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. Landerbased 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 HP 3 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. Plain Language SummaryThe InSight lander has geophysical instruments that are designed to determine the interior structure of Mars. Understanding the results from these instruments requires a geological analysis of materials beneath the landing site at Elysium Planitia. This study presents data that describe the vertical sequence of rocks and soils beneath the lander, as well as the geologic history. The results indicate that InSight rests on a 1.7-billion-year-old lava plain that is covered in a 10-30 m thick regolith that was produced by impact cratering and modified by wind. The uppermost portion of the regolith is a ∼3 m thick horizon of sand. InSight rests on sand within a degraded impact crater. The sandy material contains a slightly cohesive horizon that is only ∼1-2 cm beneath the lander and is up to 10-20 cm thick. The sandy horizon overlies rock fragments that get progressively larger with depth. Bedrock of basaltic lava exists beneath the regolith down to a depth of ∼170 m. The bedrock is interrupted by weaker materials between depths of ∼30 and 75 m. Beneath ∼170 m, the sequence is dominated by ancient (3.7-4.1 billion years old), possibly sedimentary materials.

A Minimally Cemented Shallow Crust Beneath InSight

“…At least two low velocity zones exist from 0 to 157 m and 175–300 m, where V s decreases to ∼0.4 km/s and V p decreases to ∼0.8–0.9 km/s Hobiger et al. (2021) interpreted the higher and lower velocity layers as fractured basalts and sediment, respectively (Figure 1d), consistent with geological mapping (Warner et al., 2022). Our interpretations of these seismic velocities are that sediments within the upper 300 m of the Martian crust are gas‐filled; mineral or ice cements likely do not exist at grain contacts and there is no evidence for any ice‐saturated cryosphere.…”

“…The Phoenix lander excavated ice in the upper few cm (Morgan et al., 2021). Eolian processes and impact brecciation created a 10–30 m thick regolith (including a sand horizon in the upper 3 m) at the InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport mission) landing site (Golombek et al., 2020; Warner et al., 2022). There, the rover had difficulties penetrating its heat flow probe into the subsurface owing to insufficient friction (Spohn et al., 2022).…”

Empirical H/V spectral ratios at the InSight landing site and implications for the martian subsurface structure

“…Crater statistics suggest significant erosion and landscape degradation between an Early Hesperian volcanic surface (3.6 Ga) and the early Amazonian effusive volcanism (1.7 Ga), which is estimated to be ~140 m thick (Warner et al, 2022). Furthermore, throughout the dichotomy boundary between the southern Noachian highlands and the northern plains, there are Noachian through Hesperian transition units that indicate active erosion and deposition of sedimentary materials (Tanaka et al, 2014;Pan et al, 2017Pan et al, , 2020.…”

“…The InSight landing site is located at 4.5024N/135.6234E inside a degraded impact crater, the so-called Homestead hollow, in Elysium Planitia (Golombek et al, 2017(Golombek et al, , 2020c. The surficial geology of this site was studied before the mission (Warner et al, 2017), using mainly orbital imagery and analysis of rocky ejecta craters, and also assessed after landing (Golombek et al, 2020b;Warner et al, 2022). According to the pre-landing observations, a model proposed for the shallow subsurface structure at the InSight landing site consists of a shallow fine regolith layer, a second layer of coarse ejecta, a deeper layer of fractured basalt followed by a layer of more pristine basalt and, finally, a deep layer of possible weakly-bonded sediments located at ~200 m depth below the lander (Knapmeyer-Endrun et al, 2017;Pan et al, 2020).…”

Modeling Seismic Recordings of High‐Frequency Guided Infrasound on Mars

“…Our computation is similar to Tanimoto and Wang (2019) where one does not assume the propagation velocity of pressure perturbation much slower than the shear‐wave velocity of the subsurface medium. In this study, we use a three‐layer velocity model simplified from the shallow (<100 m) geological structure under InSight (e.g., Warner et al., 2022). The first layer is made of thin soft regolith, as suggested by the analyses of the Martian atmospheric pressure drops (Kenda et al., 2020; Onodera, 2022) and of the hammerings of InSight's Heat Flow and Physical Properties (HP 3 ) instrument (Lognonné et al., 2020).…”