Mathematics – Logic
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p72a0487k&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P72A-0487
Mathematics
Logic
5410 Composition, 5464 Remote Sensing, 6225 Mars
Scientific paper
Results from the Thermal Emission Spectrometer (TES) show that Martian dark regions are composed of two distinct components. One is accepted to be basalt. The other is controversial, having been interpreted as andesite/basaltic andesite and as chemically weathered basalt. This second component, referred to here as S2, occurs most prominently in the northern plains, but it is also seen in the southern highlands, generally in higher concentrations at higher latitudes. The apparent latitudinal dependence of S2 suggests that its occurrence may be linked to water or near-surface ground ice. In the andesite hypothesis, the TES S2 spectrum has been linearly deconvolved to plagioclase feldspar, high-silica glass, and pyroxene near the TES detection limit. We accept this mineralogical interpretation of S2, but propose that the high-silica glass component can be interpreted as amorphous silica. We have performed x-ray diffraction and thermal-emission measurements on several samples of non-crystalline and microcrystalline opal. The thermal-emission spectra of silica glass and amorphous, opaline silica are highly similar in shape and the location of absorption features are identical. A good match to the S2 spectrum has been produced by linear deconvolution, using TES-derived basalt and opal as endmembers (rms = 0.340, 1300-300 cm-1). Adding constituents such as alumina will shift the opal absorption features to lower wavenumbers, which is predicted to produce better deconvolution results. We propose that S2 may be basalt coated by amorphous silica. This hypothesis is similar to the weathered-basalt model in that it requires some degree of aqueous chemical processing. However, silica coatings occur on the exterior of rocks, formed by sedimentary deposition of silica. When weathering rinds form, chemical components are dissolved from a rock. Consequently, weathering rinds tend to be mechanically weak and would be highly susceptible to wind abrasion, especially under Martian eolian conditions. Conversely, silica coatings are resilient and have been found to form on active dune sands on Earth. Additionally, the near-infrared evidence for clay minerals on Mars is lacking. While a viable hypothesis for S2, glass-rich basaltic andesite lacks a model to explain its widespread occurrence. Alternative explanations of S2 should be explored, and silica rock coating is a practicable geologic model.
Kraft Michael David
Michalski Joseph R.
Sharp Thomas G.
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