Modeling soluble salt assemblages on Mars: past aqueous history and present-day habitability

Abstract:

Soluble salt assemblages formed through aqueous processes are widespread on Mars. These minerals are important for understanding the past aqueous history of Mars and indicate critical habitability parameters such as pH, temperature, water activity, and salinity. Equilibrium models have been used to determine solution chemistry and salt precipitation sequences from aqueous chemical data; however, current models are limited by a lack of experimental data for low-temperature perchlorates, and some model predictions are clearly anomalous. To address the need for accurate equilibrium models, we have developed a comprehensive model for low-temperature perchlorate-rich brines using (1) previously neglected literature data, (2) experimental solubilities determined in low-temperature perchlorate solutions, and (3) solubility and heat capacity results determined using Differential Scanning Calorimetry (DSC). Our resulting model is a significant improvement over existing models, such as FREZCHEM, particularly for perchlorate mixtures. We have applied our model to evaporation and freezing of a nominal Wet Chemistry Laboratory (WCL) solution measured at the Phoenix site. For a freezing WCL solution, our model indicates that ice, KClO₄, hydromagnesite (3MgCO₃·Mg(OH)₂·3H₂O), calcite (CaCO₃), meridianiite (MgSO₄·11H₂O), MgCl₂·12H₂O, NaClO₄·2H₂O, and Mg(ClO₄)₂·6H₂O form at the eutectic (209 K); whereas, KClO₄, hydromagnesite, kieserite (MgSO₄·H₂O), anhydrite (CaSO₄), halite (NaCl), NaClO₄·H₂O, and Mg(ClO₄)₂·6H₂O form upon complete evaporation at 298 K. In general, evaporation yields more dehydrated mineral assemblages than salts produced by freezing. Hydrated phases that form during evaporation contain 0.3 wt. % water, which compares with 1.2 wt. % during freezing. Given independent evidence for the presence of calcite and minimum water contents in Martian soils of ~1.5 wt. %, salts at the Phoenix site, and possibly elsewhere, appear more likely to have formed during freezing. Furthermore, our model indicates that some minerals, such as halite, kieserite, epsomite, and mirabilite (Na₂SO₄·10H₂O) form only under limited temperature and water activity conditions, and could be used to constrain the past aqueous history of Mars.