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

Basalt rock heights and three-dimensional shapes at the InSight lander in Homestead hollow, Mars, provide new constraints on modification of the 0.4-0.7 Ga, highly degraded, 27 m-in-diameter impact crater. Rocks were measured from an orthomosaic digital elevation model (DEM) that allowed fragments >1 cm to be evaluated within 10 m of the lander on the hollow exterior, margin, and interior. Average rock height decreases from 6 cm outside to 1 cm within the hollow and the percentage of fragments where height comprises the short axis increases from 78% outside to 97% within the hollow. Normalizing rock heights indicates differences are not solely the result of variable rock size around and in the hollow. More exposed near-rim rocks support ejecta deflation accompanied by infilling of the interior and comparison between rock relief outside and inside the hollow with expectations of pristine ejecta thickness indicates up to 40 cm deflation occurred early in hollow history (decreasing to a few cm to one diameter beyond the rim). Transport of ~50% this inventory into the hollow by prevailing winds can account for the predicted 40% or more eolian component of infilling. Infilling associated with deflation and transport from ejecta around later occurring impacts is not required. Nevertheless, scattered, mostly buried rocks in the hollow are ejecta from nearby impacts and their continued exposure is consistent with the estimated long-term degradation rate of 10-4 m/Myr over most of hollow history. Basalt rock shapes at InSight are expected to be similar to basalt rock shapes on Earth that are generally comparable in a range of settings, but at InSight 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 partial rock embedding in the ejecta results in shapes that are quite close to terrestrial rocks. Together, degradation estimates based on current rock relief and rock shape after accounting for continued partial embedding of near-rim rocks indicates ~30-40 cm early (~0.1 Ga) near-rim deflation was followed by much slower degradation. Results demonstrate rock sizes and shapes can serve as a tool for characterizing degradation on widespread regolith-covered lava plains on Mars.