Mineralogies of Martian Soils as Indicators of Alteration

Details Mineralogies of Martian Soils as Indicators of Alteration Mineralogies of Martian Soils as Indicators of Alteration

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p12a..01m&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P12A-01

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[1060] Geochemistry / Planetary Geochemistry, [5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6225] Planetary Sciences: Solar System Objects / Mars

Scientific paper

Unlike other planetary bodies with regoliths formed by physical processes, Mars has soils potentially affected by chemical alteration processes. Although bulk chemical compositions of soils have been measured at multiple sites, their mineralogies are poorly determined, if at all. This is unfortunate because minerals provide the best means of understanding the mechanisms, conditions, and extents of chemical alteration. Using MER data, we have estimated the complete mineral assemblages of average soils at Gusev and Meridiani. We first estimated the abundances of sulfate and chloride salts from APXS analyses of S and Cl, and of silica from Mini-TES spectral deconvolution, and removed these components from the bulk chemical compositions of soils. We then determined the identities and proportions of igneous and alteration oxide minerals using a combination of Mössbauer data for iron-bearing phases and norms calculated from the adjusted APXS soil chemistry. Our results indicate that soils at Gusev and Meridiani have very similar mineralogy: ~26 wt.% alteration phases (iron oxides, salts, and silica) with the remainder comprised of the minerals of olivine-normative basalt. Our results are (almost) independent of, and agree well with, Mini-TES estimates of soil mineralogy, the only real differences being the absence of ~4 wt.% iron oxides (which Mini-TES cannot detect) and the presence of ~8 wt.% clays (which cannot be estimated by our methodology) in the latter. The similarity in the mineralogies of soils on opposite sides of the planet is somewhat surprising, given the likelihood that some local rocks must contribute minerals to the soils. That certainly happened at Meridiani, but we have excluded soils containing high concentrations of hematite concretions from our data set. Igneous minerals comprise three-quarters of these soils by weight. The intimate association of alteration phases with minerals like olivine and phosphate that are easily weathered in the Martian acidic environment indicates that soils must be physical mixtures of unrelated materials. Profoundly altered, perhaps ancient rocks have supplied one component characterized by clays, silica, salts, and nanophase iron oxides; these materials experienced hydrous alteration. They were then mixed with physically comminuted basalts, likely of younger age and degraded under anhydrous conditions.

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