Water uptake and hygroscopicity of perchlorates and implications for the existence of liquid water in some hyperarid environments

Abstract

The existence of liquid water is a prerequisite for habitability. Deliquescence of perchlorates under subsaturated conditions has been proposed to explain the occurrence of liquid water in some hyperarid environments on the earth and on the Mars. However, the amount of liquid water associated with perchlorates under different conditions is not well understood yet. In this work, we have measured deliquescence relative humidity (DRH) of three perchlorates at different temperatures from 278 to 303 K. DRH decreases from (42.8 ± 0.6)% at 278 K to (40.5 ± 0.5)% at 303 K for Mg(ClO₄)₂·6H₂O, and from (18.5 ± 0.5)% at 278 K to (15.5 ± 0.5)% at 303 K for Ca(ClO₄)₂·4H₂O; in contrast, deliquescence of KClO₄ did not occur even when RH (relative humidity) was increased to 95%. In addition, we have determined the amount of water taken up by Ca(ClO₄)₂·4H₂O and Mg(ClO₄)₂·6H₂O as a function of RH (0–90%) and temperatures (278–298 K). It is found that when both salts are deliquesced, more water (∼10% on average) is associated with Mg(ClO₄)₂·6H₂O than Ca(ClO₄)₂·4H₂O on the per mole ClO₄⁻ base. Overall, this work would significantly improve our knowledge in hygroscopicity of perchlorates, and thus may provide fundamental insights into the hydrologic cycles in some hyperarid regions on the earth and on the Mars.