Thermodynamic modeling of calcium or magnesium chloride, chlorate, and perchlorate ternary mixtures deliquescence at Mars-relevant temperatures

by Slank, Rachel A.; Chevrier, Vincent F.

The definition of habitability on Mars is intimately linked to the stability of liquid water on the surface or near sub-surface. Brines provide the best pathway to stabilize liquid water, and form through deliquescence where a solid salt crystal transitions into an aqueous solution when exposed to a humid atm. In a typical brine controlled by temperature and water relative humidity, ternary mixtures represent the best potential liquid brines. Here we modeled the deliquescence relative humidity (DRH) and the eutonic relative humidity (RH) of ternary salt mixtures Chloride, chlorate, and perchlorate were modeled with either calcium or magnesium as the cation at temperatures ranging from 223 to 273 K. For the calcium ternary mixtures, the main salt composition precipitating at the DRH was dominated by calcium chloride, and by magnesium perchlorate in the magnesium ternary system. The hydration state of the precipitating salts systematically increased as temperature decreased. The eutonic RH for the calcium mixtures ranged from 14.24% at 273 K and increased to 43.54% by the coldest temperature of 223 K. The eutonic RH for the magnesium mixtures was significantly higher than the calcium counterpart, at 49.76% at 273 K and increased to 53.09% by 223 K. Calcium perchlorate was the predominate eutonic precipitate for the calcium mixtures, and magnesium chlorate for the magnesium mixtures, This study shows that ternary mixtures bring a slight improvement to the stability of brines on Mars compared to single salts or binary mixtures

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