Degradation of Homestead Hollow at the InSight Landing Site Based on the Distribution and Properties of Local Deposits

Abstract

The InSight mission landed its scientific payload in Homestead hollow, a quasi-circular depression interpreted to be a highly degraded impact crater that is 27 m in diameter. The original pristine crater formed in a preexisting impact-generated regolith averaging~3 m thick and the surrounding ejecta deposit, consisting of coarse and mostly fine fragments, was in disequilibrium with local geomorphic thresholds. As a result, early, relatively rapid degradation by mostly eolian, and lesser impact processes and mass-wasting, stripped the rim and mostly infilled the hollow where sediments were sequestered. Early, faster degradation during the first~0.1 Ga was followed by much slower degradation over the bulk of the 0.4-0.7 Ga history of the crater. Pulses of much lesser degradation are attributed to impacts in and nearby the hollow, which emplaced some rocks as ejecta and provided small inventories of fine sediments for limited additional infilling. Even lesser sediments were derived from the very slow production of fines via weathering of resistant basaltic rocks. Nevertheless, indurated regolith caps the sediment fill within the hollow and creates a relatively stable present-day surface that further sequesters infilling sediments from remobilization. The degradation sequence at Homestead hollow is like that established at the Spirit rover landing site in Gusev crater and points to the importance of eolian, and lesser impact and mass-wasting processes, in degrading volcanic surfaces on Mars over the past~1 Ga.Plain Language Summary The InSight mission landed in a highly degraded impact crater dubbed Homestead hollow in Elysium Planitia on Mars. The hollow interior is quite flat and smooth, and mostly infilled by fine-grained sediments. Rocks are 2-3 times more numerous on the western side dubbed Rocky Field. The hollow lacks a raised rim but is marked by an increase in larger rocks. The distribution of windblown and impact materials within, around, and local to the hollow indicate degradation was mostly by wind stripping of fines from the rim and depositing them inside the hollow, with lesser contributions from impact and mass wasting processes. Rocky Field was likely formed by emplacement of ejecta during a nearby impact event occurring relatively soon after Homestead hollow formed. Most degradation occurred during the first~0.1 Ga after hollow formation. Limited modification over most of hollow history was associated with small pulses of infilling and rock emplacement during/following nearby impact events and very slow weathering of basaltic rocks. Degradation at Homestead hollow is similar to the modification of small craters at the Spirit landing site in Gusev crater, which shows common geomorphic processes occurred on comparable surfaces in different places on Mars during the last~1 Ga.

An Impact Crater Origin for the InSight Landing Site at Homestead Hollow, Mars: Implications for Near Surface Stratigraphy, Surface Processes, and Erosion Rates

“…The sedimentologic observations of Homestead hollow and the terrain surrounding InSight are consistent with the preferential preservation/accumulation of fines within a topographic depression (Grant et al, 2020, this issue). Further, the dominance of sand‐sized clasts suggests eolian transport into the hollow.…”

“…The rock density map does not indicate an unusually high rock abundance resulting from ejecta from either of these craters, although Homestead hollow may be in range of the discontinuous ejecta of the Class 6 crater. This is a possible source for Rocky Field (Grant et al, 2020, this issue). The crater data indicate that Homestead hollow postdates the 102‐m‐diameter Class 7 crater northeast of the landing site.…”

“…Slow, diffusive slope modification and more punctuated events of mass wasting result in rim height degradation, slope reduction, and net transport of material from the rim to the floor (Armitage et al, 2011; Fassett & Thomson, 2014; Sweeney et al, 2018). Impact gardening and eolian processes also operate at the InSight landing site, as evidenced by small superposing craters, nearby hollows, and abundant fines within the hollow (Grant et al, 2020, this issue). Rim height reduction occurs at a rate that nonlinearly decreases with time as slope transport diminishes and the overall landscape becomes weathering‐limited (Grant et al, 2006; Sweeney et al 2018; Grant et al, 2020, this issue).…”

“…Post‐landing observations of the hollow and the exterior plains from the Instrument Deployment Camera (IDC) and Instrument Context Camera (Maki et al, 2018) onboard InSight confirm orbital observations (Golombek et al, 2020). InSight rests on a smooth, poorly sorted, regolith‐covered plain dominated by sand‐size particles with a low abundance of meter‐size rocks (~1% to 2% cumulative fractional area; Golombek et al, 2020; Charalambous et al, 2019; Grant et al, 2020, this issue; Figures 3 and 4). The dominant modification processes that likely operated on this terrain include impact cratering, mass wasting, and eolian processes.…”

“…The observed float rocks are likely ejecta, delivered to this location by impacts. Pebble to boulder‐size clasts in the regolith are angular to sub‐angular, fine‐grained, and dark gray (Golombek et al, 2020; Weitz et al, 2019; Grant et al, 2020, this issue; Figures 4a and 4b). The texture, fabric, and color of these clasts are consistent with aphanitic basalt.…”