Hydrous silicate and salt minerals are stable in a hydrated state on the surface of Mars

Mars Odyssey Neutron spectrometer data provide evidence of heterogeneous near-surface water abundances up to ∼ 10 wt% in near-equatorial regions on Mars. The fact that water ice is unstable in these regions suggests that some of this water may be present in hydrous silicates (smectites or zeolites) or hydrated salts. Zeolites (e.g., clinoptilolite and chabazite) and smectites have been suggested to occur on the martian surface, and Viking, Pathfinder, and MER chemical data support the existence of MgSO₄ in altered surface materials. In addition, a variety of sulfate and chloride minerals have been identified in martian meteorite samples. Mg-sulfates exist on Earth in a variety of hydration states, including 7-hydrate (51 wt% water), 6-hydrate (47 wt% water), and monohydrate (13 wt% water), but occurrences on Mars may be very different and may include intermediate hydrates such as 5-, 4-, 3-, and 2-hydrates (43, 37, 31, and 23 wt% water, respectively). Other hydrated sulfates (e.g., gypsum, CaSO₄\cdot 2H₂O, 21 wt% water) or hydrated sulfate-chloride salts may also be present based on existing chemical data. Experimental data on the thermodynamics of hydration/dehydration reactions for clinoptilolite, chabazite, and smectite show that they would be partially to completely hydrated at martian surface conditions. Their hydration enthalpies are such that they have the potential to hydrate and dehydrate during diurnal temperature cycles at the martian equator. Similar data are not available for the hydrated Mg sulfates, but controlled-atmosphere X-ray diffraction measurements on the 7-, 6-, and 1-hydrate suggest that the 7-hydrate is not stable at the low partial pressures of water found on Mars. The 6-hydrate becomes amorphous under low-pressure conditions but retains ∼ 18 wt% water; it readily rehydrates when exposed to elevated water-vapor pressure. The potential suite of hydrated silicate and salt minerals is large and requires considerable additional research. However, existing data demonstrate that several of these minerals are reasonable hosts for water molecules under present martian surface conditions. One or several of these minerals may contribute to the subsurface water observed by Mars Odyssey. Some of the hydrated Mg sulfates have such high water concentrations that realistic amounts would be required to account for the observed near-equatorial martian water.