HR: 1340h
AN: P23A-1093    [Abstracts]
TI: Chemical Composition of Meridiani Sediments: Traces of Aqueous Past on Martian Surface
AU: * Brueckner, J
EM: brueckner@mpch-mainz.mpg.de
AF: Max-Planck-Institut Chemie, Abt. Geochemie, J.-J.-Becher-Weg 27, Mainz, 55020, Germany
AU: Gellert, R
AF: Univ. of Guelph, Dept. Physics, 50 Stone Road East, Guelph, On N1G2W1, Canada
AU: d'Uston, C
AF: CESR, 9 Avenue Colonel-Roche, Toulouse, 31028, France
AU: Treguier, E
AF: CESR, 9 Avenue Colonel-Roche, Toulouse, 31028, France
AU: Squyres, S W
AF: Cornell Univ., Center Radiophys. Space Res., 428 Space Science, Ithaca, NY 14853, United States
AU: Science Team, A
AF: Cornell Univ., 428 Space Science, Ithaca, NY 14853, United States
AB: Measurements of outcrop samples by the Alpha Particle X-Ray Spectrometer (APXS), onboard the NASA Mars Exploration Rover Opportunity at Meridiani, showed strong sulfur peaks in the x-ray spectra. Sulfur concentrations increased from natural (as is) rock surfaces over brushed to abraded rocks that turned out to be sulfur-loaded sediments. Along the 11-kilometer traverse of the rover many abraded surfaces could be measured by the APXS because the softness of the outcrops permitted grinding by the Rock Abrasion Tool (RAT) until today. All outcrop samples exhibited high sulfur concentrations of more than 6 weight percent; some samples exceeded 10 wt-% making S a major element and indicating a special history of these sediments. Element concentrations of all abraded rocks along the traverse were studied as function of sulfur content. A linear relationship with a negative slope was found for the silicon-sulfur pair. A similar relation holds for Al, Na, K, P, Ti, and Cr versus S. Iron shows a weak correlation with S (only a slight negative slope). Constant concentrations are exhibited by Mn and Ni. Calcium, Mg, and Zn, reveal a slight increase with increasing S contents (positive slope). During the first half of the traverse Mg and S are strongly correlated, later almost none. The formation of the sediments can be described by a two-component mixing model, where sulfur is mainly present in one component. The composition of the other component, the siliciclastic material, was extrapolated from above sample compositions to low S contents. The derived siliciclastic composition differs from encountered basaltic material, such as 'Bounce Rock' at Meridiani or the Adirondack Class rocks at Gusev crater, but, is similar to rocks discovered near Home Plate (Gusev). Best compositional matches are found for 'Masada Clod', 'Raquelme3', and others, which are significantly altered from an original basaltic composition. Apparently this composition type is wider spread on the Martian surface. The other mixing component contains various sulfates. Assuming large volcanic exhalations of sulfur, any original aqueous solution became very acidic. 'Normal' rocks were rapidly leached and gradually dissolved to form new compounds and large quantities of sulfates in an aqueous system. To bring the two components together, either wind and/or water did the transport. The small scatter of the concentration data points (mostly around a straight line) suggests that there was a concentration gradient in bodies of standing water on a kilometer-wide scale at least for a short period of time. The concentrations of many elements (Si, Al, Na, K, P, Ti, and Cr) are diluted by increasing sulfur contents. Hence, these elements were mainly part of the siliciclastic component. On the other hand, elements whose concentrations increase with increasing S (e.g. Ca, Mg, and Zn) were part of sulfates and of mafic minerals (in the siliciclastic component). Iron showing some dilution by sulfur was determined by Mössbauer spectroscopy to be present also as ferric sulfate. The above observations reveal that several elements formed sulfates in these sediments: Mg, Ca, Fe, and Zn. An aqueous system existed during the period of sediment formation and left unique traces in the sedimentary composition.
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 5410 Composition (1060, 3672)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting