In Situ and Orbital Stratigraphic Characterization of the InSight Landing Site - A Type Example of a Regolith-Covered Lava Plain on Mars

Abstract

The InSight lander rests on a regolith-covered, Hesperian to Early Amazonian lava plain in Elysium Planitia within a ∼27-m-diameter, degraded impact crater called Homestead hollow. The km to cm-scale stratigraphy beneath the lander is relevant to the mission's geophysical investigations. Geologic mapping and crater statistics indicate that ∼170 m of mostly Hesperian to Early Amazonian basaltic lavas are underlain by Noachian to Early Hesperian (∼3.6 Ga) materials of possible sedimentary origin. Up to ∼140 m of this volcanic resurfacing occurred in the Early Amazonian at 1.7 Ga, accounting for removal of craters ≤700 m in diameter. Seismic data however, suggest a clastic horizon that interrupts the volcanic sequence between depths of ∼30 and ∼75 m. Meter-scale stratigraphy beneath the lander is constrained by local and regional regolith thickness estimates that indicate up to 10–30 m of coarse-grained, brecciated regolith that fines upwards to a ∼3 m thick loosely-consolidated, sand-dominated unit. The maximum depth of Homestead hollow, at ∼3 m, indicates that the crater is entirely embedded in regolith. The hollow is filled by sand-size eolian sediments, with contributions from sand to cobble-size slope debris, and sand to cobble-size ejecta. Lander-based observations indicate that the fill at Homestead hollow contains a cohesive layer down to ∼10–20 cm depth that is visible in lander rocket-excavated pits and the HP3 mole hole. The surface of the landing site is capped by a ∼1 to 2 cm-thick loosely granular, sand-sized layer with a microns-thick surficial dust horizon.