Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p43a..03j&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P43A-03
Mathematics
Logic
6225 Mars
Scientific paper
The Rock Abrasion Tool (RAT) on the Mars rover Opportunity can brush away surface dust and grind away outcrop surface, exposing presumably less altered rock at depths of several mm. Alpha particle X-ray spectrometer (APXS) and Moessbauer spectrometer (MB) analyses of pre- and post-RAT targets, thus, provide information on the chemical nature of weathering of Meridiani outcrop rocks. To date, Opportunity has analyzed some 25 undisturbed rock surfaces, brushed and then analyzed 7 more, and ground 23 targets for IDD analysis. Panoramic camera images show that outcrop surfaces are typically either buff or purple (as viewed in bands centered at 673, 535, and 432 nm, Farrand et al., JGR, in press). Relatively flat surfaces that are approximately parallel to the ground are typically buff, whereas those that slope steeply tend to be purple. Surfaces of rock interiors ground by the RAT are also commonly purple. Spectrally, these color differences correspond to more oxidized (buff) and less oxidized (purple), and appear to relate to the degree of eolian abrasion. Flat-lying surfaces are not eroded as quickly, thus surfaces chemically weathered by exposure to tenuous atmospheric vapor may be preserved. These observations are consistent with in-situ analyses of rock surfaces and interiors. Compared to interiors, rock surfaces have about 1/3 less S, and in general, surface compositions lie between those of rock interiors and average surface soil. In detail, they differ from soil-rock mixtures as follows: surfaces are relatively depleted in Mg, Fe, Mn, Ti, and Cr, and they are enriched in Al, Na, K, P, Cl, and Si. From MB analyses, surfaces are richer (compared to soil-rock mixtures) in oxidized Fe phases and poorer in magnetite, olivine, and pyroxene. Morphologically, numerous flat-lying rocks and outcrop surfaces that are at or near the ground surface have a rind of erosionally resistant material. Such rinds are also chemically distinct from outcrop interiors. A rind/subjacent rock pair analyzed in detail was "Lemon Rind" and "Strawberry," ca. sols 555-560. The rind is depleted in S (balanced mainly by increased Si and Al) and, compared to a soil-rock mixture, it is depleted in Mg, Ti, Cr, Mn, and slightly in Fe, and it is enriched in Na, Cl, K, and P. Differences between rock surfaces and interiors, and between hardened weathering rinds and rock interiors, are consistent with loss of Mg-sulfate, oxidation of mafic minerals, enrichment of siliciclastic material, e.g., feldspar, and enrichment in chloride. These changes are consistent with slow rates of chemical weathering via interaction with small amounts of atmospheric water vapor or condensation. Erosionally resistant rinds may be related to preservation of aqueous condensate by a thin cover of soil on flat, near-surface rocks.
Farrand W.
Jolliff Brad
Knoll Alois
Sullivan Regina
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