A minimally cemented shallow crust beneath InSight

Abstract

Ice and other mineral cements in Mars' shallow subsurface affect the mechanical properties of the shallow crust, the geologic processes that shape the planet's surface, and the search for past or extant Martian life. Cements increase seismic velocities. We use rock physics models to infer cement properties from seismic velocities. Model results confirm that the upper 300 m of Mars beneath InSight is most likely composed of sediments and fractured basalts. Grains within sediment layers are unlikely to be cemented by ice or other mineral cements. Hence, any existing cements are nodular or formed away from grain contacts. Fractures within the basalt layers could be filled with gas, 2% mineral cement and 98% gas, and no more than 20% ice. Thus, no ice- or liquid water-saturated layers likely exist within the upper 300 m beneath InSight. Any past cement at grain contacts has likely been broken by impacts or marsquakes.

Lithology, Pore‐Filling Media, and Pore Closure Depth Beneath InSight on Mars Inferred From Shear Wave Velocities

“…Although we do not provide constraints on the Moho depth, we have focused on the second seismic discontinuity as both the thinner and thicker crustal density models support a seismic discontinuity at 20 ± 5 km below the surface. Our study builds upon previous studies (Heap, 2019; Li et al., 2022; Manga & Wright, 2021; Wright et al., 2022) by considering models for both fractured and granular media (Heap, 2019; Manga & Wright, 2021), using more recently constrained InSight‐derived velocities (Manga & Wright, 2021), and or interpreting seismic velocities constrained for a wider range of depths (0–20 km vs. the upper 300 m or 8 ± 2 km) (Li et al., 2022; Wright et al., 2022). Here, we infer that (a) the upper crust beneath InSight comprises layers of fractured gas‐filled basalts and weakly cemented sediments, (b) the deeper crust could be fractured basalts or more felsic igneous rocks that are either unfractured or has up to 23% porosity, (c) the pores of fractured rocks in the deeper crust could host liquid water, gas, or 2% cement and 98% liquid water or gas, and (d) no seismically detected ice‐saturated cryosphere layer exists beneath InSight.…”

“…The coexistence of the gas‐filled basalt and weakly cemented sediment layers would be resolved as one seismic velocity layer in seismic velocity models since differences in the layers' V s would not produce a large impedance contrast. Additional support for the potential coexistence of igneous rock and sediment layers in the upper crust comes from (a) Martian meteorites and images of surface‐exposed stratigraphic columns that evidence basalts, sandstones, and sediments in the upper 1 km of the crust (Carr & Head, 2002; Edwardset al., 2011; McSween, 2015; Golombek et al., 2018; Hobiger et al., 2021; Knapmeyer‐Endrun et al., 2021) and (b) InSight‐derived high‐resolution seismic velocities that are consistent with gas‐filled basalt and sediment layers down to 0.3 km below the surface of the landing site (Hobiger et al., 2021; Wright et al., 2022). Other layers with different lithologies and pore‐filling media, and hence different V s , may exist within the upper crust.…”

“…V s are 1.7–2.1 km/s in the upper crust (i.e., between 0 and ∼6–11 km) and 2–3.4 km/s in the deeper crust (i.e., between ∼6–11 and 20 ± 5 km) (Figure 1). Interpretations using self‐consistent fractured‐media rock physics models (Berryman, 1980; Te Wu, 1966) indicate that V s within the upper 6–11 km is lower than expected for a cryosphere (Manga & Wright, 2021; Wright et al., 2022). V s between 6–11 and 20 ± 5 km may be consistent with basalts whose fractures are 1%–5% filled with calcite cement (Manga & Wright, 2021).…”

Lithology, pore-filling media, and pore closure depth beneath InSight on Mars inferred from shear wave velocities

“…V s are 1.7-2.1 km/s in the upper crust (i.e., between 0 km and ∼8-11 km) and 2-3.4 km/s in the deeper crust (i.e., between ∼8-11 km and 20 km) (Figure 1). Interpretations using self-consistent fractured-media rock physics models Berryman, 1980) indicate that V s within the upper 8-11 km is lower than expected for a cryosphere (Manga & Wright, 2021;Wright et al, 2022). V s between 11 km and 20 km may be consistent with basalts whose fractures are 1-5% filled with calcite cement (Manga & Wright, 2021).…”

“…The coexistence of the gas-filled basalt and weakly- cemented sediment layers would be resolved as one seismic velocity layer in seismic velocity models since differences in the layers' V s would not produce a large impedance contrasts. Additional support for the potential coexistence of igneous rock and sediment layers in the upper crust comes from (1) Martian meteorites and images of surface-exposed stratigraphic columns that evidence basalts, sandstones, and sediments in the upper 1 km of the crust (Carr & Head, 2002;Edwards et al, 2011;McSween, 2015;Golombek et al, 2018;Hobiger et al, 2021;Knapmeyer-Endrun et al, 2021) and ( 2) Insight-derived high-resolution seismic velocities that are consistent with gas-filled basalt and sediment layers down to 0.3 km below the surface of the landing site (Hobiger et al, 2021;Wright et al, 2022). Other layers with different lithologies and pore-filling media, and hence different V s , may exist within the upper crust.…”