Rock Size‐Frequency Distributions at the InSight Landing Site, Mars

Abstract

The size-frequency distribution (SFD) of rocks on Mars is important for understanding the geologic and geomorphic history of the surface (e.g.,

Degradation at theInSightLanding Site,Homestead Hollow, Mars: Constraints From Rock Heights and Shapes

“…The panorama DEM and orthoimage were created by mosaicking 283 stereo images acquired by InSight's arm-mounted Instrument Deployment Camera (IDC, Figure 3) (angular resolution of 0.82 mrad/pixel at the center of the image, see Maki et al, 2018). As summarized in Golombek et al (2021), the scale of the IDC images varies from 0.12 cm/pixel to 2.8 cm/pixel with increasing range from the lander, and the DEM has elevation postings every 5 mm. The panorama orthomosaic was bundle adjusted (Abarca et al, 2019), except for the western region, which is separated from the rest of the panorama (Figures 3 and 5).…”

“…A convex hull, which is the smallest convex polygon enclosing the rock (representing the minimum bounding geometry), was calculated to provide the shortest and longest axes for each (Golombek et al, 2021). The longest axis is calculated as the longest distance between any 2 vertices of the minimum bounding polygon, and the shortest axis is the shortest distance between any two vertices of the minimum bounding polygon measured orthogonal to the longest axis.…”

“…These measurements represent horizontal dimensions with no elevation information, yielding the small and large axes for each rock fragment. To assess the maximum height of each rock, we started with the same data set of 2,034 rocks noted above (Golombek et al, 2021), but removed 31 rocks that were not entirely covered by the 5 mm DEM. The height (Z) of each rock was derived from the 5 mm DEM (with estimated vertical precisions better than 1 mm based on 10%-20% of the spatial elevation posting scale) using the Add Surface Information analyst tool in ArcGIS, which calculated the difference between the maximum (Z_MAX) and minimum (Z_MIN) values for each rock (as mapped by the bounding polygon from Golombek et al, (2021)).…”

“…First, prior work demonstrates that the relatively low energy impact that formed Homestead Hollow limited excavation to an impact gardened regolith "pre-processed" by ∼3 impact events (Charalambous et al, 2020;Golombek et al, , 2021 that decreased the number of high velocity spalls and large rocks on/within the pristine ejecta (Head et al, 2002;Melosh, 1984). These multiple impacts would have produced abundant sand-sized material at the expense of fewer larger rocks (Golombek et al, , 2021. Hence, regardless of whether Homestead hollow formed as a primary or secondary impact structure, the entirety of its rim is comprised of ejecta and the underlying regolith with comparable properties.…”

“…Further, to minimize potential uncertainties related to setting, we emphasize comparison of rock shape parameters for the basalt rocks in ejecta at Homestead hollow to rock shape parameters for basalt rocks in the ejecta deposit at the simple impact structure Lonar crater, in India (Kumar et al, 2014). Although the hollow is 27 m across relative to the 1.2 km-diameter Lonar crater, fragmentation processes associated with impacts at a wide range in scales produce rock distributions with exponentially more small rocks than large rocks (e.g., Brown, 1989;Golombek et al, , 2021Melosh, 1989;Schroder et al, 2020Schroder et al, , 2021 Our approach helps detail where and how much degradation occurred at the InSight landing site, and whether processes occurring locally around and within the hollow can account for the current degraded appearance or whether additional sediments from more Rock axes measured in the InSight landing site are plotted to evaluate shapes in (interior), around (margin), and outside (exterior) of Homestead hollow (see Figure 7).…”