Quantifying the volume and distribution of Mars' subsurface lithologies, mineral cements, and liquid water are critical to unraveling the planet's geologic evolution (Carr & Head, 2003Di Achille & Hynek, 2010;Scheller et al., 2021). Mars' crust comprises igneous and sedimentary rocks that are lithified and fractured to varying degrees (Golombek et al., 2018;Pan et al., 2020;Tanaka et al., 2014). Two open questions are (a) what is the depth where pores close entirely within the Martian crust and (b) what percentage of existing pores in the Martian crust host liquid water or ice, or water incorporated into mineral cements.
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).
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.
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