The InSight lander settled safely onto the Martian surface in western Elysium Planitia (4.502°N, 135.623°E) in November, 2018, and started collecting information about the surface and interior of Mars shortly thereafter (Fig. 1). Dr. Grant participated in the geology group and assisted in locating the placement of the instruments for monitoring seismic and thermal activity. Much of Dr. Grant’s research related to the InSight mission focuses on understanding the surface evolution in the vicinity of the lander and relies mostly on interpretations made using images obtained by the two cameras on the lander.
Fig. 1. (a) The InSight lander (red box) in the quasi-circular 27 m-diameter Homestead hollow degraded impact crater (red dashed circle) in western Elysium Planitia, Mars. The lander is 6 m measured across the solar panels, Yellow box shows location of (b). Subframe of HiRISE color ESP_036761_1845 (0.25 m/pixel). (b) Color view of InSight lander in Homestead hollow (red dashed circle) and numerous other hollows (white dotted circles) in the immediate vicinity. Red line cutting across the hollow is the boundary between occurrence of relatively few rocks to the east versus ~3X more rocks in the region to the west dubbed “Rocky Field”. Landing rockets removed dust from the immediate surface and caused the bright zone and surrounding dark halo around the lander.
InSight landed its scientific payload in Homestead hollow (Fig. 2), a quasi-circular depression interpreted to be a highly degraded impact crater that is 27 m in diameter1-3. The original pristine crater formed in a pre-existing impact-generated regolith averaging ~3 m thick and the surrounding ejecta deposit, consisting of some 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 sequestered1-3. 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 crater1-3. 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 remobilization1-3.
Fig. 2. Mosaics covering approximately 290 degrees around the north, east and south side of lander in Homestead hollow (top) and approximately 70 degrees around the west side of the lander (bottom) showing the nature of the surface around the InSight lander. Mosaics are from the IDC camera on the lander: D_LRGB_0014_RAS030100CYL_R__SCIPANQM1 (a) and IDC Mosaic D_LRGB_0119_RAD030100CYL_R__AUTOGENM3 (b).
Rock shapes and heights around the InSight lander (Fig. 3) support the degradation history summarized above and were examined to refine the evolution of Homestead hollow4. In summary, results document decreasing average exposed rock height and increasing percentage of rocks where height comprises the short axis from outside to within the hollow and support prior models of ejecta deflation accompanied by hollow infilling. Our estimated 0.3 m of deflation4 at the current rim that is realistic compared to rock relief, original ejecta thickness, and predicted aeolian contributions to infilling1-3. In addition, shapes of embayed basalt rocks outside the hollow appear platy, bladed, and elongate in a triangular form factor plot, and more discoidal and bladed in an axes ratio plot. By contrast, expected shapes based on terrestrial studies of basalt rocks are mostly compact, compact platy, compact bladed, compact elongated, platy, bladed, and elongate in triangular form factor plots, and equant with lesser, but significant disc- and blade-shaped rocks in axes ratio plots. Our work4 found that addition of 10 cm to the heights of rocks near the hollow rim, to account for continued partial embedding in ejecta, yields the best match between observed and expected rock shapes. Exposure of small ejecta rocks in the hollow supports degradation rates of 10-4 m/Myr during most of hollow history. Results indicate that deflation from ejecta accompanied by downwind deposition in the hollow can account for the current degraded form of the crater4.
Fig. 3. Image of the IDC orthomosaic coverage in and around Homestead hollow showing the rocks within ~10 m of the lander that were used to help evaluate the degradation of the hollow. Rocks within the interior (green), within ~1 meter of the margin to the west of the north side of the hollow (yellow), and outside of the hollow within a few meters of rim to the west and north of the lander (red). Insets show the elevation data of highlighted rock fragments.
The approach summarized here is a new tool for characterizing small crater degradation on regolith-covered lava plains on Mars. For example, the degradation sequence at Homestead hollow is like that established at the Spirit rover landing site in Gusev crater5 and points to the importance of eolian, and lesser impact processes, and mass-wasting, in degrading broad volcanic surfaces on Mars over the past ~1 Ga.
1). Golombek, M. P., N. H. Warner, J. A. Grant, E. Hauber, V. Ansan, C. M. Weitz, N. Williams, C. Charalambous, S. A. Wilson, A. DeMott, M. Kopp, H. Lethcoe-Wilson, L. Berger, R. Hausmann, E. Marteau, C. Vrettos, A. Trussell, W. Folkner, S. Le Maistre, N. Mueller, M. Grott, T. Spohn, S. Piqueux, E. Millour, F. Forget, I. Daubar, N. Murdoch, P. Lognonné, C. Perrin, S. Rodriguez, W. T. Pike, T. Parker, J. Maki, H. Abarca, R. Deen, J. Hall, P. Andres, N. Ruoff, F. Calef, S. Smrekar, M. M. Baker, M. Banks, A. Spiga, D. Banfield, J. Garvin, C. E. Newman, and W. B. Banerdt (2020), Geology of the InSight landing site on Mars: Nature Communications. https://doi.org/10.1038/s41467-020-14679-1.
2). Grant, J. A., N. H. Warner, C. M. Weitz, M. P. Golombek, S. A. Wilson, M. Baker, E. Hauber, V. Ansan, C. Charalambous, N. Williams, F. Calef, W. T. Pike, A. DeMott, M. Kopp, H. Lethcoe-Wilson, and M. E. Banks (2020), Degradation of Homestead hollow at the InSight landing site based on the distribution and properties of local deposits: J. Geophys. Res., 125. https://doi.org/10.1029/2019JE006350.
3). Warner, N. H., J. A. Grant, S. A. Wilson, M. P. Golombek, A. DeMott, C. Charalambous, E. Hauber, V. Ansan, C. Weitz, W. T. Pike, N. Williams, M. A. Banks, M. Baker, M. Kopp, M. Deahn, and H. Lethcoe-Wilson (2020), An impact crater origin for the Insight landing site at Homestead Hollow: Implications for near surface stratigraphy, surface processes, and erosion rates: J. Geophys. Res., 125. https:/doi.org/10.1029/2019JE006333.
4). Grant, J. A., S. A. Wilson, M. P. Golombek, A. Trussell, N. H. Warner, N. Williams, C. M. Weitz, H. Abarca, and R. Deen (2022). Degradation at the Insight landing site, Homestead hollow, Mars: Constraints from rock heights and shapes. Earth and Space Sciences. https://doi.org/10.1029/2021EA001953.
5). Weitz, C. M., J. A. Grant, M. P. Golombek, N. H. Warner, E. Hauber, V. Anssan, S. A., Wilson, C. Constantinos, M. Williams, F. Calef, W. T. Pike, H. Lethcoe-Wilson, J. Maki, A. DeMott, and M. Kopp (2020), Comparison of InSight Homestead hollow to hollows at the Spirit landing site: J. Geophys. Res., 125. https:/doi.org/10.1029/2020JE006435.