Gy basement crustal rocks overlain by between 200 m and ~10 km of lava flows and modified ancient terrain [START_REF] Johnson | Crustal and Time-Varying Magnetic Fields at the InSight Landing Site[END_REF]. In addition, the upper crustal structure beneath InSight seems to be layered by ~10 km thick, highly altered or fracturated volcanic material, inferred by three major marsquakes that occurred during September, 2019 and the HP 3 hammering activity [START_REF] Giardini | The seismicity of Mars[END_REF][START_REF] Lognonné | Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data[END_REF].

The structure and the depth of upper subsurface layer remains poorly understood beneath SEIS and HP 3 instruments. As the HP 3 mole started digging millimeter by millimeter into the sub-surface, using the friction between clastic material and its sides to progress [START_REF] Wippermann | Penetration and performance testing of the HP 3 Mole for the InSight Mars mission[END_REF], it began to tilt with a dip of ~15° to the southwest at a depth of 10 cm before its sharp stopping at a depth of about 30 cm. While digging was helped using the robotic arm's scoop to press on the side of the mole, the mole popped back out of the regolith on two occasions, questioning about the structure of the sub-surface.

In this context, it is crucial to better understand the nature and the spatial structure of subsurface beneath the lander and its nearby surroundings. Would it be close to a terrestrial regolith, (i.e. a surficial geological formation resulting from the in situ fragmentation of rocks by chemical and mechanical alteration without the elements having been transported), or rather to a lunar regolith where the fragmentation of the rocks is essentially due to meteoritic impacts (impact gardening)? What would be the contribution of sedimentary processes, such as winddriven sedimentation or diagenetic processes in its formation? What would be its detailed structure (homogeneous or layered) and thickness? At which depth are the pristine lava flows?

In this study, we propose to revisit the local stratigraphy of the first hundreds meters of upper crust, using a combination of orbital imagery and in situ imagery acquired by cameras on board of the InSight lander, during the first 420 Martian days (sols) of the mission. From these observations, we highlight the different lithology exposed in this area, their spatial distribution and organization to propose a stratigraphic cross-section of the Homestead hollow and its nearby surrounding. Finally, we discuss the nature, conditions of formation and timing of the various geologic units, and the potential implications for the shallow crustal structure on geophysical data acquired by SEIS and HP 3 .

Data and methods

In order to determine the stratigraphy beneath the InSight lander and its surroundings, we use both orbital and ground datasets. Firstly, the multiscale orbital visible imagery gives us a regional view of geological context. High Resolution Stereo Camera (HRSC) nadir images [START_REF] Neukum | HRSC: The High Resolution Stereo Camera of Mars Express[END_REF][START_REF] Jaumann | The High Resolution Stereo Camera (HRSC) experiment on Mars express: Instrument aspects and experiment conduct from interplanetary cruise through the nominal mission[END_REF] and ConTeXt camera (CTX) images [START_REF] Malin | Context Camera Investigation on board the Mars Reconnaissance Orbiter[END_REF] with an average resolution of 20 m.pixel -1 and 6 m.pixel -1 respectively, are used for the identification of geological features like lava flows, lava fronts, and impact craters. Their analysis is completed by HiRISE images [START_REF] Mcewen | Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE)[END_REF] for the fine details of geological features with a spatial resolution of 0.25 m.pixel -1 . This dataset is reinforced by Thermal Emission Imaging System (THEMIS) images [START_REF] Christensen | The Thermal Emission Imaging System (THEMIS) for the Mars 2001 Odyssey mission[END_REF] to discriminate the thermo-physical nature of surface material, at a resolution of 100 m.pixel -1 . This analysis is performed in association with altimetry data at different scales, including topography acquired by Mars Orbiter Laser Altimeter (MOLA) [START_REF] Smith | The global topography of Mars and Implications for surface evolution[END_REF] at ~463 m.pixel -1 , and Digital Elevation Models (DEM) made from CTX and HiRISE stereo pairs processed by [START_REF] Ferguson | Analysis of Local Slopes at the InSight Landing Site on Mars[END_REF], at spatial resolution of 20 m.pixel -1 and 1 m.pixel -1 respectively, with a vertical accuracy close to 0.13 m and 4.0 m respectively. All orbital data are implemented in GIS, using Equirectangular projection equally sampled in planetographic latitude, with elevations referenced to the MOLA-defined geoid [START_REF] Smith | Mars Orbiter Laser Altimeter: experiment summary after first year of global mapping of Mars[END_REF].

Secondly, the ground color imagery is performed both by the Instrument Context Camera (ICC) placed on the lander, showing the wide-angle view in front of lander with an angular field of view of 2.1 mrad.pixel -1 at the center of the image [START_REF] Maki | The Color Cameras on the InSight Lander[END_REF], leading to a spatial resolution of ∼6 mm.pixel -1 at a range of 3 m from the lander, and the Instrument Deployment Camera (IDC), mounted to the lander robotic arm acquiring images of the lander and surrounding terrain with an angular resolution of 0.82 mrad.pixel -1 at the center of the image, corresponding to a spatial resolution of ∼0.5 mm.pixel -1 at a range of 0.65 m from the IDC [START_REF] Maki | The Color Cameras on the InSight Lander[END_REF]. The topography of the instrument deployment workspace, a 4 m×6 m area located in front of the lander, was performed from Stereo IDC image pairs, acquired by moving the arm between images [START_REF] Trebi-Ollennu | InSight Mars Lander Robotics Instrument Deployment System[END_REF], with a horizontal post spacing ranging from 0.5 to 2mm.pixel -1 [START_REF] Maki | The Color Cameras on the InSight Lander[END_REF][START_REF] Abarca | Image and Data Processing for InSight Lander Operations and Science[END_REF]. These ground data were mosaicked and projected in Cartesian coordinates in Site or lander frame (see Abarca et al., 2019 for details).

The stratigraphy of Homestead hollow was made, using both the main concepts of lithostratigraphy [START_REF] Stenonis | De solido intra solidum naturaliter contento dissertationis prodromus[END_REF] based on the law of superposition (relative age), the principle of original horizontality and the principle of lateral continuity, and planetary chronostratigraphy based on impact crater size-frequency distribution derived from the Moon (e.g. [START_REF] Hartmann | Preliminary note on lunar crateringrates and absolute timescales[END_REF][START_REF] Neukum | A studyof lunar impact crater sizedistributions[END_REF], Neukum et al., 2001) and extrapolated to Mars (e.g. [START_REF] Ivanov | Mars/Moon cratering rate ratio estimates[END_REF][START_REF] Hartmann | Martian cratering 8: Isochron refinement and the history of Martian geologic activity[END_REF]. So, crater-counting methods were used to estimate the absolute ages of surface units. Crater counts were performed on orbital THEMIS, HRSC and CTX data. A sinusoidal projection was used for the crater counts. Buried craters (with no or partial rims exposed, often named ghost craters) were not included in the crater counts of units nor were dense fields of secondary craters. Crater statistics and crater model ages were analyzed with Craterstats2 software [START_REF] Michael | Planetary surface dating from crater size-frequency distribution measurements: Partial resurfacing events and statistical age uncertainty[END_REF][START_REF] Michael | Planetary surface dating from crater size-frequency distribution measurements: Spatial randomness and clustering[END_REF][START_REF] Michael | Planetary surface dating from crater size-frequency distribution measurements: Multiple resurfacing episodes and differential isochron fitting[END_REF], using the production function of [START_REF] Ivanov | Mars/Moon cratering rate ratio estimates[END_REF] and the chronology Hartmann and Neukum (2001).

Combining all cartographic information (spatial organization and age) with landforms and geologic features (e.g. bedforms, layers, rock/clast textures…) from orbital and in situ data, geologic cross-sections and logs were performed at the InSight landing site.

Regional Stratigraphy from orbital data

The previous geological studies showed that the region is covered by at least 200 m thick smooth volcanic material, probably consisting of basaltic composition [START_REF] Ansan | Western Elysium Planitia, What is regional geology telling us about subsurface?[END_REF][START_REF] Golombek | Geology and physical properties investigations by the InSight Lander[END_REF][START_REF] Pan | Crust stratigraphy and heterogeneities of the first kilometers at the dichotomy boundary in western Elysium planitia and implications for InSight lander[END_REF], using a wide variety of imaging during the landing site selection process. These data indicated that this volcanic material had to be overlapped by a dominantly sandy material with low rock abundance, interpreted as ~3-18 m thick, impact fragmented, unconsolidated regolith overlying coarse breccia that grades into jointed basalt [START_REF] Golombek | Selection of the InSight landing site[END_REF]2018, 2020;[START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]. A relatively thin dust layer (dust index of 0.94, [START_REF] Ruff | Bright and dark regions on Mars: Particle size and mineralogical characteristics based on Thermal Emission Spectrometer data[END_REF] covering the area was regularly swept by seasonal winds [START_REF] Spiga | Atmospheric Science with InSight[END_REF] raising dust devils [START_REF] Reiss | Dust devil track survey at Elysium Planitia, Mars: Implications for the InSight landing sites[END_REF]Reiss et al., 2016) whose traces scar the surface at metric scale of High Resolution Imaging Science Experiment (HiRISE) images [START_REF] Mcewen | Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE)[END_REF].

Geological units

Here we revisit the geological units identified in the vicinity of the landing site (~50 km),

showing that the region is essentially composed of layered volcanic material covered by clastic material whose source is both impact ejected rocks and eolian sediments.

While the western Elysium Planitia is relatively flat, the InSight lander touched down on the eastern side of ~NNW-SSE trending, 25 km wide, 80 km long ridge raising ~65 m above the surrounding plain (Fig. 1b andd), based on MOLA topography at ~500 m.pixel -1 . The topographic slope of the ridge side is lesser than 0.4°, which is in good agreement with the prelanding recommendation [START_REF] Golombek | Selection of the InSight landing site[END_REF]. This broad ridge could be either the result of volcanic edifice or tectonic structures. The first assumption seems to be rule out because we observe no obvious primary volcanic landforms at its summit, such as volcanic vents, or fractures, at THEMIS and HRSC scale.

The broad ridge is framed by two parallel, 10 km wide, 10s km long, <50 m high, sinuous wrinkle ridges, recognizable by their asymmetrical morphology and crenulation at their crest (e.g. [START_REF] Maxwell | Distribution, morphology, and origin of ridges and arches in Mare Serenitatis[END_REF][START_REF] Chicarro | Global and Regional Ridge Patterns on Mars[END_REF][START_REF] Watters | Wrinkle Ridge Assemblages on the Terrestrial Planets[END_REF][START_REF] Schultz | Localization of bedding plane slip and backthrust faults above blind thrust faults: Keys to wrinkle ridge structure[END_REF][START_REF] Golombek | Martian wrinkle topography: Evidence for subsurface faults from MOLA[END_REF]. Wrinkle ridges are usually interpreted as compressive tectonic structures (i.e. folds and/or fault-related folds (e.g. Watters, 1988, Suppe and[START_REF] Suppe | Fault-related folding on Earth with application to wrinkle ridges on Mars and the Moon[END_REF][START_REF] Allemand | Modèle fragile des rides martiennes contrait par la géométrie de surface[END_REF][START_REF] Mangold | Wrinkle ridges of Mars: Structural analysis and evidence for shallow deformation controlled by ice rich décollements[END_REF][START_REF] Mueller | Compressional structure on Mars[END_REF][START_REF] Golombek | Mars tectonics[END_REF], created during the cooling and contraction of the magmatic plume below the volcanic plain, or the whole planet. The broad elongated topographic ridge is then interpreted as an "arch", first characterized in lunar mare (e.g., [START_REF] Strom | Lunar mare ridges, rings and volcanic ring complexes[END_REF][START_REF] Bryan | Wrinkle-ridges as deformed surface crust on ponded mare lava[END_REF][START_REF] Maxwell | Distribution, morphology, and origin of ridges and arches in Mare Serenitatis[END_REF], consisting of a broad, low-relief tectonic pop-up structure. Although the formation of wrinkle ridges requires the presence of any layered upper crust, regardless of the nature of the layering, they are commonly thought, based on lunar analogy, to form in volcanic terrains composed of stacks of lava flows.

At CTX resolution, this interpretation is supported by the presence of relatively flat landforms that are lobate in plan view, characteristic of volcanic lava flow fronts [START_REF] Golombek | Geology and physical properties investigations by the InSight Lander[END_REF]. A few kilometers east of the landing site, at least three superimposed lava flows are identified (Fig. 1c), with an apparent eastward flow direction. Based on CTX DEM, the typical thickness at the front of individual flows is ranging from 5 m to 40 m (Fig. 1h), assuming a local horizontal dip, which is quite common on Elysium Planitia (e.g. [START_REF] Vaucher | The volcanic history of Central Elysium Planitia: Implications for martian magmatism[END_REF]. Due to their morphology and their extent, these lava flows were likely composed of relatively low viscosity material during their emplacement. However, it is quite difficult to find local vents or fractures from which lava flows originated.

Around the landing site, the relatively flat and shallowly sloping volcanic surface at CTX scale (Fig. 1c) appears rough and heavily cratered with craters smaller than 200 m in diameter at HiRISE scale (Fig. 1g), without reaching a saturation density (i.e. old impact craters are not packed tightly enough to be destroyed by the creation of new craters [START_REF] Chapman | Cratering of planetary satellites[END_REF][START_REF] Hartmann | Planetary cratering I . The question of multiple impactor populations: Lunar evidence[END_REF]). The large population of small craters was previously interpreted [START_REF] Golombek | Selection of the InSight landing site[END_REF] as a dominant impact-induced regolith (i.e. a layer of unconsolidated solid clastic material covering the bedrock of a planet, due to impact gardening). In this scenario, when meteorites impact the Martian surface, the surface is fragmented, the subsurface material is partly excavated forming a cavity (crater), and ejected into atmosphere before re-impacting around the crater forming an ejecta blanket [START_REF] Melosh | Impact Cratering: A Geologic Process[END_REF]. When the impact process is replicated thousands of times at the same place, a regolith layer forms with a clast distribution decreasing in size to the top of the layer [START_REF] Charalambous | On the evolution of particle fragmentation with applications to planetary surfaces[END_REF], as observed on the Moon. At HiRISE scale (Fig. 2b,c andd), the volcanic surface is cratered by 1-200 m-in-diameter impact craters, relatively spaced with rocky ejecta for those greater than 30 m in size [START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]. Although many small impact craters are surrounded by an ejecta blanket, many of them show a significant degree of degradation. At HiRISE scale (Fig. 2), numerous craters with a diameter ≥ 30m do not show a classic radiating boulder-rich ejecta but a remnant discontinuous ejecta blanket, testifying to significant degradation by aeolian processes in privileged directions, leading to the preservation of a star pattern in the ejecta (Fig. 2d, impact crater in box labelled c and Fig. 4g). This suggests that the area is subjected to winds carrying sandy materials that cause ejecta abrasion.

Sand accumulated near the crater rims forms either leeward wind streaks (e.g. three bright wind streaks (WS) radiating from fresh impact craters, as in the lower right corner of Fig. 1c), or bright crescent-shaped, 10s m long, 10 m wide, ~1 m high dunes on the windward side (Fig. 2). These dunes appear immobile if we compare their location between May 2014 and December 2018, which seems to be in agreement with the light tone of the dunes surface (Baker et al., 2019). Indeed, most mobile dunes display a low albedo, due to their mineralogical composition. Dark dunes are composed of basaltic igneous minerals such as olivine, pyroxene and plagioclase, as detected by orbiter-based thermal infrared and visible/shortwave-infrared (VSWIR) spectroscopic measurements (e.g. [START_REF] Christensen | Identification of a basaltic component on the Martian surface from thermal emission spectrometer data[END_REF][START_REF] Bibring | Mars surface diversity as revealed by the OMEGA/Mars Express observations[END_REF][START_REF] Poulet | Quantitative compositional analysis of martian mafic regions using the MEx/OMEGA reflectance data 1. Methodology, uncertainties and examples of application[END_REF][START_REF] Rogers | Mineralogical characterization of Mars Science Laboratory candidate landing sites from THEMIS and TES data[END_REF][START_REF] Tirsch | Dark aeolian sediments in Martian craters: Composition and sources[END_REF] and rover X-ray diffraction (XRD)/X-ray fluorescence measurements (e.g. [START_REF] Achilles | Mineralogy of an active eolian sediment from the Namib dune, 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]. Dark dunes become lighter-toned due to a reddish dust mantling and internal grain size and organization [START_REF] Blake | Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow[END_REF], although some dunes, such as small ripple dunes and transverse aeolian dunes, are suspected to be light-toned and formed by dust clumps (e.g [START_REF] Balme | Transverse Aeolian Ridges (TARs) on Mars[END_REF][START_REF] Zimbelman | Transverse Aeolian Ridges on Mars: First results from HiRISE images[END_REF][START_REF] Geissler | The birth and death of transverse aeolian ridges on Mars[END_REF]. In addition, ripple and dune fields are locally observed inside relatively larger impact craters (Fig. 2c).

For the small craters and intercrater areas near the landing site, the infilling material is characterized by a relatively smooth, medium grey-toned surface (Fig. 2). This surface appears scoured by 100s of meter long, <10 m wide, dark, rectilinear features, although these features continue in the surrounding rougher areas with a more subdued tone (Fig. 2a,b,d). In HiRISE image acquired in May 2014 (Fig. 2 a,b) and in December 2018 (Fig. 2d), numerous rectilinear dark albedo features are visible with a dominant trend of NW-SE. They are the result of dust devils (e.g. [START_REF] Balme | Dust devils on Earth and Mars[END_REF][START_REF] Cantor | Mars Orbiter Camera observations of Martian dust devils and their tracks (September 1997 to January 446 2006) and evaluation of theoretical vortex models[END_REF]Reiss et al., 2016), raising the dust accumulated from regional and global Martian storms (e.g. [START_REF] Mckim | The dust storms on Mars[END_REF][START_REF] Cantor | MOC observations of the 2001 Mars planet encircling dust storm[END_REF][START_REF] Guzewich | Mars Science Laboratory Observations of the 2018/Mars Year 34 Global Dust Storm[END_REF] During the first 420 sols of the InSight mission, many new linear dark tracks caused by dust devils have been detected near the InSight landing site in orbital images, showing seasonal variations of their frequency and directions [START_REF] Perrin | Monitoring of Dust-Devil Tracks Around the InSight Landing Site, Mars, and Comparison with in-situ Atmospheric Data[END_REF].

In summary, the InSight landing area is composed of two main units: a volcanic unit consisting of stacked volcanic flows cratered by small impact craters, and a surficial unit consisting of aeolian deposits (sand and dust) mixed with rocky ejecta formed by impact gardening, as distinct from a sensu stricto lunar regolith.

Age of geological units

As we observed previously, the volcanic surface is characterized both by a small population of >1km-wide fresh impact craters (i.e. with ejecta blanket) in an area between 4°N and 5.6°N and 135° and 137°E (Fig. 1b,f,Fig. 3). In order to determine the surface retention age, we selected all well-preserved impact craters with ejecta blanket (fresh impact craters) post-dating the surface. On the area (SHRSC) covering 10,437.099 km 2 (Fig. 3), 14 impact craters have been identified with a diameter ranging from 1.08 km to 8.83 km, based on THEMIS and HRSC images. Within this widespread area, a smaller area has been selected from CTX images centered on the InSight landing site (Fig. 3, SCTX, 1,642.369 km 2 ) in which 11,496 impact craters are identified with a diameter ranging from 20 m to 1.50 km. The crater count has not been performed for <150 m-wide craters, due to the high degree of surface degradation.

Based on this dataset, crater counts were carried out on the volcanic unit to estimate its individual absolute model age of formation (Fig. 3). Based on THEMIS and HRSC images, the regional volcanic surface has a crater retention age of 3.6 + 0.1/-0.2 Ga (i.e. the boundary between Early Hesperian and Late Hesperian in Neukum-Ivanov's chronology system; Neukum and Ivanov, 2001), following well the isochrons of [START_REF] Ivanov | Mars/Moon cratering rate ratio estimates[END_REF] for impact craters greater than 2 km in diameter (black line in Fig. 3).

Using the smaller crater population enables the actual surface age of the volcanic surface, after a degradation event, to be characterized. Indeed, a kink in the size-frequency plot occurs for crater diameters ranging between 1.5 km and 2 km. Resurfacing process (red curve in Fig. 3) then overprinted the entire studied area until 2.6 + 0.5/-0.7 Ga (i.e. Early Amazonian in the chronology system of Neukum and Ivanov (2001)), following well the isochron of [START_REF] Ivanov | Mars/Moon cratering rate ratio estimates[END_REF] for impact craters ranging from 200 m to 2.0 km-in-diameter.

Interestingly, this secondary retention age is of the same order as that of the volcanic flows estimated from a cumulative crater size-frequency distribution at CTX scale (i.e. 2.6 ± 0.1 Ga, green curve in Fig. 3, for impact craters ranging from 150 m to 700-in-diameter), showing that resurfacing probably occurred as a volcanic episode during the Early Amazonian. This last widespread volcanic event buried most impact craters smaller than 2 km-in-diameter.

Since this period of time, the area has been subjected to a continuous degradation process affecting <200 m-in-diameter impact craters because the crater size distribution below this diameter crosscuts all isochrons younger than 2.6 Ga. With respect to geological processes, the degradation is most likely due to aeolian erosion and deposition (dust and sand). At CTX scale, there is no difference in retention age between the surface of the volcanic ridge (western area) and the surface of the eastern plain, which prevents the determination of the ages of each lava flow identified on the eastern side of ridge (Fig. 1c).

Thickness of geological unis

In the case of western Elysium Planitia where InSight landed, the difficulty for determining the thickness of geological units is the lack of exposed sections of the stratigraphy, such as topographic scarps. However, impact craters are windows into the subsurface that enable the extraction of geological information crucial to the understanding of geological structure, with a few precautions. Using relative chronology relationships (craters crosscutting or overlain by a formation, Fig. 1e,d) and crater morphometry relationships (Table 1), it is possible to estimate the thickness of volcanic material at InSight landing site, discriminating the Early Amazonian volcanic surface due to regional resurfacing from the probable Hesperian volcanic bedrock.

Then, the thickness of sedimentary units will be estimated by the same method.

Assuming that the Early Amazonian volcanic material within the ridge belongs to volcanic material covering the regional area (i.e. area viewed in Fig. 1b andf), its minimum thickness could be estimated from the buried impact craters. Indeed, the minimum thickness of volcanic flow may be estimated from the rim heights of several buried craters emerging above the lava surface (Fig. 1e, crater #1). These rim heights had to be reached before the lava could fill the impact crater (Fig. 1e, craters #2 and 3). The crater rims heights (h) above the surrounding level are then estimated, using the average relationships between rim heights and diameters (D) for fresh impact craters on Mars (Table 1), based on the measurement of 6000 impact craters in MOLA data [START_REF] Garvin | Craters on Mars: Global geometric 1142 properties from gridded MOLA topography[END_REF].

We identified 6 partly to completely buried large impact craters in the Early Amazonian volcanic material of the surrounding of landing site (Fig. 1f) and we measured their rim-to-rim diameter from HRSC images (Table 2). Although the population of filled craters is small around the landing site, three buried craters frame the area to the north and south, providing an estimate of minimum Amazonian lava thickness (T) between 56 and 186 m (Table 2). We could compare it with that expected for partly filled craters using the same dataset (i.e. MOLA altimetry).

We calculated the remnant relief of crater rim (hr) from the difference of the maximum MOLA elevation of rim (hr_max) and the mean plain elevation (hplain) measured at a distance of one crater diameter from the crater rim. Then we calculated the minimum Amazonian lava thickness (T) as the difference between the expected rim height as calculated with Garvin's morphometry relationship (h) and the measured remnant (or current) rim height (hr, Table 2).

Except the crater c6, the minimum lava thickness is of the same order (~60 m) as that calculated from the crater rim of buried craters. Due to the lack of completely filled craters very close to the landing site, it can be estimated that the minimum lava thickness is at least 60 m.

In addition, this estimated lava thickness is consistent with that deduced from the maximum size of degraded craters due to resurfacing process by volcanic flows occurred at 2.6 Ga (red curve in Fig. 3). Indeed, a kink appeared in the cumulative size-frequency distribution of impact craters for craters smaller than 2 km-in-diameter that have an average rim height of ~60 m, calculated from Garvin's crater morphometry for simple bowl-shaped impact craters (i.e. T = h =49.5 m, Table 1) and from Watters et al.'s relationship for small impact craters (i.e. T = h = 71.6 m, Table 1). So, the average estimation of lava thickness for the last volcanic event (i.e. at ~2.6 Ga Early Amazonian) is ~60 m.

Although the partly buried impact craters show inner degradation such as sedimentary filling, rim slumps and landslides, they give us some information about the nature and the thickness of the Hesperian bedrock (Fig. 4) on which the Early Amazonian lava was deposited.

For example, the c#6 crater shows a 900 m deep central depression with relatively well preserved inner rim (Fig. 4b) and flat floor. Below the crater rim, ~500 m thick dark layered material is identified in the crater side (L in Fig. 4d). It is also observed in the inner side of an impact crater of the same size, located 50 km north of c#6, post-dating Early Amazonian lava, with 400m of stratified dark layered material under its rim (Fig. 4c,e). For impact craters larger than 9 km, no data are available to observe the bedrock due to degradation or modification of the inner crater edges. This suggests that the bedrock would consist of a series of volcanic lava flows at least 500 m thick. Unfortunately, the lithology cannot be determined mineralogically by near-infrared data due to the presence of dust.

In addition, all craters with an ejecta blanket post-date the Early Amazonian volcanic surface. Depending on their diameter, they can penetrate the Early Amazonian volcanic material and the underlying Hesperian volcanic bedrock, with no distinction between the two, e.g. crater #4 in Fig. 1d). Indeed, impact craters greater than 300 m and smaller than around 1 km, postdating the Early Amazonian lava flows, have dark rocky ejecta blankets both in daytime THEMIS images (Fig. 1c) and in HiRISE images (Fig. 2c,4g), interpreted as rocky volcanic material [START_REF] Golombek | Selection of the InSight landing site[END_REF][START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]. Using the crater morphometric relationships for relatively pristine impact craters (Table 2), the depth of these craters is thus ranging from 81 m using Sweeney et al.'s morphometry relationship (Table 1) to 210 m using Garvin et al.'s morphometry relationship (Table 1). Therefore, these craters have impacted volcanic material with a thickness exceeding 200 m, which is very consistent with the thickness of volcanic material of undifferentiated age as previously estimated from large impact craters [START_REF] Golombek | Selection of the InSight landing site[END_REF]2018).

Since the Early Amazonian, the area is subjected both to aeolian erosion and sedimentation (this study, [START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]2020;[START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF]. Many impact craters ≤ 200 m-in-diameter show degraded impact rims with boulders [START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF] and scours from aeolian abrasion. Based on this observation and rim crater morphometry relationships (Table 1, [START_REF] Watters | Morphometry of small recent impact craters on Mars: Size and terrain dependence, short-term modification[END_REF][START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF], the surface could have lost ~6m in thickness by aeolian erosion.

The thickness of the surficial unit, consisting of aeolian deposits (sand and dust) mixed with rocky ejecta, is estimated from very small impact craters observed on HiRISE images, using crater morphometry relationships (Table 1). The smaller fresh impact crater whose excavation depth can be calculated by [START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF] is 10m, giving a excavation depth of 0.8 m, comparable to that measured from HiRiSE DEM (Fig. 4f,g). All impact craters smaller than 30 m, observed within a radius of 1 km around the landing site, show no boulders within their partly preserved ejecta (Fig. 2 c,d), which suggests that the impacted material is composed essentially of a loosely consolidated, fine-grained material with particle sizes smaller than boulders [START_REF] Golombek | Selection of the InSight landing site[END_REF][START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF]). Using crater morphometry relationship (Table 1, [START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF], the thickness of fine-grained material without boulders is 2.5 m. This thickness is consistent with the previously estimated thickness of regional surficial finegrained "regolith" (i.e. 2-5 m [START_REF] Warner | Near surface stratigraphy and regolith production in southwestern Elysium Planitia, Mars: Implications for Hesperian-Amazonian terrains and the InSight lander mission[END_REF] and i.e. ~3 m [START_REF] Warner | An impact crater origin for the InSight landing site at Homestead Hollow, Mars: Implications for near surface stratigraphy, surface Processes, and erosion rates[END_REF]).

The surface of Homestead hollow is covered by dust mantling whose thickness was evaluated only from the analysis of TES and THEMIS thermal inertia data, showing an average thermal inertia of around 200 Jm -2 K -1 s -1/2 that is consistent with surface grain sizes of fine sand and a <1-2 mm thin dust layer [START_REF] Golombek | Selection of the InSight landing site[END_REF][START_REF] Golombek | Geology and physical properties investigations by the InSight Lander[END_REF].

Stratigraphy from orbital data

Based on the geomorphological, geological and chronological analyses of orbital data covering the landing site, we propose the following stratigraphy (Fig. 5): The area was covered by very thick (> 500m) volcanic material dated at 3.6 Ga corresponding to the boundary of Early/Late Hesperian.

A regional resurfacing event occurred at 2.6 Ga during the Early Amazonian, interpreted to be the last widespread volcanic flows covering the highly cratered Early/Late Hesperian volcanic surface, with a thickness of ~60 m, covering most craters below ~2km-in-diameter.

Although this volcanic material was then deformed by wrinkle ridges, and degraded by small impact craters, its layered structure and its volcanic landforms (lobate fronts) are relatively well preserved.

Above this volcanic material, a granular, unconsolidated unit covers the surface with a variation in grain size and thickness. This surficial unit is composed both of clasts coarser than boulders ejected from ≤200 m-in-diameter impact craters and smaller clasts of eolian origin. This surficial unit shows notable thickness variations (i.e. 3-17m at regional scale (Warner et al., 2017), but the thickness of the upper part of surficial unit containing only clasts smaller than boulders seems to be limited to 2.2-3 m in Homestead hollow, based on the surrounding no rocky ejecta crater ≤30 m-in-diameter.

Stratigraphy from in situ data

On December 18, 2018, InSight landed at 4.502°N, 135.623°E at an elevation of -2613.43 m, on a relatively flat area slightly inclined 1.3° to the SE (Fig. 1g, 2), in the northwestern part of a ~27 m wide near circular depression, interpreted to be a degraded impact crater [START_REF] Warner | An impact crater origin for the InSight landing site at Homestead Hollow, Mars: Implications for near surface stratigraphy, surface Processes, and erosion rates[END_REF], informally named Homestead hollow. The depth of depression is estimated to be ≤ 0.8 m from HiRISE DEM (Fig. 6c andd). At HiRiSE image scale, the surface texture of the depression is very smooth with very few boulders and small impact craters lesser than 3 m in diameter (Fig. 2e,f), suggesting that the sub-surface is composed of fine-grained material smaller than boulders.

During the landing, the dust layer was lifted into the atmosphere by retro-propulsive rockets enabling to slow the lander's descent and perform a safe soft landing, which led to cleaning the surface and redistributing primarily fine-grained materials. The surface alteration is easily identified as "halo" of albedo and/or color variations in HiRISE images (Fig. 2d,f,g). These images reveal a relative brighter inner halo extending up to 8-11 m from the lander that has an 18% lower relative albedo compared to unaltered background, surrounded by a relative darker outer halo extending from the edge of the high reflectance halo out to 15-21 m from the lander that has up to a 35% lower relative albedo compared to the unaltered surface [START_REF] Williams | Surface alteration from landing Insight on Mars and its implications for shallow regolith structure. 50 th LPSC[END_REF].

The relative darker outer halo also extends more weakly much farther to the southeast along the prevailing wind direction [START_REF] Forget | Improved general circulation models of the Martian atmosphere from the surface to above 80 km[END_REF][START_REF] Spiga | Atmospheric Science with InSight[END_REF]Bandfield et al., 2020).

Additionally, a discontinuous pattern of low-reflectance rays (dark-blueish tones in false-color (Infrared, Red and Blue) IRB HiRISE image, Fig. 2g) extends primarily towards the north up to 5 m from the lander. The relative darkening of the surface is thus consistent with the expected removal of a thin layer (microns) of dust during landing [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF], similar to previous Mars landing sites (e.g. [START_REF] Johnson | ChemCam passive reflectance spectroscopy of surface materials at the Curiosity landing site[END_REF][START_REF] Daubar | Changes in blast zone albedo patterns around new Martian impact craters[END_REF] and analogous to the formation of typical dust devil tracks [START_REF] Reiss | Dust devil track survey at Elysium Planitia, Mars: Implications for the InSight landing sites[END_REF]. Although much brighter than the outer halo, the relative brighter inner halo is still darker than the unaltered surface and it was likely covered by a veneer of dust that is now removed.

The presence of dust at the time of landing is also demonstrated by the first images acquired by the two cameras on the lander board, in particular by the wide field of view (ICC) whose cover did not sufficiently protect the optics (Fig. 7). A few days after landing, the dust particles fell off the optics and the images were clear (Fig. 7), due to a cleaning by the wind activity in the area as measured by wind sensors (e.g. Bandfield et al., 2020, Charalambous et al., 2020).

Landing site viewed from the ground

In the following sections, we describe the landforms and the surface materials observed in front of and beneath the lander, based on IDC images and DEMs. InSight lander touched down the surface with a low tilt of 3.975° towards the SE (133.408 degrees clockwise from the North) (Fig. 6), interfering with the data acquisition on the ground, near the lander, in particular in the NW part where the radiometer (RAD) instrument acquires the surface temperature. As expected, the solar lander panels were deployed and oriented ~E-W (Fig. 2f,g), enabling the ICC camera looking southward (Fig. 7). In this south view of Homestead hollow, the landscape appears relatively flat, with a smooth surface on which no sedimentary bedforms (ripples, dunes) are present, except for rocks that are typically smaller than cobbles and scattered randomly (Fig. 7).

InSight's Instrument Deployment Camera (IDC) located on the lander's robotic arm acquired red/green/blue (RGB) color images from a height of ~1.5 m above the surface, including a panorama of mid-and far-field terrain around the lander on sol 14. In the mid-field panoramic view around the lander looking eastward (Fig. 6a), the dark halo and the brighter central part of Homestead hollow are still recognizable. The landscape is relatively flat with approximately a dozen bright-reddish 1-10m wide circular shallow depressions (c# in Fig. 6a) within 20 m of the lander, interpreted as impact-generated depressions filled with granular material [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF]. No outcrops are present within the very shallow (~ 0.8 m deep in IDC DEM, Fig. 6b) Homestead hollow, delineated by a darker, ~0.4 m tall, gentle northfacing rise of 3° in the SSE looking (Fig. 6a,b,e), beyond which many boulders and rocks strew the surface (Fig. 8).

Within the Homestead hollow, the surface is relatively smoother and brighter to the far east of the lander than in its western and southwestern parts where rocks, ranging from cobbles to pebbles, are randomly distributed and spaced apart (Fig. 6a,8), as discussed in details by [START_REF] Grant | Modification of Homestead hollow at the Insight landing site based on the distribution and properties of local deposits[END_REF]. The rocks partially cleaned by the rocket blast show different shades, shapes and textures (Fig. 8): There are either dark-toned rocks probably composed of volcanic material such as basalt, or light-toned rocks, often smaller than the former, possibly composed either of brighter volcanic material such as andesite or cohesive sedimentary material excavated by impact from a near-surface sedimentary layer. However, these lithological proposals should be taken with caution since no microscopic and mineralogical analysis instruments are included in the InSight instrument package. Most of these rocks show relatively angular equant shapes and pyramidal shapes typical of wind erosion (i.e. wind-faceted rocks w/o ventifacts, Fig. 8c,e). Some of them are cross-cut by sharp fractures possibly due to thermal weathering (thermosclasty) and/or shock from ejecta fall. Although the rock texture is quite difficult to define at this distance away from lander, few rocks show an assembly of cohesive centimeter-sized components, suggesting a brecciated texture (Fig. 8f). Note that many rocks has a redder area on the ground radially outward from the lander, suggesting that a thin layer of material may have remained in place during the blowing produced by the lander retrorockets.

Landforms and geology in front of lander

A large number of color images, including stereo coverage, were acquired by the armmounted IDC camera, imaging the ~6 m × ~4 m workspace tilted ~4° to the southeast in front of the lander (Fig. 6b,9), to select the locations to place the geophysical instruments. The smooth workspace surface is covered by a majority of sand-sized grains with scattered >cm scale clasts (e.g. [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF][START_REF] Grant | Modification of homestead hollow at the insight landing site based on the distribution and properties of local deposits[END_REF][START_REF] Grant | Modification of Homestead hollow at the Insight landing site based on the distribution and properties of local deposits[END_REF][START_REF] Hauber | Geology of the Insight Landing Site, Mars: Initial Results[END_REF]Weitz et al., 2019Weitz et al., , 2020)), ranging from pebbles to very few cobbles in Wentworth's granulometry classification [START_REF] Wentworth | Fundamental limits to the sizes of clastic grains[END_REF]. Large clasts show two types of materials: one is dark-toned, grey aphanitic at the IDC resolution of ~1 mm at this distance (Fig. 9a, b, c andk), probably corresponding to a basaltic composition, and the other one is light-toned with no visible texture and unknown composition (Fig. 9i,j) possibly a cohesive mix of sand and dust. The dark clasts show sub-equant shape with sub-angular edges while the light clasts are close to thin, platy shape with angular to sub-angular edges (Fig. 9). Some of dark clasts seem to be composed of different aggregated elements, suggesting that they may form a brecciated material (Fig. 9a,d,f).

In addition, the clasts are usually covered by very thin reddish material whose nature remains unclear (layer or coating), suggesting they have been on the surface for a long time despite the cleaning of the retrorockets. Numerous clasts are partially buried (Fig. 9d,h,j), showing that sedimentary processes are or were active after their deposition. Finally, the retrorockets not only cleaned the surface of the dust, but also dislodged small pebbles and even threw them a few tens of centimeters from their original position (Fig. 9e, h, i) [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF], either by dragging or skipping and rolling leaving their footprints in the ground (white arrows), especially near the western lander foot as observed on ICC images (Fig. 7).

In a more restricted area of the workspace very close to the lander (Fig. 10), a mosaic of IDC ortho-rectified images with a very high resolution (scale:1 mm.pixel -1 ) enables to determine the size and the shape of 8,252 clasts. Their edges are digitized as polygonal outlines in GIS package, and the length and the width of each clast are measured in metric coordinates. The majority of measured clasts belongs to very small pebble class, ranging from 2mm to 50 mm with a mean size of ~6 mm (Fig. 10b). The shape of these clasts in 2D is close to be circular with almost the same length in two orthogonal trendings (i.e. equant to sub-equant, Fig. 10c) (e.g. [START_REF] Hauber | Geology of the Insight Landing Site, Mars: Initial Results[END_REF][START_REF] Weitz | Clast sizes and shapes at the InSight landing site[END_REF]Weitz et al., , 2020;;[START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF].

While the surface between the rocks appeared smooth just a few meters from the lander, the ground surface is striated and scoured, with multi-millimeter relief ridges and troughs that extend radially away from the lander (Fig. 11). Some granules and small pebbles have tails extending away from the lander, suggesting that material behind small clasts has been preserved from the erosive rocket blast. The removed clastic material is thus smaller than millimeter (i.e. sand at least), but its grain size cannot be precisely determined due to the resolution limits of the IDC camera (i.e. >0.5 mm.pixel -1 ).

In addition, this sandy material is not cohesive, enabling the displacement of clasts by rocket blast. This is also supported by (Fig. 12): 1) the displacement of coarse sand/granule grains (2.5±0.7 mm) over a distance of ~2 cm, during an aeolian gust occurred sol 283 (Fig. 12f,g);2) the "splash" features on the surface of HP 3 foot that are oriented parallel to the measured wind direction during this event, suggesting that sandy particles impinge on the surface after a transport of saltation (Fig. 12h,i); 3) the mass-wasting of scour sides previously formed by retrorockets, leading to partial removal and new arrangement of fine-grained material transported by creeping towards NE trending as marked by the reshaping of scours, and imprinted by faint striation into the ground (Fig. 12d,e). See more detailed information in Charalambous et al., 2020, Charalambous et al., submitted in GRL and Baker et al. submitted in JGR. Despite the lack of aeolian bedforms such as ripples, the surface of Homestead hollow is subjected to wind activity, reshaping the surface roughness sporadically. The dust fallen few hours/days after the landing and during the regional dust storm (sol40-sol52) (e.g. [START_REF] Viudez-Moreiras | Effects of a Large Dust Event in the near-surface atmosphere as measured by InSight in Elysium Planitia, Mars[END_REF] was removed also on the small pebbles (Fig. 12) and on solar panels (Lorenz, 2020) during different gusts monitored at landing site (Bandfield et al., 2020;Spiga et al., 2020). However, no image has captured a dust devil to date.

Sub-surface at location of HP 3 mole penetration

During sol 76 (February, 12, 2019), the HP 3 instrument was placed on the ground ~1.5 m from the lander in a SSE direction. Few days laer, the hammering test started, but the 2.7 cm diameter mole only partially penetrated into the sub-surface up to ~30 cm deep with a tilt of ~15°. Due to the stalling of the mole penetration, it was decided to lift the HP 3 cover, in order to visualize the hole made by the mole.

A steep-sided open pit ~5 cm wide and ~5 cm deep appeared between circular imprints of the HP 3 feet (Fig. 13a,14). These imprints with a deep of a few mm, showed a very finegrained sandy layer with sparse coarse sand grains. This layer was sampled, lifted up to 50 cm above the ground and redeposited on the ground by the east rear HP 3 foot, without losing its cohesion, suggesting that this fine sand is weakly indurated or that the sand particles are very fine in order to be held together by electrostatic forces as flour would behave (Fig. 13e-g).

In addition, the open pit did not show any significant piles of excavated granular material around the hole, which suggests the underlying material has a low density and/or high porosity because it was compressed by the mole hammering. The steep pit walls showed also a layered material composed of relatively dark fine sand with sparse coarse sand grains in the uppermost ~2 cm as in the imprints of HP 3 feet. In order to support the mole penetration, several pushing scoop tests have been performed close to the mole (Fig. 13c,d). The first imprint of the scoop at the surface (Fig. 13c) was around 0.5 mm deep with a relatively flat surface without cracks.

After several scoop pushing tests, the top surface remained relatively flat and horizontal, but mass-wasting and collapsing with relatively flat slope occurred in the upper granular material in the west side of the hole, showing the very fine layering controlled by the grain size (Fig. 13d). The topographic profiles across the scoop indentation shows the upper layer has been compacted vertically by ~1 cm (Fig. 14), indicating that the material is unconsolidated granular and its porosity would be quite high.

This fine-grained layer overlapped a layered material composed of granules/small pebbles and sand, interstratified with very fine sand or dust. The laminae containing granules/small pebbles seem to have a higher degree of cohesion than finer ones, because the coarse clasts remained in overhangs (Fig. 13a,b). However, when a last pushing test was made, these granules, small pebbles and clumps collapsed and partially filled the hole (Fig. 14b,c), suggesting that the cohesion is quite low in this coarser material. Note that a few dark-toned, rounded, ~5 mm in diameter pebbles seem to be aggregated together without any visible cement (Fig. 13b). In addition, the clumps lining the bottom of the hole appear to consist of an aggregate of coarse sand/granule grains in a finer-grained matrix, suggesting an apparent degree of cohesion, whereas they come from upper material that is not very cohesive and friable (Fig. 14c).

Geology beneath the lander

Thanks to the articulation of the arm, the IDC camera was able to acquire images under the lander (Fig. 15) with sufficient precision to create a mosaic of ortho-rectified images on a digital model of spatial resolution of 2mm.pixel -1 (Fig. 16). The first surprise is the presence of two large rocks that the lander was able to avoid during its landing (Fig. 15a). Both boulder/cobblesized rocks, called "Ace of spades, ~30 cm wide" and "Turtle, ~20 cm wide" by team [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF], show a sub-equant shape with sub-angular edges. They have darkgrey color and no mineral texture is visible at the IDC resolution, suggesting that they are composed of volcanic material. The "Turtle" cobble displays a relatively dark, wavy top surface that can be interpreted as a lava-corded surface or a surface shaped by the wind (ventifacts) suggesting an aeolian abrasion (Fig. 15b). As observed on the workspace area, several light-toned pebbles show planar-facets characteristics of wind abrasion (wind-faceted pebbles) (Fig. 15c,e).

The IDC images display the partial sinking of the two front feet of the lander in a loose material, showing evidence for slight sliding into place, creating a bulge in the ~south trending and a depression on other side (Fig. 15c,d). In spite of the loose material, the feet of the lander were able to stabilize, probably thanks to a more load-bearing sub-surface material.

During the landing, the retrorockets disturbed the surface under the lander, excavating up to ~10 cm deep, ~50 cm-in-diameter pits providing thus views into subsurface materials and their physical properties (Fig. 15a). The sub-surface material is mainly composed of dark-toned aphanitic pebbles within a reddish finer-grained matrix [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF].

In the very shallow depression d3 (Fig. 15d andf), the sub-surface is also composed of subangular pebbles, arranged in layers, within a reddish fine-grained matrix [START_REF] Ansan | InSight landing site: subsurface stratigraphy and implications for formation processes[END_REF].

But a few pebbles show a different texture and color, notably pebble made up of light-toned, millimeter-sized, prismatic elements within a darker homogeneous matrix whose elements/crystals are not visible at the IDC resolution. These prismatic elements could correspond to feldspar minerals (phenocrysts) within a microlitic to microgranular matrix (Fig. 15g), suggesting a longer crystallization time. These pebbles would come from volcanic material requiring longer crystallization time, i.e. deeper magmatic sources.

Based on the mosaic of ortho-rectified IDC images (Fig. 16a), a map of clasts present at the surface (excluding the inner pit surfaces due to lack of stereo images in these areas) has been performed taking into account their shape, using the same methodology as on the workspace area. The median size of the clasts under the lander is 6.5 mm except the two cobbles, and they are relatively sub-equant (Fig. 16b,c) over the mapped area, while the size of clast increases near the pits with a mean of 13 mm (Fig. 16d), which suggests that these clasts are ejected clasts due to their coarse size.

Using both ortho-rectified IDC images and DEMs, a detailed stratigraphy of the pits is made.

The ~8 cm deep depression d2, located in the forward part under the lander between its two feet, shows three sub-horizontal superimposed units (Fig. 17), from top to down: R0) ~2 cm thick unit composed of small pebbles-granules mixed reddish fine-grained matrix showing a gentle surface slope of 10°, suggesting that this material is relative loose; R1) ~2 cm thick, very fine-grained relatively reddish unit, showing lateral and vertical organization with resistant laminae marked by a bench (black arrow in Fig. 17b) interstratified within a softer material (Fig. 17b,d profile 2). Laterally, the softer material appears lighter (white arrow in Fig. 17b) within a small scalloped depression topped with a resistant laminae well marked in topographic profile 1 (Fig. 17d). These resistant laminae would have a higher degree of cohesion than the softer material. They might be indurated by a cement, suggesting the local possible presence of duricrust at depth of 2 cm below the surface; R4) ~3 cm thick bedrock, composed of poorly sorted dark-toned, aphanitic pebbles within loose, fine-grained, reddish matrix. Note that the R0 unit would extend laterally to the eastern lander footpad in which it is partially buried. Material R1 would have stopped the footpad sinking and maintained its stability through its greater load-bearing.

The ~50cm-in-diameter, >10 cm deep pit P1 located in the northeast part beneath the lander (Fig. 15a) shows a very different morphology from the previous one. It is bordered by three superposed clastic units (Fig. 18), from top to bottom: R2) ~3 cm thick unit composed of sandy to poorly sorted, sub-angular to angular, pebble material; R3) ~2-6 cm thick unit composed of poorly sorted, sub-equant, angular, >4cm wide pebbles. Some pebbles are longer than thick (white arrow), suggesting that they are pieces of a larger pebble broken vertically. This thick unit has a steep slope, greater than the angle of repose, up to 60°-86° (Fig. 18c) which can be explained either by the arrangement of pebbles such as dry stone walls, or by the presence of cement forming a sedimentary breccia. In the absence of better image resolution and chemical analysis, it is not possible to conclude; R4) >3 cm thick unit of cohesionless, poorly sorted, subangular pebbles within a fine-grained matrix. Note that the left side of the pit collapsed with a very small slope of 15° eastward, suggesting that the upper layer R2 at least is no cohesive.

The third shallower ~50 cm-in-diameter pit P2 located in the northwest part beneath the lander (Fig. 19) shows only to two units, from top to bottom: R2) sandy to poorly sorted, subangular, gravel material and R3) poorly sorted, sub-angular pebbles arranged in a ~5 cm thick, sub-horizontal unit. Many pebbles are longer than wide (white arrows), with their longer side arranged sub-vertically and parallel to each other, suggesting that they have been sharply broken either by the retro-rockets. Some large clasts have been ejected out of the pit (white arrow in the left foreground, Fig. 19a), suggesting that this thick unit is composed of cohesionless, 5 cm wide clasts, despite the relatively steep slope on the northern pit side. Note firstly that this unit R3 is in continuity with that observed in depression P1, suggesting that these large pebbles are not indurated but rather arranged like dry stone walls giving them a great slope stability.

Secondly, this unit would be sub-horizontal with a dip of ~ 2° toward SSE, using its spatial correlation of upper boundary the in DEM.

Stratigraphy in the Homestead hollow

As we saw previously the NW part of Homestead hollow is filled by clastic material, dominated by sand size grains. From all observations acquired in the workspace and beneath the lander, a schematic sub-surface stratigraphy is proposed from lander to HP 3 location [START_REF] Ansan | InSight landing site: subsurface stratigraphy and implications for formation processes[END_REF], from top to bottom (Fig. 20a): i) a thin dust layer, probably < few microns, with cobbles and small boulder (Turtle and Ace of Spades), pebbles and granules at the surface; ii) a few cm-thick unit consisting of light-toned, cohesionless, sand or smaller (<1mm) grains. This material was sculpted into <cm deep troughs and ridges radial to the lander by the descent rockets; iii) a possibly fine-grained indurated unit, called duricrust, at least in shallow depression d2 in front the lander, showing lateral variations of thickness (ranging from a few mm in front of lander up to ~2 cm beneath the lander). Laterally, a coarse-grained material (i.e. composed of poorly-sorted, angular to sub-rounded dark-toned clasts, ranging from granules to pebbles contained in lighter-toned, finer-grained (i.e. <0.5 mm) matrix) is present within depression d3 near the western footpad; iv) a ≥~5 cm-thick, cohesionless, granular material comprised of a likely sandy matrix with poorly sorted clasts of sub-angular, dark-toned pebbles are present with different lateral arrangement due to the proportional amount of clasts. In the depression d2, the sandy matrix seems more important while it is almost absent in the pits (P1 and P2) in the back of the lander. This suggests that all stratigraphic units are spatially distributed in lens-shaped features.

By plotting the different stratigraphic columns established from the ortho-rectified images and DEM on the local map (Fig. 20b) and using the stratigraphic relationships, we notice that the units located under the lander are made of coarser clastic materials (pebbles) than those located in the workspace. These units reinforce the small slope on which the lander touched down. The finer material distributed within the workspace show a variety of stratified grain sizes. A geologic cross-section shows the spatial distribution of the different units between the pit P1 under the lander and the indentation made at HP 3 (Fig. 20c), with lens-shaped deposit.

Unfortunately it is difficult to deduce what type of clastic material is under the first few centimeters of sand at HP 3 location. We can assume that it would be a mixture of granules and small gravels in a relatively porous sand matrix which has allowed the penetration of the mole over the first ~30 cm of depth.

At the scale of Homestead hollow, the surficial clastic material shows a great variety of grain sizes, ranging from cobbles to dust, with a dominant sandy size, distributed in sub-horizontal, meter long, centimeter thick lens-shaped features, at least in its NW embankment area. The geometry of these clastic materials suggests a sedimentary origin, at least for fine-grained material transported by wind. For coarser clasts, their shape suggests that they have undergone very short ground transport. Their spatial accumulation suggests that they came from nearby impact ejected rocks, with vertical breaks created when they re-impacted the ground. However, those at the surface of the depression were subjected to wind abrasion by the sand particles after their ejecta fall, forming a residual lag (see [START_REF] Grant | Modification of Homestead hollow at the Insight landing site based on the distribution and properties of local deposits[END_REF].

Based on the analysis of orbital data, the maximum thickness of the surficial clastic material without boulders would be 2.5 m, using the crater morphometry relationships [START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF] for no rocky ejecta 30 m-in-diameter impact craters, corresponding to the crater size beyond which the impacts excavated the volcanic bedrock within a one-kilometer radius of the landing site. If we assume that Homestead hollow is a 27 m-in-diameter, degraded impact crater, the maximum thickness of fine-grained clastic material would be ~2.2 m, using crater morphometry relationship for pristine small impact craters [START_REF] Sweeney | Degradation of 100-m-scale rocky ejecta craters at the InSight landing site on Mars and implications for surface processes and erosion rates in the Hesperian and Amazonian[END_REF] up to 3 m from model of crater shape evolution [START_REF] Warner | An impact crater origin for the InSight landing site at Homestead Hollow, Mars: Implications for near surface stratigraphy, surface Processes, and erosion rates[END_REF]. Based on in situ data, the thickness of clastic material is not constrain beyond a depth of ~10 cm, however the <meter-scale hollows viewed in the smoother part of Homestead hollow are surrounded by small sandy rims (Fig. 6c) whereas those, in rockier part of Homestead hollow and its boundary, are ringed by pebbles and cobbles (Fig. 6a and8).

Beyond this maximum meter-scale thickness of fine-grained material, the Early Amazonian, fractured, layered lava flows would be present, as suggested by >50m-in-diameter rocky impact craters surrounding the landing site, from which remnant ejected boulders at the southern boundary of Homestead hollow could come from (Fig. 21).

Discussion

In this section, we will discuss about the nature and preservation of bedrock, the nature and processes at the origin of surficial unit (regolith).

Volcanic bedrock and clasts

The InSight landing area is characterized by Early Amazonian, widespread, <60m thick, volcanic flows, whose morphologic elements, such as lobate fronts, are well-preserved, despite their age of 2.6 Ga. Underneath them, >500 m layered dark-toned material have been identified in the inner walls of >2km large impact craters, interpreted as superimposition of volcanic flows, dated of Early/Late Hesperian, from orbital data. The surface retention age, at InSight landing, is consistent with previous regional chronology of Elysium Planitia and Elysium Mons, for which the emplacement of the main edifice occurred at the latest 3.5 Ga ago and the emplacement of major Elysium planitia occurred from 3.7 to 3.0 Ga with locally very young volcanic events such as those in Central Elysium planitia (e.g. [START_REF] Werner | The global Martian volcanic evolutionary history[END_REF][START_REF] Vaucher | The volcanic history of Central Elysium Planitia: Implications for martian magmatism[END_REF] Although the area is quite close to Elysium Mons, their relative regular layered organization suggests that they formed by fissural magmatic processes, as observed in Deccan traps on Earth.

Due to their large extension and morphological elements, these lavas could be composed of relatively low viscosity flood basalts, as observed in Hesperian volcanic regions on Mars, such as Lunae and Hesperia Planitiae [START_REF] Carr | The Surface of Mars[END_REF][START_REF] Hartmann | Martian cratering 8: Isochron refinement and the history of Martian geologic activity[END_REF][START_REF] Werner | The global Martian volcanic evolutionary history[END_REF]. This is supported by visual analysis of the large ejected clasts present on the surface of Homestead hollow and under the lander, showing a majority of dark clasts with no mineralogical texture (aphanitic at ~0.5 mm IDC scale) and no vugg, relatively resistant to wind erosion, typical of basaltic lava flows. Nevertheless, some large clasts show a light-grey color with an aphanitic texture that may correspond to andesites. In addition, a few clasts show a different texture and color, with light-toned prismatic minerals that may be feldspars within a darker matrix (Fig. 15g), suggesting they would come from deeper magmatic sources or lava flows with distinct chemistry. These clasts could have been ejected from nearby craters sampling deeper section of the volcanic flows. In absence of chemical and mineralogical analytical instruments, we cannot conclude definitively about their nature. The dominant type of rocks shows that this region was characterized by homogeneously basaltic, effusive flows.

Although undergoing cratering since 3.6 Ga, the volcanic materials as a whole have preserved their overall layered structure, not developing a megaregolith (i.e. a larger-scale disrupted crustal structure by impacts in reference to that on the Moon, e.g. [START_REF] Hartmann | Ancient lunar megaregolith and subsurface structure[END_REF][START_REF] Short | Thickness of impact crater ejecta on the lunar surface[END_REF][START_REF] Hôrz | Large-scale cratering of the lunar highlands: some Monte Carlo model considerations[END_REF]) at the km scale.

Regolith formation processes and meter-scale structure

Since the last volcanic flows dated of 2.6 Ga, the region has been cratered forming a majority of small craters less than 200 m-in-diameter. These craters and those down to 50 m-in-diameter are surrounded by rocky ejecta coming from disruption of lava flows by impacts. The ejected clasts could comprise not only piece of lava flows; as mentioned previously, but also meteorite fragments and impactites consisting of shocked and shock-melted materials in larger, angular clasts. In Homestead hollow, none of these types of clasts have been identified, unlike those found within Gale Crater [START_REF] Yingst | Characteristics of pebble-and cobble-sized clasts along the Curiosity rover traverse from Bradbury Landing to Rocknest[END_REF], even if it is not ruled out in the Homestead home surroundings.

For the smaller <200 m-in-diameter craters, the ejecta appear to be composed of finer material, suggesting that they could mainly result from ejecta reworking by impacts at the origin of the formation of "lunar regolith". Indeed, on the Moon, impacts have been fragmented the original surface and distributed it as ejecta, which intermixed with successive cratering products (fragmental layer of broken, melted, and otherwise altered debris). By iterative process, the numerous small down to micro-impacts only disturbed and mixed (gardened) the regolith layer already present, and the regolith thickness increased more slowly [START_REF] Mckay | The lunar regolith[END_REF]. The lunar regolith consists thus of clasts <1 cm in size although larger cobbles and boulders are commonly found at the surface. This unconsolidated, heterogeneous and fragmented lunar material is generally about 4-5 m thick in the mare but may average about 10-15 m in older highland regions (Oberbeck & Quaide, 1967[START_REF] Oberbeck | Genetic implications of lunar regolith thickness variations[END_REF][START_REF] Mckay | The lunar regolith[END_REF].

However, the presence of such a "lunar-type" regolith is quite inconceivable on Mars due to the presence of an atmosphere, coupled with diurnal and seasonal thermal cycles, which prevents or reduces micrometeoroid penetration to the surface. In addition, due to the existence of a dynamic hydrosphere in early Mars's history, there is evidence for the action of liquid water in terms of weathering, erosion and deposition of sediments, whether intermittent or continuous, as well as the formation of sedimentary rocks (e.g. Squyres et al.2004[START_REF] Malin | Sedimentary rocks of early Mars[END_REF][START_REF] Dromart | Stratigraphic architectures spotted in southern Melas Chasma, Valles Marineris, Mars[END_REF][START_REF] Ansan | Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater[END_REF][START_REF] Grotzinger | The Sedimentary Rock Record of Mars: Distribution, Origins, and Global Stratigraphy[END_REF]. In addition, the thin Martian atmosphere has driven wind systems that have resulted in aeolian transport and deposition (e.g., dust storms [START_REF] Cantor | MOC observations of the 2001 Mars planet encircling dust storm[END_REF], dunes and ripples (e.g. [START_REF] Kocurek | Source-to-sink: an Earth/Mars comparison of boundary conditions for eolian dune systems[END_REF]), which continue to the present day. In summary, not only Martian surface materials may comprise disaggregated precursor rocks, but they will also comprise unconsolidated materials derived from fluvial, lacustrine, aeolian, and glacial activity that postdate and drape earlier impact-dominated processes of Noachian (pre-3.7Ga) age. The Martian regolith (i.e. the uppermost unconsolidated, fragmented material resting on bedrock) is therefore more complex than that of the Moon.

In the equatorial region of western Elysium Planitia, no evidence of hydrological and glacial activity has been identified from orbital data. Only dunes, wind streaks, and dust devils give evidence of relatively old wind activity, still active until today. In Homestead hollow, the presence of sand and dust attests its partially infilling by aeolian process, even if no aeolian features such as sand sheet, sand drift, ripples and dunes, nor internal texture (e.g. crossbedding) has been observed. The source of this sandy material could be attributed either to local sources such as the remobilization of surface granular material from the regolith itseft [START_REF] Bourke | Aeolian sediment transport pathways and aerodynamics at troughs on Mars[END_REF][START_REF] Fenton | Potential sand sources for the dune fields in Noachis Terra, Mars[END_REF], the walls of deeper impact craters within layered lava flows (Tirsh et al., 2011), or more regional sources such as ashes and sedimentary material within Medusae Fossae formation [START_REF] Mandt | Origin of the Medusae Fossae Formation, Mars: Insights from a synoptic approach[END_REF][START_REF] Zimbelman | HiRISE images of yardangs and sinuous ridges in the lower member of the Medusae Fossae Formation[END_REF]. Although dust covers the whole planet, the source of dust storms is usually found in the southern hemisphere in areas such as Hellas, Argyre and Solis within a latitudinal zone ranging from 20° to 50°S.

Dust is then transported via regional to global dust storms preferentially occurring during southern spring to summer (e.g., [START_REF] Martin | An analysis of the history of dust activity on Mars[END_REF][START_REF] Mckim | The dust storms on Mars[END_REF] and settled with microscopic thickness over the entire Martian surface.

Although the measured grain size determined at the landing site is ≥0.5 mm in diameter due to the resolution of the IDC camera at close range, it is likely that the grain size is very smaller than ~100 µm, due to their low mobility during wind gusts. In reference to previous robotic missions (e.g. MER and Curiosity rovers), the size of fine grains was considered to be around 200 μm ± 50 (fine sand) before landing [START_REF] Golombek | Selection of the InSight landing site[END_REF]. However, the granulometry of the Rocknest sand shadow in Gale crater [START_REF] Minitti | MAHLI at the Rocknet sand shadow: science and scienceenabling activities[END_REF], considered as a representative surficial regolith sample on Mars, has <10% of 0.5-2 mm-in-diameter grains, 40-60% of grains between 100-150 μm, and 30-50% of finer grains (i.e. <31 μm, because smaller grain size could not be resolved by MAHLI imager [START_REF] Edgett | Curiosity's Mars Hand Lens Imager (MAHLI) investigation[END_REF]) would be similar within Homestead home. Although the size of the dust grains has not been measured directly on Mars, it is probably less than a few microns. Combining optical and atomic force microscopy, grains less than 4 μm in diameter would be found in regolith at Phoenix landing site [START_REF] Pike | Quantification of the dry history of the Martian soil inferred from in situ microscopy[END_REF].

So, the grains ranging from coarse sand to dust could be present within Homestead hollow, in various proportions, as found in other Martian regolith samplings.

Lithology and structure of clasts

In reference to geological context, the regolith would have a basaltic composition, as found in others landing sites (Yen et al., 2005;[START_REF] Achilles | Mineralogy of an active eolian sediment from the Namib dune, Gale crater, Mars[END_REF], derived from a globally basaltic crust (McSween et al., 2009). As within regolith in Gale crater, it cannot be ruled out that the fine grain component comprises 50% glass fragments mixed in crystalline phases and a dust component, with a mineralogy dominated by feldspar, pyroxene, olivine, with minor magnetite, anhydrite, hematite and their weathered derivatives, as revealed by the CheMin and APXS instruments on the Mars Science Laboratory (MSL) Curiosity rover [START_REF] Bish | X-ray diffraction results from Mars science laboratory: mineralogy of rocknest at Gale Crater[END_REF][START_REF] Blake | Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow[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]. The glass component would contain a water abundance of 1.5 to 3 weight percent, which is supported by the isotopic analyses, suggesting an atmospheric source of water [START_REF] Leshin | Volatile, isotope, and organic analysis of martian fines with the Mars curiosity rover[END_REF]. The possible presence of minor amount of sulfates and chlorides at Homestead hollow could explain partly the presence of soft, light-toned, fine-grained material observed in the depression d2 underneath the lander (R1 in Fig. 17b). In addition, the magnetite content could be higher than that analyzed in Gale Crater, if it is assumed that the particles came from airborne magnetic dust captured by magnets on the board of MER [START_REF] Bertelsen | Magnetic properties experiments on the Mars Exploration Rover Spirit at Gusev Crater[END_REF] and from the local abrasion of volcanic materials in the region. This could give it different physical properties from those of other regoliths studied at different landing sites.

At the meter scale, the lens-shaped organization of the different sub-surface clastic components was discovered thanks to the artificial depressions created by the lander retrorockets and the HP 3 mole (Fig. 20,21). The clastic lenses have meter horizontal extent and a thickness of ten centimeters, which gives a strong spatial heterogeneity into clastic deposits. This could explain the difficulty of the HP 3 mole penetration in the subsurface.

InSight landed on the inner gentle slope of Homestead hollow, reinforced by a sub-surface more than 10 cm thick cluster of ejected pebbles/cobbles, whose southeastward extension could continue deeply until the location of HP 3 . The thin-layered, heterogeneous fine-grained material, found at HP 3 location, would be the core deposit of the hollow center with spatial heterogeneities due to the differentiated content of dust and sand. This spatial organization will be refined by the data acquired by the seismometer SEIS during its mission extending.

In summary, the InSight landing site is characterized by aoelian sediments (sand and dust) trapped in impact craters, hollows and intercrater areas, interstratified into ejected clasts and ejecta blankets (this study, [START_REF] Golombek | Geology of the InSight landing site on Mars[END_REF][START_REF] Grant | Modification of Homestead hollow at the Insight landing site based on the distribution and properties of local deposits[END_REF][START_REF] Warner | An impact crater origin for the InSight landing site at Homestead Hollow, Mars: Implications for near surface stratigraphy, surface Processes, and erosion rates[END_REF] rather than a sensus stricto impacted-related regolith (i.e. in situ fragmented material).

Cohesion, lithification of fine-grained material at Homestead hollow?

On Earth, diagenesis is typically divided into two processes: lithification and epigenesis.

Lithification is the primary conversion of sediment to sedimentary rock. This involves compaction (via burial), the expulsion of fluids, the reduction of porosity, and the precipitation of a binding medium (cement). There may be further grain growth, and replacement and generation of secondary interstitial phases, process called epigenesis.

On Mars, the contribution of sedimentary processes is regionally significant. In many places, the surface consequently encompasses a variety of fine clastic components identified at landing sites, such as dust cover, sandy bedforms, and more cohesive features as bedform armors and "cemented" or "indurated" sand, usually called duricrust. Due to their relative cohesion, sand grains bonded to each other with/o fine matrix were interpreted as glued together by cement, which would imply the presence of a liquid fluid transporting soluble salts or oxides/hydroxides. As observed in Gale crater and Gusev crater, the presence of such processes might have been helped by hydrothermal fluid containing sulfates and Fe-oxides (e.g. [START_REF] Wang | Sulfate deposition in subsurface regolith in Gusev crater, Mars[END_REF][START_REF] Yen | Hydrothermal processes at Gusev Crater: An evaluation of Paso Robles class soils[END_REF][START_REF] Blake | Curiosity at Gale Crater, Mars: Characterization and Analysis of the Rocknest Sand Shadow[END_REF]. In other places in Gusev crater, crusty, flaky, cemented, and cohesive materials partly consisting of composite grains (millimeter to centimeter aggregates of clast smaller than ~30 μm) broke with clean-cut fractures when Môssbauer Spectrometer was applied to their surface [START_REF] Cabrol | Sands at Gusev Crater, Mars[END_REF]. This behavior was also observed in fined-grained surface material within basalt plains unit of Gusev, in which trenches made by rover wheels, showing the material frequently behaves like a sheet of indurated fine-grained material a few mm thick [START_REF] Arvidson | Localization and physical properties experiments conducted by Spirit at Gusev Crater[END_REF][START_REF] Herkenhoff | Textures of the soils and rocks at Gusev Crater from Spirit's Microscopic Imager[END_REF].

The very fine-grained subsurface material showed also modest degrees of cohesion, with values of <1 kilopascal, perhaps resulting from electrostatic forces or a modest degree of cementation [START_REF] Arvidson | Localization and physical properties experiments conducted by Spirit at Gusev Crater[END_REF].

The presence of cement remains debated within Homestead hollow. Near the lander, the ~2 cm thick, very fine-grained relatively reddish unit R1 shows variations in the lateral and vertical organization with resistant sandy laminae marked by a bench (black arrow in Fig. 17b) interstratified within a softer brighter material (Fig. 17b, d profile 2), suggesting a variation of degree of cohesion in materials buried at a depth of 2 cm. This cohesion could result from the interaction between the S-and Cl-rich components of the granular material and the atmospheric water vapor absorbed at the surface of the grains, occurring during recent changes of Mars obliquity, as observed in Spirit hollows within Gusev crater (Weitz et al., 2020). But without chemical analysis at InSight landing site, the question remains open.

Within Homestead hollow, the surficial granular material is composed of very fine grains mixed within coarse sand and granules, at HP 3 location at least. Its relative low degree of cohesion is supported by the grain flows along the flat planar collapsing pit wall (Fig. 13,14). Some aggregates of coarse sands within a very fine grained matrix were also observed after several tests of HP 3 hammering. Although the presence of cement cannot be ruled out, the amount of water vapor in the current atmosphere at ground level near the equator appears to be very low, in comparison to that measured at higher latitude and altitude (e.g. [START_REF] Vincendon | New near-IR observationsof mesospheric CO2and H2O clouds on Mars[END_REF]Federova et al., 2018;Savijarvi et al., 2019;[START_REF] Aoki | Water vapor vertical profiles on Mars in dust storms observed by TGO/NOMAD[END_REF], which would inhibit chemical reactions at the surface-atmosphere boundary.

To explain the apparent cohesion of the granular material near HP 3 , the most relevant parameters would be the very fine size of the particles, their shape, their composition and the vibration or compaction processes generated during the HP 3 hammering. For parameters associated with grains, they remain unknown due to the absence of microscopic optics and chemical and mineralogical analysis instruments in the InSight mission. However, the amount of dust contained in the granular material could greatly influence this cohesion. On Mars, the dust is composed of charged-particles (e.g. [START_REF] Bertelsen | Magnetic properties experiments on the Mars Exploration Rover Spirit at Gusev Crater[END_REF]Ardvison et al., 2006), promoting the electrostatic cohesion. The compaction induced by hammering tests would have favored the apparent cohesion of the fine-grained material thanks to electrostatic forces, while keeping its weakness. The process would be all the most efficient if we consider the possible important amount of dust mixed to sand. Coupled to compaction/vibration, the effect of lower gravity on cohesive behavior on fine powders would be greater than on Earth [START_REF] Walton | Effects of gravity on cohesive behavior of fine powders: implications for processing Lunar regolith[END_REF], which would promote the apparent cohesion. This apparent cohesion induced by the vibrations/compaction of the HP 3 hammering would be localized not only around the mole but also to the whole surficial (cm-scale) granular unit of the hollow, inhibiting the formation of dust devils in the field of view of the cameras, while their traces are observed a few hundred meters from the lander.

Conclusion

Homestead hollow is a unique place of Mars, in which InSight lander touched down in 2018.

The combination of orbital and in situ data enabled to understand the structure of upper crust over the first hundreds meters of depth. It consists of two major units: 1) the bedrock unit composed of 60 m thick, Early Amazonian lava flows overlapping >500m thick Late Hesperian layered volcanic floods and 2) ~3 m thick, surficial unit consisting of superimposition of lensshaped clastic features filling the hollow. The latter corresponds to the Martian regolith, whose formation processes are cratering, aeolian sedimentation and thermal alteration to a lesser extent, at Insight landing site, and it differs from the lunar (cratering) and terrestrial (in situ alteration and biological activity) regoliths by sedimentary processes. This is the first time that the lens-shaped structure of the Martian regolith has been observed and analysed. In Homestead hollow,the lens-shaped features are composed of volcanic ≤ boulder-sized clasts ejected by nearby impacts interstratified within aeolian sedimentary materials essentially composed of loose centimeter pebbles, granules, sand and dust at nearsurface. The surficial clastic unit did not appear to have been transformed by early diagenetic processes at least over the first 10 cm of depth, since the fine sandy eolian granular material is characterized by high porosity and a very low cohesion. Then, as on the entire Martian surface, the area was subjected to aeolian exhumation, leaving coarse clasts in place and forming a residual lag. [START_REF] Garvin | Craters on Mars: Global geometric 1142 properties from gridded MOLA topography[END_REF] equations for fresh crater (see Table 1). Lava thickness is then deduced for buried (B) impact craters and estimated for partly buried (PB) craters, using the relief between the Garvin's rim height (h) and the remnant rim height (hr). See the text for explanation. The size of clastic patch or moat is around mm scale. f and g) Displacement of coarse sand/granule grain (2.5±0.7 mm) of around 2 cm, marked by black and white arrow. This grain could be a candidate for saltation process because no striation path appears on the ground. Notice that the cleaning of coating pebble occurred during this aeolian event (white circle). h and g) Potential displacement of sand grain by saltation around 2 cm (white arrows on the ground). Note the black elliptical features on the surface of HP3 foot that are oriented parallel to the measured wind direction, suggested that small particles impinges the surface like a "splash". which steep edges are irregular, composed of few mm thick, indurated, light-toned, finegrained material (white arrow), and partly cementing cm-scale clasts locally, and covering a clastic material composed of dark-toned, angular to sub-rounded pebbles poorly sorted in a very fine grained material. c) IDC image (D001R0018_598130890CPG_F0707_0010M1, sol 18 at 13h17), shows evidence for slight sliding and digging of the western footpad into place, creating a depression on one side and clods of material in left side, suggesting the sub-surface consists of surficial dust mixed with thin, cohesionless, fine-grained material. Black arrows show three pyramidal stones sculpted by wind (ventifacts). d) IDC image (D002R0018_598131254CPG_F0707_0010M1, sol 18 at 13h22) shows the front side of lander and the eastern footpad completely sunk in ground. Behind the Turtle cobble, the small pit shows a diversity of materials. e) Zoom on ventifacts seen in C, showing that their base was preserved of wind action before the removal of fine-grained material. The thickness of removal material is estimated to some centimeters. f) Close-up of pebbles beneath the lander showing a variety of sizes and lithologies (see text). Tenuous plane beddings are observed (white arrows and lines). g) Close-up of pebble composed of mm-scale, coarse, light-toned minerals.

Figures :Figure 1 :

 1 Figures :

Figure 2 :

 2 Figure 2: Geological features observed from HiRISE images. a, b, and c correspond to closeups of the InSight landing site before the landing (May, 6, 2014, ESP_036761_1845). Star shows the InSight location. a) Highly cratered surface on which remain relics of degraded ~100m impact rim and relatively circular hollows smaller than 30 m in diameter. Some of them display smooth inner surface on which NW-SE trending, dark, linear features occurred (white arrows) by wind erosion (dust devil) and small bright linear structures (dunes) can be observed in the orthogonal direction (red arrows) on rim relics. b) Close-up centered on 135.6227°E/4.5128°N showing more preserved degraded impact craters, dust devils tracks and dunes. c) Crescent dunes (red arrows) settled on the NW side of impact crater located at 135.6304°E/4.5013°N. A ripple field is observed inside the crater (orange arrow). R

Figure 3 :

 3 Figure 3: Cumulative crater size-frequency distribution plots for InSight landing site (yellow star). Solid black line: Isochron fitted to diameter bins (2.0-9.0 km) interpreted as containing primarily craters, which have accumulated on the volcanic surface (S HRSC: 10,437.099 km 2 above 3.95°N, based on HRSC and THEMIS data. Solid red line: Isochron fitted to diameter bins (0.7-2.0 km) interpreted as containing primarily craters, which have accumulated after a resurfacing of the volcanic unit from the HRSC and THEMIS data. Solid green line: Isochron fitted to diameter bins (0.15-0.7 km) interpreted as containing primarily craters, which have accumulated on the volcanic surface (Sctx: 1642.369 km 2 ), based on CTX images.

Figure 4 :

 4 Figure 4: Morphology of impact craters and implication for nature and thickness of geological units. a) Buried impact crater c#3 in figure 1f. b) partly buried ~9 km-in-diameter impact crater c#6 in figure 1e. c) Relatively fresh ~8 km-in-diameter post-dating the last event of Early Amazonian lava flows. d and e) Inner crater sides showing dark layered material (L) up to 500m below the crater rims marked by white arrows. a, b, c, d and e) are extracted from CTX image D18_034071_1842. f) HiRISE topographic profile extracted across a ~11 m relatively fresh impact crater viewed in box of h. g) 100m-in-diameter impact crater showing boulder-rich degraded ejecta corona located 430m east of InSight site (box c Fig. 2d). b: boulders. White arrow shows a 20m impact crater with no boulder around it. h) Morphologies of impact craters in function of their diameter on the smooth surface of landing area. Note that the 50m impact crater, located at 190m to the south of InSight site has very few boulders and the smaller ones display no boulder. g and h are extracted from HiRISE image ESP_036761_1845.

Figure 5 :

 5 Figure5: The stratigraphy at the landing site (yellow star) deduced from orbital data. On the left, geologic cross-section based on MOLA topographic profile viewed in Fig.1c. The vertical scale is exaggerated. On the right, the first 100m thick log. See the text for explanation.

Figure 6 :

 6 Figure 6: a) InSight sol 14 (December 10, 2018) IDC panorama of the mid-field terrain around the lander from NNW to SSW in clockwise view (D_LRGB_0014_RAS030100CYL_R__SCIPANQM1). Black triangles show the outer halo line of dust deposit and white triangles show the inner halo boundary. The c# symbol indicates the location of impact craters. b) Mosaic of individual IDC images projected in local equirectangular projection centered on the lander (0, 0). The grid unit is meter and North is up. The color lines are local height contours with a height interval of 10 cm with a height origin located at lander desk. White areas are without datum. Black line corresponds to the location of topographic profile viewed in E. Area underlined by black line corresponds to workstation in which SEIS seismometer and HP3 probe will be installed. c) Close-up of panorama, showing 6cm-in-diameter impact crater c and ~meter-in-diameter hollow h probably formed by impact. Note that their rims are without rocks. d) HiRISE image (12/6/2018), on which MOLA elevation contours are reported. White line corresponds to

Figure 7 :

 7 Figure 7: Evolution of the southern area of Homestead hollow (landing site) viewed by the ICC. Each ICC image has a field of view of 124 x 124 degrees. a) The first image (C000M0000_596533559EDR_F0000_0106M8) acquired on November 26, 2018 (Sol 0 of the InSight mission) where the local true solar time for the image exposure was 13h34 PM. b) This image (C000R0063_602124400EDR_F0000_0121M2) was acquired on January 30, 2019, (Sol 63 at 12h48 PM). Note the camera lens is cleaner because the dusty particles were removed by wind. c) ICC image acquired on December 12, 2018 (sol 16) at local true solar time of 13h09 PM (C000M0016_597952811EDR_F0000_0500M2) and d. The same area viewed sol 16 at 16h11 PM (C000M0016_597964044EDR_F0000_0461M2), in which shadows of microstructures are enhanced. At the foreground, the western lander foot (right lower corner) has sunk into a cohessionless material. The hollow surface appears relatively flat with randomly sparse stones. In three images (b, c and d): a. The inner slope of Homestead hollow, ~17 m from the InSight lander; b. Light-toned, flat, pebble; c. and e. Dark-toned, sub-angular pebbles; f. Dark-toned cobble showing a corrugated surface; g. Dark-toned, rounded boulder outside the Homestead hollow; h. Small clod, few cm away from the western lander foot.

Figure 8 :

 8 Figure 8: a) Central part of the first panorama acquired sol 14, covering the western Elysium planitia between SE and WSW trends. Notice that the southeastern part of Homestead hollow is brighter and smoother than the southwestern part with coarser clasts (pebbles to cobbles). The south area shows well defined slope of ~3° towards the center of hollow whereas the western edge of Homestead hollow shows a slight slope of ~1.5° towards the east. In the latter, see the ~1.5 m in diameter, shallow depression (dashed white circle) due to meteorite impact, surrounded by a pebble circle. The white arrows show other hollows outside Homestead hollow filled by brighter and probably sandy material. The rocks (b to f) scattered on the surface show different colors, shapes and textures: b. Boulder at the edge of Homestead hollow (~1.5 m long, <1m high). Note the ventifact, dark-toned, cobble at the front of boulder; c. ~10 cm long, ~5 cm thick, light-toned, flat top pebbles; d. ~10 cm equant, dark-toned pebble showing a sharp oblique fracture; e. Scatter of pebbles with different colors. Note that the dark one is characterized by typical aeolian erosion (ventifact); f. ~20 cm long cobble with a heterogeneous texture (several indurated clasts? breccia?).

Figure 9 .

 9 Figure 9. Ejected pebbles and cobbles on the InSight landing workspace: a) 8 cm wide piece of breccia composed of cm-scale angular, dark-toned clasts (D036R0012_597605746CPG_F0101_0060M2) and b) Same cobble viewed from east (D000M0128_607888894CPG_F0000_0846M3). c) partlyburied piece of breccia (D000M0076_603267350CPG_F0000_2699M1). d) partlyburied piece of breccia composed of light-toned and dark-toned cm clasts (D036R0012_597605746CPG_F0101_0060M2). e) Rolling stone in cohesionless sandy material (D001L0016_597958573EDR_FS0S0SS080M1). White arrows display prod mark (at the foreground) and 52 cm long sinuous roll marks. f) Relatively light-toned, brecciated stone (D000M0070_602751416CPG_F0000_0250M1). g) Equi-rectangularly projected workspace IDC images in front of lander with a spatial resolution of 1 mm. Color lines are relative height contours with an intervals of 2 cm. Labeled circles indicate the location of pebbles. h) Displaced pebbles (black arrows) near the west lander foot with their roll and groove marks (white arrow) in sandy material (D003R0012_597602303CPG_F0101_0060M2). i) Up lifted and few cm pushed away light-toned pebble remaining a trough mark (white arrow) (D003R0012_597602303CPG_F0101_0060M2). j) partly buried, light-toned pebble with saddle sharp crestline (D003R0012_597602303CPG_F0101_0060M2). k) Dark-toned, subangular pebble showing oblique planar fracture (D016R0014_597779204CPG_F0103_0100M1). All images are oriented North up.

Figure 10 .

 10 Figure 10. a) Clast distribution into workspace (N=8252; area 5.339m 2 ). b) Particles sizes over the measured diameter range (range=2 mm to 50 mm; mean=5.96 mm) on which modified Wentworth classes are superimposed (sand, granule and pebble). c) Diagram showing 2D clast shape (equant to sub-equant).

Figure 11 .

 11 Figure 11. Sandy surface near lander in workspace. a). The Instrument Deployment Camera acquired this image (D000M0008_597253045EDR_F0000_0463M1) on December 5, 2018 (Sol 8) when the local true solar time was 16h01.The ground surface is striated and scoured, with multi-millimeter relief ridges and troughs that extend radially away from the lander. Some granules and small pebbles have tails extending away from the lander (see insert), suggesting that material behind small clasts has been preserved from erosive rocket blast. Note partly buried sub-rounded pebble (black arrow) and angular, very flat pebble (white arrow) on the darker ground near the lander.

Figure 12 .

 12 Figure 12. Aeolian changes on the Martian ground surface near HP3 mole. All images are oriented with the North down. a) IDC image acquired sol 383 at 16h around (D000M0383_630540698CPG_F0000_0250M1). b) IDC image acquired sol 385 at the same time (D000M0385_630718248CPG_F0000_0250M1). c) Location of Aeolian changes (yellow point is the starting location and green point the ending one). No plot into the scoop, although coarse grain displacement has been observed and very fine particles have been added between the two snapshots (white arrow in b). d and e) Close-ups in the western side of observed area: Notice that the mass-wasting of scour sides occurred (white circle), leading to partial removal and new arrangement of clastic material transported by creeping towards NE trending as imprinted by faint striation into the ground. It is not possible to determine the diameter of individual transported particles.The size of clastic patch or moat is around mm scale. f and g) Displacement of coarse sand/granule grain (2.5±0.7 mm) of around 2 cm, marked by black and white arrow. This grain could be a candidate for saltation process because no striation path appears on the ground. Notice that the cleaning of coating pebble occurred during this aeolian event (white circle). h and g) Potential displacement of sand grain by saltation around 2 cm (white arrows on the ground). Note the black elliptical features on the surface of HP3 foot that are oriented parallel to the measured wind direction, suggested that small particles impinges the surface like a "splash".

Figure 13 .

 13 Figure 13. Ground texture near HP3. a). IDC image acquired (D000M0235_617388344CPG_F0000_0250M1, sol 235) b) . Close-up of few centimeter deep cross-section at HP3 mole hole shows a fine layered material composed of very fine sand, granules, small pebbles and aggregates of small pebbles (white arrows). Notice that the ground is darker than the red clastic lamina at the surface. c) IDC image acquired (D001L0243_618107964CPG_F0101_0028M3, sol 243) after the first scoop push. d) IDC image acquired (D003L0250_618729516CPG_F0101_0028M2, sol 250) after the test scoop. e) HP3 left rear foot with a sample of indurated fine sand (white arrow). f) During HP3 lift, the sample remains at the same place although the foot is tilted. (D000M0203_614553629CPG_F0000_0824M1) G. After three lifts, the sample of indurated fine sand keeps its location on the foot. (D000M0209_615090548CPG_F0000_0817M1, sol 209).

Figure 14 .

 14 Figure 14. Sub-surface near HP3 after scoop scraping and pushing tests. a) IDC image acquired sol 417 at 16h10 (D000M0417_633558206CPG_F0000_0473M1). b) The same area viewed three days after IDC image acquired sol 421 at 13h54 ( D000M0421_633904709CPG_F000_0250M1). c) Close-up of the hole showing pebbles down the hole. Location indicated by black arrow in b. For previous images, north is down. d) Ortho-rectified image and DEM acquired sol 417. Color lines are height contours at intervals of 5 mm. White areas have no data. e and f) Topographic profiles across the hole with relative distance. Gray ones correspond to pristine profiles acquired sol 14. Dark one corresponds to topographic profiles 1 and 3. Green one is profile 2 on Fig. 14d.

  Figure 15. Geologic surface beneath the lander. a) IDC image (D001R0018_598131636CPG_F0606_0010M1), acquired sol 18 at 13h29, shows terrain disturbed by retro-rockets (seen at the top of image) under the lander: shallow depressions (d#) composed of poorly sorted, dark-toned, angular gravels and pebbles at the forefront, and pits (p#) bordered by a cm-scale deep, steep slope (greater than the angle of repose) clastic material in the background. Two cobbles are present near the western lander foot (Ace of Spades) and in front of lander (Turtle). b) IDC image (D001L0014_597774194CPG_F0909_0010M1, sol 14 at 12h52) shows few cm-deep left pit

Figure 16 .

 16 Figure 16. Clast distribution under the lander (N = 3213; area = 2.046 m 2 ). b) Particles sizes over the measured diameter range (range=2 mm to 350 mm; mean=9.08 mm) on which modified Wentworth classes are superimposed (sand, granule and pebble). c) Diagram showing 2D clast shape (equant to sub-equant). A. corresponds to "Ace of Spades" cobble and T to "Turtle" cobble. d) Clast distribution into three pits p1, p2 and p3 (N = 377; area = 0.408 m 2 ).

  Figure 17: a) IDC image acquired sol 18 under the lander (D002R0018_598131254CPG_F0707_0010M1), showing the shallow depression d1 between the two lander fore feet. Note that the eastern foot (at the upper right corner of image) is sunk into cohesionless, fine, clastic material. The two cobbles (Ace of spades and Turtle) are visible at the front. White arrow indicates small depression behind "Turtle". b) Zoom on the depression d2 (dashed line), viewed by IDC sol 14 (D001L0014_597774194CPG_F0909_0010M1). Three superposed clastic material units are visible: R0, R1 and R4. Notice that the middle unit displays two erosive shapes: small concave scalloped edge with white sides (white arrow) in softer material, and serrated ledge (black arrow) composed of more erosion resistant, tabular, layered material. c) Equirectangularly projected IDC image with height contours spaced of 4 cm. White lines correspond to location of topographic profiles. d) Topographic profiles across the depression d2, showing the location of three clastic units viewed in B.

Figure 18 .

 18 Figure18. a) Close-up of pit P1 located at the central part under the lander. It is bordered by three superposed clastic units, from top to bottom: R2, sandy to poorly sorted, sub-angular, pebble material; R3, poorly sorted, sub-equant, sub-angular pebbles arranged in a ~5 cm thick, sub-horizontal layer. Some pebbles are longer than wide (white arrow); and R4, cohesionless, poorly sorted, sub-angular gravels. Note that the left side of the pit collapsed (black arrow). b) Mosaic of projected IDC images on which colored lines correspond to elevation contours with an interval of 4 cm. The pit is deep of ~16 cm. The white lines correspond to location of labeled topographic profiles. c) Topographic profiles extracted from DEM on which three units and topographic slopes have been reported. Note that the R3 unit,

Figure 19 .

 19 Figure 19. a). Close-up of pits P2 (right) and P3 (left) located at the western side under the lander. They are bordered by two superposed clastic units, from top to bottom: R2, sandy to poorly sorted, sub-angular, gravel material; and R3, poorly sorted, sub-angular pebbles arranged in a ~5 cm thick, sub-horizontal layer. Many pebbles are longer than wide (white arrows), with their longer side arranged sub-vertically and parallel to each other. b) Mosaic of projected IDC images on which colored lines correspond to elevation contours with an

Figure 20 .

 20 Figure 20. Stratigraphy beneath and in front of Lander. a) Schematic view of the stratigraphy based on distribution of clasts. b). Mosaic of projected IDC images acquired sol 18 on which IDC DEM height contours are plotted each 2 cm of intervals, the location of pits and cross-section (red line). Stratigraphy logs are plotted for each pit around the IDC mosaic. c). Geologic cross-section established from clast distribution and relative height, showing a spatial lens-shaped organization.

Figure 21 :

 21 Figure 21: Schematic geologic cross-section across Homestead hollow. a) At CTX scale, the WNW-ESE cross-section shows the superimposition of ~60 m thick, layered lava flows, cratered and fractured by meteoritic impacts, overlapping very thick Early/Late Hesperian volcanic material. A meter thick sedimentary layer covers these lava flows. b) Close-up of the upper layer centered on InSight lander, showing the complex structure of this layer, composed of ejecta blanket and aeolian sediments arranged in fine-fined layered lens-shaped features. Note the vertical axis is exaggerated in this cross-section.

Tables : Table 1 .

 :1 Morphometric laws for Martian impact craters. D: diameter, d: depth and h: crater rim. See figure 1d for sketch.

	Author	D	d	h	unit
	Garvin (2003)	3 km ≤ D< 7 km	0.21*D 0.81	0.04 * D 0.31	h and D in km
	Garvin (2003)	7 km ≤ D < 100 km	0.36*D 0.49	0.02 * D 0.84	h and D in km
	Watters et al., 2014	50 m ≤ D < 2 km	0.223 *D 1.012	0.0354 * D 1.017	h and D in km
	Sweeney et al., 2018 10 m< D < 1.2 km	0.081*D	0.029*D	h and D in m

Table 2 .

 2 Minimum lava thickness T calculated from MOLA data. Crater diameter D is measured, depth d and rim height h are calculated, using

Acknowledgments. We thank the French agencies, CNES and CNRS, which have supported and funded this work. We thank IPGP and JPL employees for the logistics and the very good hospitality during the operations of the mission.