Ferric sulfates on Mars: A combined mission data analysis of salty soils at Gusev crater and laboratory experimental investigations

A temporal visible near-infrared (VIS-NIR) spectral variation was observed from Tyrone yellowish salty soils based on seven periodic Pancam 13 filter observations made by the Spirit rover. The major change was the reduction of spectral slope from 434 nm to 753 nm. Based on the results from a set of systematic laboratory experiments on the stability field and phase transition pathway of typical ferric sulfates, we suggest that the strong dehydration processes of ferricopiapite, either through amorphization or chemical alteration, could be the reasons for the spectral changes of Tyrone yellowish salty soils, excavated from a deep trench. The change of soil property suggests that they were originally not in equilibrium with the surface atmospheric conditions, that there is a relative humidity (RH) gradient existing in the upper few tens of centimeters depth below the surface. A layer of salt-rich regolith beneath the surface will change the underground temperature profile, especially to keep a low-temperature zone with a small temperature oscillation (than diurnal cycle at surface) in a salt-enriched regolith layer. This temperature profile will provide a relatively high RH and small RH variation and thus will facilitate the preservation of hydrous sulfates with high degree of hydration during the moderate obliquity period on Mars. Additionally, the sulfates with high degrees of hydration are excellent RH buffers in a local environment. The subsurface hydrous sulfates can be the sources for high level of water-equivalent hydrogen found at two large equatorial regions on Mars by Neutron Spectrometer on Mars Odyssey Orbiter.