Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p52b..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P52B-02
Mathematics
Logic
[5415] Planetary Sciences: Solid Surface Planets / Erosion And Weathering, [5460] Planetary Sciences: Solid Surface Planets / Physical Properties Of Materials, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
High-resolution images of surface soils in Gusev Crater from the Spirit Microscopic Imager allow four textural classes to be distinguished. Each soil class has a distinctive distribution of grain sizes and shapes, based on statistically significant measurements down to the effective resolution limit (100 μm) (McGlynn et al., JGR in review). All measurable grains are poorly sorted and have variable shapes and roundness. The measurable grains (sand and pebbles) have been deposited along with varying proportions of dust, which is too fine-grained to be texturally characterized. Textures indicate that surface soils cannot represent the uppermost layer of a Martian weathering profile, as has sometimes been proposed. Rather, they constitute a highly modified, transitory but widespread eolian layer that retains no textural record of its original comminution process. In contrast, subsurface soil exposed in a trench has a size probability distribution that matches that of physically crushed rocks - a textural record that points to its impact formation. The APXS-measured chemical compositions of Martian soils at different sites are remarkably uniform, supporting the view that soils record the bulk composition of the exposed Martian crust. Estimates of the mineralogic compositions of these soils can be derived from analyzed elemental abundances and Mössbauer and Mini-TES spectra (McSween et al., JGR in press). All soils are composed of ~3/4 (by weight) primary minerals (olivine, pyroxenes, plagioclase, spinel) derived from comminution of basaltic lavas, and ~1/4 secondary minerals formed by chemical weathering. Alteration phases include clays, amorphous silica, hematite, nanophase oxide, and some combination of sulfate and chloride or schwertmannite and akagneite. The proportions of sulfur- and chlorine-bearing phases and nanophase oxide correlate roughly with the approximated proportion of dust, but not with mean grain size of the larger particles in the various soil classes. Our method of estimating mineralogy cannot resolve whether other alteration phases are also concentrated in the fine fraction. We conclude that the igneous and alteration components of soils were derived from rocks having different histories (and probably ages). The mineralogy of soils thus constrains the nature of the protoliths, and possibly their relative proportions in the exposed Martian crust. However, the textures of soils on the windswept Martian surface have undergone pervasive eolian modification, thus limiting their usefulness in constraining soil formation mechanisms. More complete understanding of sediment formation and the alteration environment will require textural analyses of soils from the subsurface using higher resolution images, and more rigorous mineral determinations.
Fedo C.
McGlynn I.
McSween Harry Y.
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