Sulfates hydrating bulk soil in the Martian low and middle latitudes

The evidence for sulfate-bearing strata, across Late-Noachian to Amazonian eons, suggests a central role for sulfates in acidity and salinity of Martian paleofluids and the planet's habitability. However, details remain unclear owing to shallow sampling and the limited ability of visible/near-infrared spectroscopy to distinguish among some sulfates. Using chemical data from the Mars Odyssey gamma ray spectrometer, including the sulfur map of Mars, we confirm the possibility of hydrous sulfates acting as key hydrates throughout the southern midlatitudinal soil at decimeter depths. An H₂O:S molar ratio between 2.4 and 4.0 for 80% of the midlatitudes is also consistent with hydrous sulfate phases, including the many Fe sulfates hydrated in this range or mixtures of Ca and Mg sulfates. Nevertheless, hydrous Fe sulfates could explain our observations in a simpler manner relative to Ca/Mg mixtures. Furthermore, phyllosilicates, zeolites, amorphous phases, and H₂O(s) do not seem to be strong candidates to explain the H-S variations. Consequently, we speculate that sulfates, as the primary contributor of H₂O in bulk soil, may influence modern aqueous processes including warm-season slope lineae in the southern hemisphere.