Soils, Salts, and Water: Geochemical and Astrobiological Investigations of Briny Martian Regolith

Abstract

Despite its present cold and dry surface, Mars may once have been able to support life—and perhaps still could. Liquid water is essential for life as we know it, and there is ample physical and chemical evidence proving that water flowed in Mars’ distant past. Today, what H2O remains exists primarily as ice or vapor, but some liquid water may form on the surface through interactions with salts and regolith (i.e., soil). This dissertation explores how salt, soil, and water shape the chemistry, mechanical properties, and astrobiological potential of Mars’ surface.The first part of this thesis investigates modern surface processes through laboratory analog experiments with Martian regolith simulant. Mars’ soil is rich in salts (such as perchlorates) that can form liquid brine, but prior investigations of the formation, stability, and habitability of these brines frequently neglected the soil component. Experimental results presented here demonstrate that regolith has a strong effect on brine properties, increasing the water content and stabilizing perchlorate brine at Mars-relevant conditions. Specifically, we find that regolith can inhibit salt and ice crystallization in brines, which likely also improves the habitability of those brines. These results greatly expand the geographic and temporal extent where potentially habitable brines could exist on Mars. Related experiments test a hypothesized formation mechanism for mysterious seasonal features dubbed Recurring Slope Lineae (RSL). RSL are apparently linked to water because of their appearance during warm seasons, yet simultaneously exhibit characteristics of dry granular flows. This work reconciles these seemingly contradictory observations by testing a semi-dry model of RSL formation, in which seasonal humidity cycles alternatively stabilize and destabilize slopes, periodically triggering mass wasting. Experimental results illustrate a correlation between relative humidity, soil hydration, and slope stability, which supports the hypothesis that RSL are desorption-triggered granular flows. Findings demonstrate that neither liquid water nor high abundances of salt are required to explain RSL, which raises questions about their protected status and presumed astrobiological potential. This thesis also reports novel geochemical measurements of salty Martian regolith made in situ using the Planetary Instrument for X-ray Lithochemistry (PIXL) on NASA’s Perseverance rover. PIXL’s investigations map the elemental chemistry of regolith at a higher spatial resolution than any previous measurement on Mars. We interpret the results to infer the mineralogy and provenance of various regolith components that appear to be contributed from local, regional, and global sources. Abundances of S and Cl, which may form soluble species, deviate from the global trend for Martian soils, which suggests a unique local aqueous history. The results provide compelling new motivation for a future mission to return a sample of Martian regolith—two of which have been cached by Perseverance. As a whole, this dissertation elucidates how water interacts with salty soil, which improves our understanding of the various roles of water on Mars, the potential habitability of the surface, and possible resources and/or hazards for future human explorers.