CHEMICAL COMPOSITION OF UNCONSOLIDATED SEDIMENTS ON MARS

Large portions of the Martian surface are covered by deposits of sand and dust. These “soil” materials are commonly believed to comprise a global stratigraphic unit that has been compositionally homogenized by aeolian processes. This would imply that its chemical composition reflects the average of the crust, in the same way that shales do on Earth. Variations on the global homogenization theme allow local or regional differences in soil composition resulting from aeolian fractionations of heavy minerals, admixture of local rock fragments, or addition of salt cements. These hypotheses can be tested by chemical analyses of soils from different sites.

XRF and APXS instruments on landers and rovers of the Viking, Mars Pathfinder, and Mars Exploration Rover missions have provided ~60 sediment analyses from 6 distinct sites sometimes separated by thousands of kilometers. Soils at all but one site appear to contain a common chemical component, but soils in Gusev Crater are distinct. Unlike other sites, Gusev soil compositions show neither fractionation of heavy Fe-Ti oxides nor admixture of local rocks. Ni abundances limit the meteoritic component in Gusev and other soils to only a few percent. Trenched soils at Gusev do contain abundant salts, especially Mg-sulfate, also inferred in Viking duricrusts. Gusev soils refute the hypothesis of global homogenization of sediments. The gross similarities in composition may simply reflect a relatively uniform petrologic (basaltic) character of the Martian surface.