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

“…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).…”

“…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.…”

“…Based on analogy with basaltic fragments in a range of environments (including impact) on the Earth (e.g., Craddock & Golombek, 2016; Ehlmann et al., 2017; Kumar et al., 2014), many of the dark rocks should be compact/equant with significant numbers of platy/disc‐shaped and bladed fragments. Their current more platy/disc‐shaped, bladed, or elongate expressions owe to the fact they are partially buried (Grant et al., 2022). Lighter reddish clasts may be equant but are also sometimes thin and platy, with angular to sub‐angular edges.…”

“…These observations suggest a supply‐limited system where the original sand in the ejecta was produced by the impact process itself (Golombek, Charalambous, et al., 2020; McGlynn et al., 2011), either through fragmentation of bedrock and/or excavation of the upper column of sandy regolith. Freshly exhumed sand is quickly transported into the nearest topographic depression that is downwind of its location, which for some of the sand in the ejecta, is the crater itself (Grant et al., 2020, 2022). Once the locally‐derived sand is exhausted, crater fill may be supplied, at lower rates, by more distal sand sources.…”

“…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).…”