Ansan, V.

Banerdt, William B.

Calef, Fred

Charalambous, C.

Deen, R.

Garvin, J.

Golombek, M.

Grant, J.A.

Hauber, Ernst

Kopp, M.

Lethcoe, H.

Lorenz, R. D.

Maki, J.N.

Pike, T.

Smrekar, S.

Warner, N.

Weitz, C.M.

Williams, Nathan

Wilson, S.

Institute of Transport Research:Publications

INSIGHT LANDING SITE: STRATIGRAPHY OF THE REGOLITH BENEATH THE LANDER AND IN ITS SURROUNDINGS, AND IMPLICATIONS FOR FORMATION PROCESSES. V. Ansan1, E. Hauber2, M. Golombek3, N. Warner4, J. Grant5, J. Maki3, R. Deen3, F. Calef3, C. Weitz6, J. Garvin3, S. Wilson5, N. Williams3, C. Charalambous7, T. Pike7, H. Lethcoe3, M. Kopp4, A. De Mott4, S. Smrekar3, B. Banerdt3, and R. Lorenz8 1LPG-UMR CNRS 6112, University of Nantes, 2 rue de la houssinière, BP 92208, 44322 Nantes Cedex 3, France (veronique.ansan@univ-nantes.fr), 2German Aerospace Center (DLR), Institute of Planetary Research, 3Jet Propul-sion Laboratory, California Institute of Technology, Pasadena, CA, 4SUNY Geneseo, Department of Geological Sci-ences, 5Smithsonian National Air and Space Museum, Center for Earth and Planetary Studies, 6Planetary Science Institute, 7Imperial College, London, Department of Electrical and Electronic Engineering, 8Johns Hopkins University Applied Physics Lab, Laurel, MD.   Introduction:  On November 26, 2018, the InSight lander touched down at 4.50°N/135.62°E within Homestead hollow, a subdued and filled depression on Late Hesperian, highly cratered volcanic plains of west-ern Elysium Planitia, Mars [1,2,3,4].  Surface Terrain: Both using the lander-mounted Instrument Context Camera (ICC) and the robotic arm-mounted Instrument Deployment Camera (IDC) [5], the terrain surrounding and beneath the lander was im-aged, showing a smooth, sandy surface with additional >cm scale clasts, ranging from pebbles to very few cobbles [6,7,8] (Fig. 1). Close to the lander, pebbles and cobbles show two distinct types of materials: dark-toned, grey aphanitic  one probably corresponding to a basaltic composition, and light-toned one with un-known composition and origin. Some particles are par-tially buried, showing that sedimentary processes are or were active after their deposition (i.e. sedimentation or aeolian erosion is very efficient at present-day).   Fig. 1: A. South view of Homestead hollow, showing the flat sandy terrain covered by spaced sub-angular, pebbles and cobbles (a,b,c), and boulders (e) over the hollow. The right lander footpad (f) is partially buried in cohesionless, dark-toned, fine material, the “turtle” cobble consisting of ~20 cm in size, dark-toned material (probably mafic basalt), eroded by wind (deflation scour at its summit), lifted and pushed away by rockets during landing forming a shallow depression (d) consisting of <cm scale dark particles.  ICC color image C000M004_596888328EDR _F0000_0461M1 obtained on sol 5. B. Detailed view in front of lander marked by white box in A, obtained by IDC, sol 14, (D001L0014_597774194CPG _F0909_0010M1), showing few cm-deep left pit which steep edges are irregular, composed of few mm thick, indurated, light-toned, fine-grained material (white arrow), and partially cementing cm-scale clasts locally,  and covering a breccia composed of dark-toned, angular to sub-rounded pebbles poorly sorted in a very fine grained material.  Sub-surface terrain (~10 cm deep): The texture and near surface structure of regolith  have been ex-posed by landing rocket-induced excavations under the lander (Fig. 2), showing a variety of clast sizes, ar-rangements and texture at horizontal meter-scale (dis-tance between lander feet).   Fig. 2: A.  B. Two footpads show evidence for slight slid-ing into place, creating a depression on one side and bulge in direction of travel, suggesting the sub-surface consists of 1310.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)surficial dust mixed with thin, cohesionless, fine-grained ma-terial. White arrow shows the “Ace of Spades” cobble.  IDC images acquired sol 18, D001L0018_598130765CPG_F0707 _0010M1 and D002L0018_598131151CPG_F0707_0010 M1. C. Zoom on pits under the lander (D001L0018_ 598131526CPG_F0606_0010M) showing a cm-scale deep, steep slope (greater than the angle of repose) composed of cemented soils, small angular gravels and pebbles indicating a cemented duricrust. Smaller clods and pieces of this material are littered within the pits and adjacent to the pits, showing imbricated texture (black arrow).   Stratigraphy of regolith: From these observations, stratigraphic sequences are proposed as following from top to bottom (Fig. 3): i) a cm-scale thick layer consist-ing of light-toned, cohesionless, <mm-scale grain sizes ranging from clay to fine sand. It was partially removed by the rocket blast as shown by cm-scale erosive streaks and divots around the lander [6,7]; ii) a mm-to cm scale thick, light-toned layer, consisting of indurated material “duricrust”, easily fragmented in flat polygons by lander feet.  This duricrust shows lateral variations of textures, from fine-grained (i.e. <mm scale) to coarse-grained material (i.e.  composed of poorly-sorted, angular to sub-rounded clasts, ranging from granule to cobbles with a majority of pebbles contained in fine-grained cement; iii) a cm-scale thick, cohesion-less, granular material comprised of either dark-toned sand or poorly sorted material with sub-angular, dark-toned pebbles showing a weakly developed sub-horizontal planar layering, containing in a fine-grained matrix.    Fig. 3: A. Granulometry (c: clay, si: silt, s: sand, g: gran-ule, p: pebble, co: cobble). B. synthetic cross-section of rego-lith beneath and in surroundings of InSight lander.   If we extrapolate this at meter-scale, an idealized geologic cross-section would show the distribution of fragmented-rocks with depth (Fig. 4), due to impact gardening, before reaching Hesperian lava flows a few meters deep.    Fig. 4: Idealized geologic cross-section in Homestead hol-low. Lander foot at right side.    Discussion: This stratigraphic succession is quite similar to what was seen at the other Martian landing sites [9,10,11], notably if compared to the regolith in Gusev crater which consists of similar Hesperian vol-canic host material:  There is a cohesive duricrust near the sub-surface. It differs by a greater thickness (cm-scale). The stratigraphy of regolith in Homestead hollow suggests that several processes modified the Late Hes-perian/Early Amazonian lava flows of western Elysium Planitia into this clastic regolith during the last 3 billion years: impact gardening as the origin of clasts; aeolian erosion, transport and sedimentation, filling impact craters and intercratered plains, though the small exca-vations beneath the lander do not display sand cross-bedding; weathering as the change of clast shape (e.g. wind abrasion) and  the cementation to form duricrust.   References: [1] Golombek et al., this issue. [2] Golombek et al. (2018) SSR. [3] Warner, N., et al., this issue. [4] Parker, T., et al., this issue. [5] Maki, J., et al., this issue. [6] Grant et al., this issue. [7] Weitz, C., this issue. [8] Charalambous et al., this issue. [9] Arvid-son et al. (2006) Journal of Geophysic Res., 111, E02S01, doi:10.1029/2005JE002499. [10] Herkenhoff et al. (2008) The Martian Surface: Composition, Min-eralogy, and Physical Properties, Cambridge University Press. [11] Golombek et al. (2008) The Martian Sur-face: Composition, Mineralogy, and Physical Proper-ties, Cambridge University Press.  1310.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

Insight Landing Site: Stratigraphy of the Regolith Beneath the Lander and in Its Surroundings, and Implications for Formation Processes

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Insight Landing Site: Stratigraphy of the Regolith Beneath the Lander and in Its Surroundings, and Implications for Formation Processes

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