A model for formation of dust, soil, and rock coatings on Mars: Physical and chemical processes on the Martian surface

[1] A model is presented here to explain the generation of surface material on Mars using chemical, magnetic, and spectroscopic data from Mars and geologic analogs from terrestrial sites. One basic premise is that the dust/soil units are not derived exclusively from local rocks, but are rather a product of global, and possibly remote, weathering processes. Another assumption in this model is that there are both physical and chemical interactions of the atmosphere with dust particles and that these two processes create distinctly different products. Physical processes distribute dust particles on rocks and drift units, forming physically aggregated layers; these are reversible processes. Chemical reactions of the dust/soil particles create alteration rinds on rock surfaces and cohesive, crusted surface units between rocks, both of which are relatively permanent materials. According to this model the dominant components of the dust/soil particles are derived from alteration of volcanic ash and tephra and contain primarily nanophase and poorly crystalline ferric oxide/oxyhydroxide phases as well as silicates. These phases are the alteration products that formed in a low-moisture environment. These dust/soil particles also contain a smaller amount of material that was exposed to more water and contains crystalline ferric oxides/oxyhydroxides, sulfates, and clay silicates. These components could have formed through hydrothermal alteration at steam vents or fumeroles, thermal fluids, or through evaporite deposits. Wet/dry cycling experiments are presented here on Mars soil analogue mixtures containing poorly crystalline and crystalline components dominated by nanophase to 2 mm diameter particles. Cemented products of these soil mixtures are formed in these experiments, and variation in the surface texture was observed for samples containing smectites, nonhydrated silicates, or sulfates. Reflectance spectra were measured of the initial particulate mixtures, the cemented products, and ground versions of the cemented material. The spectral contrast in the visible/near-infrared and midinfrared regions is significantly reduced for the cemented material compared to the initial soil and somewhat reduced for the ground, cemented soil compared to the initial soil. The results of this study suggest that transient fluvial activity on Mars will have a profound effect on the texture and spectral properties of the dust/soil particles on the surface. The model developed in this study provides an explanation for the generation of cemented or crusted soil units and rock coatings on Mars and may explain albedo variations on the surface observed near large rocks or crater rims.