Stability of Mg‐sulfates at-10°C and the rates of dehydration/rehydration processes under conditions relevant to Mars

摘要

We report the results of low temperature (-10°C) experiments on the stability fields and phase transition pathways of five hydrous Mg‐sulfates. A low temperature form of MgSO_4·7H_2O (LT‐7w) was found to have a wide stability field that extends to low relative humidity (～13% RH at -10°C). Using information on the timing of phase transitions,we extracted information on the reaction rates of five important dehydration and rehydration processes. We found that the temperature dependencies of rate constants for dehydration processes differ from those of rehydration, which reflect differences in reaction mechanisms. By extrapolating these rate constants versus T correlations into the T range relevant to Mars, we can evaluate the possibility of occurrence of specific processes and the presence of common Mg‐sulfate species present on Mars in different periods and locations. We anticipate in a moderate obliquity period, starkeyite and LH‐MgSO_4·H_2O should be two common Mg‐sulfates at the surface, another polymorph MH‐MgSO_4·H_2O can exist at the locations where hydrothermal processes may have occurred. In polar regions or within the subsurface of other regions, meridianiite (coexisting with water ice, near 100% RH) and LT‐7w (over a large RH range) are the stable phases. During a high obliquity period, meridianiite and LT‐7w should exhibit widespread occurrence. The correlations of reaction rates versus temperature found in this study imply that dehydration and rehydration of hydrous Mg‐sulfates would always be slower than the sublimation and crystallization of water ice, which would be supported by mission observations from Odyssey and by Mars Exploration Rovers.