Thermodynamic modelling of perchlorate/chloride and perchlorate/chlorate deliquescence at Mars-relevant temperatures

Abstract

Perchlorate (ClO4−) salts were discovered on Mars and are known to absorb water vapor from the atmosphere and deliquesce into the aqueous phase. Other species such as chlorides (Cl−) and chlorates (ClO3−) were also identified; these species can affect the deliquescence of perchlorates. Here we generate phase diagrams of perchlorate/chloride and perchlorate/chlorate binary mixtures for K, Na, Mg and Ca in the temperature range 223–273 K. Using a new approach based on thermodynamic modelling of evaporation, we determined the deliquescence relative humidity (the minimum relative humidity at which a salt converts into a liquid by absorbing atmospheric water vapor) and the eutonic relative humidity (the minimum relative humidity at which two salts are in equilibrium with liquid) for binary salt mixtures. Our modelling results show that the deliquescence relative humidity values of all salt mixtures is always lower than that of each individual end-member salt at a fixed temperature, typically a few percent lower. The closer the eutonic is to one of the end-member, the smaller the decrease in relative humidity compared to the pure pole. Thus, only eutonics which are far from both poles exhibit a significant drop in relative humidity. Moreover, the eutonic relative humidity always increases with decreasing temperature, which does not favor liquids in the dry and cold Martian environment. Finally, the increased stability of water ice at the lowest temperatures always reduces or even eliminates the stability of liquids. Therefore, the favorable temperature and relative humidity conditions under which binary salt liquid mixtures exist are generally not significantly improved compared to single salts.