REMNANTS OF CLASTIC DEPOSITS BENEATH THE INSIGHT LANDER ON MARS?

The InSight spacecraft landed in western Elysium Planitia at 4.502°N, 135.623°E on an Early Amazonian basaltic lava plain [1,2]. Seismic investigations revealed a low-density unit of possible clastic origin ~30 m below the lander, within the Early Amazonian to Hesperian basaltic sequence [3,4]. The estimated thickness of this unit is 15 m to 50 m. Geological mapping has yet to reveal evidence for clastic materials in the landing region. Here, we investigate the possibility of a more extensive clastic unit across Elysium Planitia and whether it could reside, stratigraphically, within the lava units. Our mapping reveals several candidate outcrops south of InSight’s landing ellipse, all of which pre-date the Early Amazonian lavas. The clastic deposits have a diversity of morphologies, but share similar albedo, crater retention, layering characteristics, and erosional expressions to the nearby Medusa Fossae Formation (MFF) and Aeolis Mons in Gale Crater, suggesting that these deposits are related and may extend beneath InSight. We also re-visit the morphology of 100-m-scale impact craters near InSight to evaluate whether the target materials contain more friable, easily-weathered materials. Preliminary results indicate that larger, 400 m to 1-km-size fresh craters here have volumetrically larger excavated cavities than what can be accounted for by the volume of material in the continuous ejecta. This could indicate net volume losses from the ejecta via mobilization of fines. Similar losses are not indicated for fresh craters below ~400 m in size. These preliminary results might suggest a target strength transition to a weaker, more-easily fragmented unit at depths consistent with the seismic observations.

References: [1] Golombek, M.P. et al., (2020), Nat. Comm., 11. [2] Warner, N.H., et al., (2022), JGR-Planets, 127. [3] Hobiger, M., et al., (2021), Nat. Comm., 12. [4] Carrasco, S. et al., (2023), Geo. J. Intern., 232.