Quantitative in situ determination of hydration of bright high-sulfate Martian soils

The total water content of soils and rocks encountered by the Spirit rover has been determined by a new analysis method applied to the existing data from the Alpha Particle X-Ray Spectrometer (APXS). This approach employs Monte Carlo simulation of the intensities of the photon scatter peaks in the APXS spectra, together with extraction of these intensities from the spectra. For any individual sample, the water detection limits (similar to 6 wt %) and error bars are high due to low counting statistics in the spectra, but combining the data from a well-defined group of similar samples improves the error bars and lowers the limit. Thus typical basaltic surface soils are found to be essentially dry (< 1 wt % water) and basaltic rocks are very close to dry (< 3.5 wt % water). For four bright subsurface soils in Gusev Crater the water content lies in the range 6-18 wt %; these soils contain sulfur at unusually high levels (> 12 wt %, 30 wt % SO3) relative to the soils common at other landing sites. Mass balance mixing calculations of available cations infer the presence of Fe-, Mg-, and Ca- sulfates in these bright soils. Together with constraints from mineralogy, our results imply that highly hydrated ferric sulfates are the most important carrier of the bound water found in these four spots. In conjunction with the complementary available chemical and mineralogical information they reveal additional information about present bound water reservoirs on Mars, their mineralogy and their spatial and lateral distribution along the Spirit rover's traverse.