Solid-solid hydration and dehydration of Mars-relevant chlorine salts: Implications for Gale Crater and RSL locations

Water vapor is likely being exchanged between the regolith and the atmosphere on Mars, according to evidence from multiple landing sites (including Gale Crater), satellite measurements and numerical modeling. The mechanism of this exchange is largely unknown but could involve the formation of water frost, the adsorption of thin films of water onto mineral surfaces, or the deliquescence or hydration of salts. Hydration, a solid-solid phase transition during which water molecules are incorporated into a crystal structure in stoichiometric amounts, is possible for many salts found at Gale Crater and elsewhere on Mars. These salts may therefore be acting as a source and sink for water vapor in the Martian regolith. Furthermore, salt hydration state may be used as a marker for the presence of liquid water on present-day Mars. For example, the hydrated, crystalline perchlorate and chloride salts detected at recurring slope lineae (RSL) locations have been proposed to form from liquid brines rather than by experiencing hydration by atmospheric water vapor. Here we use an environmental cell coupled to a Raman microscope to experimentally study the hydration and dehydration of magnesium chloride (MgCl₂), calcium perchlorate (Ca(ClO₄)₂), and calcium chloride (CaCl₂) with the goal of determining which of these salts are capable of experiencing hydration on diurnal time scales. Other potential hydration phase transitions of chlorine-containing salts are thought to be less likely. Specifically, we study the transition between magnesium chloride tetrahydrate and hexahydrate, anhydrous calcium perchlorate and hydrated calcium perchlorate, and calcium chloride dihydrate and hexahydrate. We find that under conditions measured by the REMS instrument at Gale Crater, some chlorine-containing salts can readily hydrate and other salts can readily dehydrate, but no salt system studied here is likely to undergo both processes at the surface on diurnal time scales. With respect to RSL formation, these experiments suggest that atmospheric hydration of these salt systems may be too slow or otherwise not feasible. Hence, hydrated salts formed recently at active RSL locations on Mars may indeed be an indicator of the presence of liquid water.