Hydrous Perchlorates and Their Relation to Humidity at the Phoenix Landing

Introduction: Phoenix showed the presence of ~1% perchlorate in the landing site regolith [1]. This ion is associated to sodium and magnesium [2], making these compounds ideal candidates for liquid brines on the surface. In addition, hydration-dehydration cycles may control the humidity. We study the stability of perchlorates (liquids and salts) under Martian conditions using a combination of experiments and models, which is then applied to Phoenix observations [3]. The previously determined phase diagrams [3] for NaClO4 and Mg(ClO4)2 show that the eutectic temperature is 236 K for 52 wt% NaClO4 and 206 K for 44 wt% Mg(ClO4)2. Using a combination of thermodynamic (Pitzer) and water evaporation kinetic models [3], we calculated evaporation rates for the Phoenix landing site conditions. Liquid Stability: While temperatures are more favorable for liquid solutions on the warmest day, equilibrium vapor pressure values are also higher, so liquid