A revised Pitzer model for low-temperature soluble salt assemblages at the Phoenix site, Mars

作者：Toner J D^*; Catling D C; Light B

来源：Geochimica et Cosmochimica Acta, 2015, 166: 327-343.

DOI：10.1016/j.gca.2015.06.011

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+, SO42+; 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 (ClO4-), 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 (3MgCO(3)center dot Mg(OH)(2)center dot 3H(2)O), calcite (CaCO3), meridianiite (MgSO4 center dot 11H(2)O), MgCl2 center dot 12H(2)O, NaClO4 center dot 2H(2)O, and Mg(ClO4)(2)center dot 6H(2)O at the eutectic (209 K). The total water held in hydrated salt phases at the eutectic is similar to 1.2 wt.%, which is much greater than hydrated water contents when evaporation is modeled at 298.15 K (similar to 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 center dot H2O) instead of meridianiite. Potentially habitable brines, with water activity aw >0.6, can occur when soil temperatures are above 220 K and when the soil liquid water content is greater than 0.4 wt.% (100 x g(H2O) g(soil)(-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 indicators of past environmental conditions.

• 出版日期2015-10-1