Magnesium and calcium sulfate stabilities and the water budget of Mars

• Published in: Journal of Geophysical Research - Planets Vol. 112; no. E11; pp. E11004 - n/a
• Main Authors: Chou, I-Ming, Seal II, Robert R.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Geophysical Union 01-11-2007; Blackwell Publishing Ltd
• Subjects: Abundance; Calcium sulfate; Composition; Dehydration; Earth sciences; Earth, ocean, space; Erosion and weathering; Exact sciences and technology; Hydration; Hydrology and fluvial processes; Magnesium; magnesium sulfate; Magnesium sulfates; Mars; phase equilibria; Planetary Sciences; Relative humidity; Solid Surface Planets; Surface materials and properties; temperature; water; temperature; Mars; relative humidity; water; magnesium sulfate; phase equilibria; atmosphere; experimental studies; Relative humidity; abundance; epsomite; sulfates; kieserite; diurnal variations; Stability limit; dehydration; starkeyite; calcium sulfate; stability; hydrates; magnesium; water balance; concentration; phase relations; hydrologic cycle; hexahydrite; hydration; erosion; ice caps
• ISSN: 0148-0227; 2169-897X; 2156-2202; 2169-8996
• DOI: 10.1029/2007JE002898

Magnesium sulfate probably plays a dominant role in the water cycle of Mars away from the polar ice caps through hydration and dehydration reactions. This prominence is due to its abundance, its occurrence in numerous hydration states, and its ability to hydrate and dehydrate rapidly. New experimental studies on the metastable reaction between hexahydrite (MgSO4·6H2O) and starkeyite (MgSO4·4H2O) as a function of temperature and relative humidity, supplemented by recent investigations of the stable reaction between epsomite (MgSO4·7H2O) and hexahydrite and by phase equilibrium calculations, suggest that the most important magnesium sulfate phases involved in the Martian water cycle are MgSO4·11H2O, epsomite, starkeyite, and possibly kieserite (MgSO4·H2O). Hexahydrite is not predicted to be stable on the surface of Mars. During diurnal variations in temperature and relative humidity, 1 kg of MgSO4 can release or remove from the atmosphere 1.5 kg of H2O by cycling between kieserite and MgSO4·11H2O. Despite subequal abundances of calcium sulfate, calcium sulfates are not likely to be important in the water cycle of the planet because of sluggish rates of hydration and dehydration and a more limited range of H2O concentrations per kilogram of CaSO4 (0.00 to 0.26 kg kg−1). Modern or recent erosion on Mars attributed to liquid water may be due to the dehydration of MgSO4·11H2O because of the inferred abundance and likelihood of occurrence of this phase and its limited stability relative to known variations in temperature and relative humidity.