Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43b1404b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P43B-1404
Mathematics
Logic
1051 Sedimentary Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5410 Composition (1060, 3672), 6225 Mars
Scientific paper
During the last four and a half years, the two NASA Mars Exploration Rovers (MER), Spirit and Opportunity, have been making scientific measurements on the Martian surface. One of the instruments is the Alpha Particle X-Ray Spectrometer (APXS) located on the rover arm. Most rock samples were abraded and their fresh surfaces were measured by several instruments. Once a target spot of a soil or rock surface was selected, the APXS was properly positioned to accumulate X-ray spectra. From these data, the concentration of all major and many minor and trace elements are derived. Sulfur is an element that could be detected in all samples measured, so far, which makes the surface of Mars a unique place, as its surface is somewhat dominated by sulfur chemistry. Whenever some moisture or even larger quantities of liquid water were present, the surface of soils and rocks were effected by sulfuric acid reactions. In addition, sulfur-rich deposits could be observed with the APXS. At Gusev crater, soils have been deeply disturbed by rover wheels andthe exposed subsurface samples were investigated that had high S contents, such as Paso Robles (Sol 401), Arad Samra (Sol 723), Berkner Island (Sol 1013), and Mount Darwin (Sol 1098). In these soils, S concentrations between 9 to 14 weight-percent were determined by the APXS. As the APXS method only allows determining elemental concentrations, the speciation of S, such as elemental sulfur, sulfate, pyrite, or sulfide, cannot be derived, directly. Mass balance considerations and mineralogical results by the Mössbauer Spectrometer (MB) strongly support the assumption of sulfate being the predominant oxide of S. The high S content of Paso Robles is accompanied by a high Fe content, consistent with the identification of ferric sulfate by MB as the main iron-bearing mineral at Paso Robles. At Meridiani Planum, varying S contents were observed by the APXS: basaltic soils that contained a few percent S as found for Gusev soils and sedimentary outcrops that revealed very high amounts of S (up to 11 wt.-%). When the rover Opportunity drove inside the crater Endurance, a stratigraphical chemical profile was obtained by frequent APXS measurements of abraded rocks. These interior rocks show a very good correlation of Mg and S, which suggests that MgSO4 is a major component of the outcrop. Since S concentrations are markedly high in all measured outcrops, the concentration of major elements can provide constraints on their sulfur compounds. In a two-component mixing model it can be shown that certain elements form a sulfur-bearing phase, while others not: Silicon and Al do not occur as S-bearing phases, while, Mg, Ca, and Fe form S-rich phases. Assuming all SO3 is bound to Mg and Ca sulfates, and ferric sulfates, according to MB data, these outcrops contain large amounts of sulfates. The ingress of rover Opportunity into the Victoria crater also revealed a correlation of S and Mg contents with depth as was observed previously. Assuming that sample Cha (Sol 962), found at the rim of the crater, is part of the deeper ejecta, its S and Mg contents fit the general trend of the interior samples. Maybe, we have evidence at two locations that in deeper layers Mg-sulfates are somewhat lower than in upper ones pointing to a possible change in the Martian environment during the formation of the sediments.
Brueckner Johannes
Dreibus Gerlind
Gellert Ralf
Klingelhoefer G.
Science Team
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