Stability of phases in the Mg(ClO4)2·nH2O system and implications for perchlorate occurrences on Mars

Abstract

[1] Salt hydrates have been identified on the Martian surface and in some instances can affect or be affected by the H2O flux between the atmosphere and regolith. Recently, perchlorates were identified in the northern plains of Mars by the Phoenix lander, in association with Ca2+, Mg2+, and Na+ cations. Solid perchlorates are rare on Earth, and the stability of perchlorate phases at low temperatures and relative humidities is poorly constrained. We have examined the stability of hydrated phases in the magnesium perchlorate system, Mg(ClO4)2·nH2O, using thermogravimetric and controlled temperature and humidity X-ray diffraction experiments from 253 to 523 K. Our data revealed the existence of three distinct hydrate phases and an anhydrous phase, and the data suggest that the stable form on Mars would be Mg(ClO4)2·6H2O. Rehydration experiments on the anhydrous form at 263 K and 0.5% relative humidity (RH) show complete hydration to the hexahydrate after 12 h. Given the hygroscopic nature of this material and its resistance to dehydration, we conclude that Mg(ClO4)2·6H2O will not undergo dehydration/rehydration-related phase changes on diurnal or seasonal cycles on Mars. Recent evidence suggests a more widespread distribution of perchlorate in the Martian regolith than previously thought, and Mg(ClO4)2·6H2O could thus represent another potential reservoir for H2O.