A Minimally Cemented Shallow Crust Beneath InSight

Abstract

Cements in the Martian crust can have multiple origins, including ice frozen from liquid water or condensed from vapor, hydrated minerals formed in situ, or minerals precipitated from aqueous fluids (e.g., salts, carbonates, and sulfates). The presence, amount, and composition of ice and other mineral cements in the shallowest sections of the Martian crust have implications for robotic and human exploration of Mars, the processes that shape and shaped the surface, and the search for past or extant life. Research on these topics is central to determining if Mars ever supported life, to understand the climate history and processes, to understand Mars as a geological system, and to prepare for human exploration.Cementation affects and records geological processes. Cement can strengthen sediments (herein defined to include regolith and all other granular media layers) by creating stiffer contacts between particles. Cementation affects the permeability and porosity of sediments and fractured rocks, which impacts gas transport driven by atmospheric pressure changes (Morgan et al., 2021). Pores and fractures filled with ice or other mineral cement could confine any deeper liquid water, creating aquifers (Carr, 1979). Ground ice can promote weak explosive eruptions at rootless cones on lava flows (Brož et al., 2021) and may promote phreatomagmatic eruptions (Moitra et al., 2021). Cemented sediments are less prone to eolian and fluvial transport and erosion. The distribution of cements in the Martian sediments may record the accumulation and transport of volatiles in geologically recent times (Dundas et al., 2021). Cements may also preserve organic compounds diagnostic of past or present biological activity (Rivera-Valentín et al., 2020).Cementation impacts human exploration, and a primary motivation for the Mars Ice Mapper mission concept is to map ice in the shallowest crust (Davis & Haltigin, 2021). The presence of ice and hydrated minerals in shallow sediments and fractured rocks could provide a source of water for in situ resource utilization (Piqueux et al., 2019). Cementation-induced strengthening of sediments affects foundations used for engineering infrastructure (Kalapodis et al., 2020). Cemented sediments can be used as a construction material (Liu et al., 2021) and have prompted studies of a range of Mars simulants in preparation for future human missions (Karl et al., 2021).

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.…”

“…Last, we identify the ranges of ϕ, α, and pore-filling media that best explain measured V s . In selecting our aspect ratio ranges, we followed the examples from previous studies that considered aspect ratios of 0.03-1 (Heap, 2019), 0.03-1 (Manga & Wright, 2021;Wright et al, 2022), and 0.01 (Li et al, 2022). While both our study and Heap (2019) use fractured media models, Heap (2019) used the Kuster Toksöz (KT) fractured media model that approximates the elastic moduli through a first-order, long-wavelength scattering theory.…”

“…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).…”

Seasonal Variations of Soil Thermal Conductivity at the InSight Landing Site

“…They found velocities of v P = 119 +45 −21 m s −1 and v S = 63 +11 −7 m s −1 , consistent with values typically encountered in low-density unconsolidated sands. It has also been speculated that any cement at grain contacts within sediment layer at the InSight landing site may have been broken up by impacts or marsquakes (Wright et al, 2022), although this may be more relevant for deeper soil layers not probed by the HP 3 mole. Nagihara et al (2022) studied the dependence of thermal conductivity on atmospheric pressure in the lab using the low-cohesion Mojave Mars simulant (Peters et al, 2008) as an analogue for the martian soil.…”