Degradation at the InSight Landing Site, Homestead Hollow, Mars: Constraints from Rock Heights and Shapes

Abstract

Rock heights and three-dimensional shapes around the InSight lander in Homestead hollow, Mars, provide new constraints on modification of the degraded 27 m in diameter impact crater and are a tool for characterizing degradation on regolith-covered lava plains on Mars. Decreasing average rock height and increasing percentage of fragments where height comprises the short axis from outside to within the hollow supports significant ejecta deflation accompanied by infilling of the interior. Rock relief outside the hollow is compared with expectations of pristine ejecta thickness and indicates up to~40 cm of near-rim early deflation (decreasing to a few cm out to one diameter) can account for the predicted eolian component of infilling and that other eolian infilling sources are not required. Scattered rocks in the hollow are ejecta from subsequent nearby impacts and their mostly buried expression is consistent with subsequent longterm degradation estimated to be 10 -4 m/Myr. Basalt rock shapes at InSight are likely similar to basalt rock shapes on Earth, but appear more platy, bladed, and elongate in a triangular form factor plot and more discoidal and bladed in an axes ratio plot. Nevertheless, addition of 10 cm to near rim rock heights to account for continued partial embedding in ejecta would result in rock shapes quite similar to terrestrial rocks. Consistency between degradation estimates based on current rock relief and rock shape after accounting for partial embedding in ejecta indicates up to~30-40 cm early (~0.1 Ga) near-rim deflation was followed by much lesser long-term degradation. Key PointsRock heights and shapes at Homestead hollow indicate early transport of deflated ejecta sediments can account for infilling of the crater.Near-rim rock heights and shapes relative to the expected original versus remnant ejecta thickness indicates~40 cm deflation occurred.

In Situ and Orbital Stratigraphic Characterization of the InSight Landing Site—A Type Example of a Regolith‐Covered Lava Plain on Mars

“…The fines at the surface of the workspace, by comparison to the RAD spot, are also likely dominated by fine sand. Examination of the prominence of rocks within and outside of the hollow indicate that rocks interior to the hollow, including in the workspace, are partially buried in the matrix of fines, consistent with a model of sediment trapping inside Homestead hollow (Grant et al., 2022). Imagery and DEMs of the workspace also reveal multi‐millimeter relief that forms ridges and troughs that extend radial from the lander (Figure 18).…”

“…Large, cobble to boulder size rocks are rare on the surface. When present, they are angular and are more commonly partially buried by fines (Grant et al., 2020, 2022). Where dust coatings were removed, the clasts are medium gray with a presumed fine‐grained texture (grains below the resolution limit of the IDC and ICC).…”

“…A small scattering of larger pebbles extends from the west across the smooth surface. This scattering, known as Rocky Field , superposes the smooth material and shows limited evidence for post emplacement burial (Grant et al., 2020, 2022) (Figure 4).…”

“…Within the ejecta, the sand is out of equilibrium with the surrounding landscape. Over time, the sand organizes against topographic obstacles (e.g., rocks and the rim) and eventually either migrates away or into the crater (Warner, Grant, et al., 2020; Grant et al., 2020, 2022). Similar to slope processes, eolian infilling is not a steady state process.…”

“…Understanding the local stratigraphy has implications for shallow seismic velocity measurements from SEIS (Kedar et al., 2017; Lognonné et al., 2020; Murdoch et al., 2021), models and observations of seismic signals from nearby crater impacts (Daubar et al., 2020), thermal conductivity and soil porosity estimates from HP 3 (Grott et al., 2021), and thermal inertia and conductivity measurements from the onboard Radiometer (RAD) (Mueller et al., 2021; Piqueux et al., 2021). Multiple pre‐landing and post‐landing geological investigations characterized the landing site by providing geomorphologic observations of local landforms (Golombek et al., 2017, 2018; Golombek, Warner, et al., 2020; Grant et al., 2020; Sweeney et al., 2018; Warner, Golombek, et al., 2017; Warner, Grant, et al., 2020), stratigraphic and sedimentological characteristics of the regolith and lava plains (Grant et al., 2020, 2022; Pan et al., 2020; Warner, Golombek, et al., 2017; Weitz et al., 2020), measurements of the local and regional rock size distributions (Golombek et al., 2017, 2021), observations of active surface processes (e.g., eolian processes) (Baker et al., 2021; Charalambous et al., 2021), and comparisons to other landing sites (Golombek, Kass, et al., 2020; Warner, Schuyler, et al., 2020; Weitz et al., 2020).…”