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Infrared, Thermophysical, and Geomorphic Studies of Aqueously Altered Surfaces on Mars

Thursday, April 24, 2008
Michalski (CNES, France)

Clay mineral-bearing deposits previously discovered on Mars with near infrared (l=1-3 mm) remote sensing data are of major significance for understanding the aqueous history, geological evolution, and past habitability of Mars . In this study, I analyzed the thermal infrared (l=6-35 mm) surface properties of the most extensive phyllosilicate deposit on Mars: the Mawrth Vallis area. Clay mineral-bearing units, which in visible images appear to be relatively light-toned, layered bedrock surfaces, have thermal inertia values ranging from 150 to 460 J m-2 K-1 s-1/2. This suggests the deposits are composed of a mixture of rock with sand and dust at 100-meter scales. Dark-toned materials that mantle the clay-bearing surfaces have thermal inertia values ranging from 150 to 800, indicating variable degrees of rockiness or induration of this younger sedimentary unit. Spectral analysis with Thermal Emission Spectrometer (TES) data indicates that the light-toned rocks have a higher silica-content than the dark-toned surfaces (63-65% SiO2 compared to 48-50%). While the dark-toned unit is similar to typical slightly weathered basaltic compositions seen previously with TES, the light-toned units are different; these surfaces are spectrally dominated by plagioclase and amorphous silica, with minor abundances of clay minerals, oxides, and possibly zeolites. Unlike near infrared data, the thermal infrared data do not show evidence for abundant clay minerals. I propose three hypotheses for why this spectroscopic disconnect occurs, including effects due to surface roughness, sub-pixel mixing of multiple surface temperatures, and low absolute mineral abundances combined with differences in spatial sampling between instruments. The evidence for a large component of feldspar in the light-toned rocks indicates that a single origin of these materials as pyroclastic deposits is not likely unless the feldspars are secondary. The average mineralogy of these complex, interbedded deposits suggests either aqueous sedimentary origins or a combination of sedimentary and pyroclastic processes.

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