Phase transitions and hygroscopic growth of Mg(ClO4)2, NaClO4, and NaClO4∙H2O: implications for the stability of aqueous water in hyperarid environments on Mars and on Earth

In general pure liquid water is not thermodynamically stable on Mars due to the extremely cold and dry environment. The presence in the soil of perchlorates, which could lower the freezing point of water and form aqueous solutions by taking up water vapor even under subsaturated conditions, has been proposed to explain the possible existence of liquid water on Mars and in some hyperarid environments on Earth. In this work, the phase transitions and hygroscopic growth of Mg(ClO4)2, NaClO4, and NaClO4·H2O were investigated between 278 and 303 K. In this temperature range, we found that anhydrous Mg(ClO4)2 was completely converted to Mg(ClO4)2·6H2O at a relative humidity (RH) as low as <1%. In contrast, anhydrous NaClO4 was stable at RH below 20%, and NaClO4·H2O was completely transformed to anhydrous NaClO4 at <1% RH; when RH was increased to 30%, anhydrous NaClO4 was transformed to NaClO4·H2O. We also found that the deliquescence relative humidity (DRH) of NaClO4·H2O decreased from ∼51.5% at 278 K to ∼43.5...