HR: 11:20h
AN: P12A-05 INVITED     [Abstracts]
TI: Recent Results from the Mars Exploration Rover Opportunity Mission
AU: * Squyres, S
EM: squyres@astro.cornell.edu
AF: Cornell University, Space Sciences Building, Ithaca, NY 14853 United States
AU: Arvidson, R
EM: arvidson@wunder.wustl.edu
AF: Washington University, Dept. of Earth and Planetary Sciences, St. Louis, MO 63130 United States
AB: The MER rover Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks, exposed in craters and along fissures in Meridiani Planum, are sandstones formed via the erosion and re-deposition of fine grained siliciclastics and evaporites derived from the chemical weathering of olivine basalts by acidic waters. A stratigraphic section more than seven meters thick measured in Endurance crater is dominated by eolian dune and sand sheet facies; the uppermost half meter, however, exhibits festoon cross lamination at a length scale that indicates subaqueous deposition, likely in a playa-like interdune setting. Silicates and sulfate minerals dominate outcrop geochemistry, but hematite and Fe₃D₃ (another ferric iron phase) make up as much as 11% of the rocks by weight. Jarosite in the outcrop matrix indicates precipitation at low pH. Cements, hematitic concretions, and crystal molds attest to a complex history of early diagenesis, mediated by ambient ground waters. The depositional and early diagenetic paleoenvironment at Meridiani was arid, acidic, and oxidizing, a characterization that places strong constraints on astrobiologial inference. Since leaving Endurance crater, Opportunity has traversed southward approximately 3 km over undulating plains characterized by basaltic sand, a thin surface lag of hematite-rich concretions, and subparallel eolian ripples that have increased in size toward the south. Mottled terrain to the south has now been revealed to consist largely of flat-lying expanses of sulfate-rich bedrock similar to the materials found in Endurance crater. The distribution of hematite in these southern exposures is different, however, with concretions that show a wider diversity of sizes and shapes than at Endurance. Dark 'rinds' on outcrop rock show a distinctive chemistry that is similar to the outcrop in many respects but with changes in the concentrations of some elements likely to be present as salts. Notably, Na and Cl are enhanced in rind material in an approximate 1:1 molar ratio. Some small cobbles are found to be martian rather than meteoritic in origin, with a unique composition that represents a material not yet found as bedrock.
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005