Water quality-evaporation modeling of microbial habitability of potential Martian brines

Abstract:

Information about the Martian surface has increased substantially in the last two decades and has facilitated the identification of Martian minerals including various sulfates, carbonates, chlorides, and perchlorates. Martian mineralogy could provide critical insight to constrain the composition of potential aqueous Martian fluids, allowing us to evaluate important characteristics of ancient and potentially habitable Martian brines including; the activity of water (aw); the activity of specific ions (ai); ionic strength (I); and the chaotropicity vs. kosmotropicity effects of brine compositions. We have evaluated water chemistries in equilibrium with a number of possible Martian mineral assemblages to help determine geochemical constraints on microbial life in a hyperarid Martian environment.

Modeling of brines initially in equilibrium with various possible Martian mineral assemblages was performed with Geochemist’s Workbench using Pitzer parameters at 25°C and a fixed PCO2 of 6 mbar, followed by evaporation with concentration factors up to 1000X. Mineral assemblages include various combinations of Ca-Mg-Na-K-Fe sulfates, chlorides, and carbonates.

Waters developed into Mg-SO4, Na-SO4, Mg-Cl, K-Cl, and Na-Cl dominant hypersaline brines. In almost all cases, aw remains above 0.61 (the minimum aw that allows for microbial life), except for waters in equilibrium with jarosite, gypsum, and Mg-Ca carbonates, when aw decreases to 0.34. The final value of I ranges from approximately 6 - 18 mol/L. The negative effects of low aw and high I may be counteracted by the presence of high concentrations of kosmotropic solutes such as NaCl, KCl, or Na-Fe sulfates, while the presence of high concentrations of chaotropic solutes such as Ca-Fe-Mg chlorides, together will have a negative synergistic effect on microbial activity. The calculations suggest that microbial activity is feasible under hyperarid conditions when brines are in equilibrium with some mineral assemblages, but not with others. Continued modeling efforts will investigate the characteristics of water chemistries in equilibrium with other sulfate, chloride, and perchlorate salts at temperatures and environmental conditions relevant to the past and present Martian surface.