LAUSR

Abstract Oxychlorine salts (chlorates and perchlorates) are globally important components of surface soils on Mars, and could form liquid water in concentrated salt solutions despite prevailing cold and dry conditions. Although perchlorate salts are well-characterized, basic thermodynamic properties of chlorate solutions, such as water activity ( a w ) and even solubility, are poorly known. To address this knowledge-gap, we measured water activities and solubilities in the Na–Ca–Mg–ClO 3 system at 25 °C using the isopiestic method, and fit the data to an aqueous ion-interaction Pitzer model. We find that chlorate solutions have extremely low water activities that could allow liquid water to form on the surface of Mars. Compared to perchlorates, chlorates generally have higher water activities at the same concentration; however, saturated Mg(ClO 3 ) 2 solutions, in particular, are extremely concentrated (7.59 mol kg −1 ) and have a w = 0.2 at 25 °C, substantially below saturated Mg(ClO 4 ) 2 solutions ( a w = 0.4 ) . If Mg(ClO 3 ) 2 salts are present on Mars' surface, then our results suggest a much greater potential for liquid water formation in soils due to freezing point depression or deliquescence than with perchlorates.