The preservation of subsurface sulfates with mid-to-high degree of hydration in equatorial regions on Mars

At low temperature (T), sulfates with high degree of hydration (epsomite and ferricopiapite) have the enlarged stability fields toward low relative humidity (RH). They are capable of maintaining high RH in a closed environment and keeping an aqueous film at their grain surfaces. More importantly, their dehydration processes are much slower than H2O ice sublimation. Using the activation energy (Ea) of epsomite dehydration, derived experimentally at Mars relevant atmospheric pressure (P) and partial pressure of H2O (PH2O), we estimated the half-life (t1/2) of epsomite dehydration within a model predicted T range of subsurface sulfate-rich layer at Mars equator. We found that subsurface hydrous sulfates formed during past high obliquity periods (>45°) have a high probability to maintain mid-to-high degrees of hydration even until the present epoch. This prediction based on experiments is consistent with an observation made by the Spirit rover at Gusev on Mars, and with a field observation at a hyperarid site on Tibet Plateau. Our study supports the hypothesis that subsurface hydrated sulfates are the major host for the high water-equivalent-hydrogen (WEH) values observed at two equatorial regions on Mars.