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Searching for Life in Salt: An Investigation of the Stability and Microbial Habitability of Possible Martian Brines, and Experimental Determination of the Chaotropicity and Kosmotropicity of Inorganic Solutes
AuthorSmith, Sara Marie
AdvisorPoulson, Simon R
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Liquid water is typically unstable at the surface of Mars due to the combination of low temperatures and pressures. However, present-day surface conditions are close to the triple point of water and the presence of dissolved salts on the Martian surface may allow metastable brines to persist for periods of time. Polyhydrated sulfates, chlorides, perchlorates, and carbonates have been detected by various Mars missions and the dissolution of these salts to form high salinity brines could substantially depress the eutectic temperature of aqueous solutions and allow liquid water to become stable in modern Martian environmental conditions. The presence of water is considered the most important prerequisite for life, so understanding how brines form and evolve on Mars could help us understand the microbial habitability of Mars in the past as well as the present. This study has performed geochemical modeling to investigate the evaporation and/or freezing trajectories of water chemistries in initial equilibrium with potential Martian mineral assemblages and atmospheric chemistries, as well as the water chemistry at a terrestrial Mars-analog site in Badwater Basin, Death Valley to explore: 1) the stability and habitability of evolved brine mixtures as a possible control upon microbial activity vs. extent of evaporation at Death Valley; 2) how modern and ancient Martian environmental conditions affect brine evolution; and 3) the stability and possible habitability of evolved brine compositions, in order to determine if microbial life could potentially exist in the past or at present on Mars. Results of this study indicate that concentrated chloride brines on Mars could be adequate microbial habitats for xerophiles that would also have to be highly psychrophilic, and be able to function at lower temperatures than has been observed in terrestrial environments. Contrary to chaotropicity at ambient temperatures, a chaotropic environment (such as high MgCl2 concentrations) may help maintain macromolecular flexibility at low temperatures to facilitate life in cold environments.In addition, an experimental study was performed to quantify the chaotropicity and kosmotropicity of additional inorganic solutes and briny solutions relevant to the Mars surface. Chaotropicity and kosmotropicity are additional stress parameters that differ from absolute parameters such as the activity of water (or water available for biologic reactions) because they are defined by the effects that they induce on macromolecular systems rather than the effects of ions in solution. Therefore, evaluation of chao-/kosmo-tropic activity (CKA) of pure salts and potential mixtures could help determine the window for potential life on the Martian surface. Measured values indicate that the chao-/kosmo-tropic behavior of inorganic salts are generally conservative, or additive/subtractive, except for mixtures with elevated concentrations of MgCl2, and results will facilitate the prediction of CKA values for a range of inorganic natural brine compositions, with implications for the feasibility of microbial life in hyper-arid environments on Earth and on Mars.