Determining the modal mineralogy of Martian soils

A method for identifying the major and minor minerals and quantifying their proportions (by weight) in soils at the Mars Exploration Rovers landing sites is presented. The procedure utilizes modeled Mössbauer data for iron-bearing minerals, chemical calculations of Alpha Particle X-ray Spectrometer data for iron-absent minerals, and modeling of MiniTES thermal infrared spectra for silica and clays. Two models are formulated, using different assumptions about the mineralogy of sulfur and chlorine: sulfate + chloride, or incorporated in or adsorbed onto schwertmannite + akaganeite. The actual soil mineralogy may be bracketed by these results. Using either model, average dark soils on the Gusev crater plains and on Meridiani Planum are shown to be composed of a mixture of igneous (olivine, pyroxene, plagioclase, Fe-Ti-Cr spinels, phosphate) and alteration (amorphous silica, hematite, nanophase oxides, clays, plus sulfate + chloride or oxysulfate + oxychloride) minerals. This assemblage suggests that soil alteration did not occur in situ and that the igneous and alteration components were likely derived from different sources. The (model dependent) mixing ratio of igneous and alteration components is identical at the two sites on opposite sides of the planet, implying that Martian soils may provide a representative mineralogical sampling of the exposed crust.