¹J.D. Toner, ¹D.C. Catling, ² B. Light
¹University of Washington, Box 351310, Dept. Earth & Space Sciences, Seattle, WA 98195, USA
²Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, USA

The Wet Chemistry Laboratory (WCL) on the Mars Phoenix Lander measured ions in a soil-water extraction and found Na+, K+, H+ (pH), Ca2+, Mg2+, View the MathML sourceSO42-,ClO4-, and Cl–. Equilibrium models offer insights into salt phases that were originally present in the Phoenix soil, which dissolved to form the measured WCL solution; however, there are few experimental datasets for single cation perchlorates (View the MathML sourceClO4-), and none for mixed perchlorates, at low temperatures, which are needed to build models. In this study, we measure ice and salt solubilities in binary and ternary solutions in the Na-Ca-Mg-ClO4 system, and then use this data, along with existing data, to construct a low-temperature Pitzer model for perchlorate brines. We then apply our model to a nominal WCL solution. Previous studies have modeled either freezing of a WCL solution or evaporation at a single temperature. For the first time, we model evaporation at subzero temperatures, which is relevant for dehydration conditions that might occur at the Phoenix site. Our model indicates that a freezing WCL solution will form ice, KClO4, hydromagnesite (3MgCO3·Mg(OH)2·3H2O), calcite (CaCO3), meridianiite (MgSO4·11H2O), MgCl2·12H2O, NaClO4·2H2O, and Mg(ClO4)2·6H2O at the eutectic (209 K). The total water held in hydrated salt phases at the eutectic is ∼1.2 wt. %, which is much greater than hydrated water contents when evaporation is modeled at 298.15 K (∼0.3 wt. %). Evaporation of WCL solutions at lower temperatures (down to 210 K) results in lower water activities and the formation of more dehydrated minerals, e.g. kieserite (MgSO4·H2O) instead of meridianiite. Potentially habitable brines, with awaw>0.6, can occur when soil temperatures are above 220 K and when the soil liquid water content is greater than 0.4 wt. % (View the MathML source100×gH2Ogsoil-1). In general, modeling indicates that mineral assemblages derived from WCL-type solutions are characteristic of the soil temperature, water content, and water activity conditions under which they formed, and are useful as indicators of past environmental conditions.

Reference
Toner JD, Catling DC, Light B (2015) A revised Pitzer model for low-temperature soluble salt assemblages at the Phoenix Site, Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.06.011]

Copyright Elsevier