Effect of evaporation and freezing on the salt paragenesis and habitability of brines at the Phoenix landing site

•Common minerals precipitated by freezing or evaporation regardless the initial concentration.•Precipitation of meridianiite indicate freezing pathway.•Precipitation of kieserite and epsomite indicate evaporation.•The present hydration states reflect the ongoing thermal processing at the landing site.•Ca-perchlorate did not precipitate due to formation of KClO4 and gypsum.

The WCL (Wet Chemistry Lab) instrument on board the Phoenix Lander identified the soluble ionic composition of the soil at the landing site. However, few studies have been conducted to understand the parent salts of these soluble ions. Here we studied the possible salt assemblages at the Phoenix landing site using two different thermodynamic models: FREZCHEM and Geochemist's Workbench (GWB). Two precipitation pathways were used: evaporation (T>0 °C using both FREZCHEM and GWB) and freezing (T<0 °C using only FREZCHEM). Through applying three different models of initial ionic concentrations (from sulfate to chlorate/perchlorate dominated), we calculated the resulting precipitated minerals. The results-through both freezing and evaporation-showed some common minerals that precipitated regardless of the ionic initial concentration. These ubiquitous minerals are magnesium chlorate hexahydrate Mg(ClO3)2⋅6H2O, potassium perchlorate (KClO4) and gypsum (CaSO4⋅2H2O). Other minerals evidence specific precipitation pathway. Precipitation of highly hydrated salts such as meridianiite (MgSO4⋅11H2O) and MgCl2⋅12H2O indicate freezing pathway, while precipitation of the low hydrated salts (anhydrite, kieserite and epsomite) indicate evaporation. The present hydration states of the precipitated hydrated minerals probably reflect the ongoing thermal processing and recent seasonally varying humidity conditions at the landing site, but these hydration states might not reflect the original depositional conditions. The simulations also showed the absence of Ca-perchlorate in all models, mainly because of the formation of two main salts: KClO4 and gypsum which are major sinks for ClO−4 and Ca2+ respectively. Finally, in consideration to the Martian life, it might survive at the very low temperatures and low water activities of the liquids formed. However, besides the big and widely recognized challenges to life posed by those extreme environmental parameters (especially low water activity), another main challenge for any form of life in such an environment is to maintain contact with the small droplets of the stable liquids in the regolith and to interact with life in other isolated droplets.