Mineralogic and compositional properties of Martian soil and dust: Resultsfrom Mars Pathfinder

Mars Pathfinder obtained multispectral, elemental, magnetic, and physical m easurements of soil and dust at the Sagan Memorial Station during the cours e of its 83 sol mission. We describe initial results from these measurement s, concentrating on multispectral and elemental data, and use these data, a long with previous Viking, SNC meteorite, and telescopic results, to help c onstrain the origin and evolution of Martian soil and dust. We find that so ils and dust can be divided into at least eight distinct spectral units, ba sed on parameterization of Imager for Mars Pathfinder (IMP) 400 to 1000 nm multispectral images. The most distinctive spectral parameters for soils an d dust are the reflectivity in the red, the red/blue reflectivity ratio, th e near-IR spectral slope, and the strength of the 800 to 1000 nm absorption feature. Most of the Pathfinder spectra are consistent with the presence o f poorly crystalline or nanophase ferric oxide(s), sometimes mixed with sma ll but varying degrees of well-crystalline ferric and ferrous phases. Darke r soil units appear to be coarser-grained. compacted. and/or mixed with a l arger amount of dark ferrous materials relative to bright soils. Nanophase goethite. akaganeite, schwertmannite, and maghemite are leading candidates for the origin of the absorption centered near 900 nm in IMP spectra. The f errous component in the soil cannot be well-constrained based on IMP data. Alpha proton X-ray spectrometer (APXS) measurements of six soil units show little variability within the landing site and show remarkable overall simi larity to the average Viking-derived soil elemental composition. Difference s exist between Viking and Pathfinder soils, however, including significant ly higher S and Cl abundances and lower Si abundances in Viking soils and t he lack of a correlation between Ti and Fe in Pathfinder soils. No signific ant linear correlations were observed between IMP spectral properties and A PXS elemental chemistry. Attempts at constraining the mineralogy of soils a nd dust using normative calculations involving mixtures of smectites and si licate and oxide minerals did not yield physically acceptable solutions. We attempted to use the Pathfinder results to constrain a number of putative soil and dust formation scenarios, including palagonitization and acid-fog weathering. While the Pathfinder soils cannot be chemically linked to the P athfinder rocks by palagonitization, this study and McSween er al. [1999] s uggest that palagonitic alteration of a Martian basaltic rock, plus mixture with a minor component of locally derived andesitic rock fragments, could be consistent with the observed soil APXS and IMP properties.