HR: 15:15h
AN: P13B-07    [Abstracts]
TI: Rock Rinds and Coatings: Indicators of Aqueous Processes
AU: * Yen, A
EM: Albert.Yen@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109
AU: Bartlett, P
EM: bartlett@honeybeerobotics.com
AF: Honeybee Robotics, 204 Elizabeth Street, New York, NY 10012
AU: Clark, B
EM: Benton.C.Clark@lmco.com
AF: Lockheed Martin Corporation, 12257 State Highway 121, Littleton, CO 80127
AU: Gellert, R
EM: gellert@mpch-mainz.mpg.de
AF: Max Planck Institute for Chemistry, Saarstrasse 23, Mainz, 55122 Germany
AU: Klingelh\"{o}fer, G
EM: klingel@mail.uni-mainz.de
AF: Johannes Gutenberg University, Staudinger Weg 9, Mainz, 55099 Germany
AB: The Rock Abrasion Tools (RAT) onboard the Mars Exploration Rovers enable chemical and mineralogical comparisons of undisturbed surfaces, brushed surfaces, and interiors of rocks at the Gusev Crater and Meridiani Planum landing sites. The variability of Alpha-Particle X-ray Spectrometer (APXS) and M\"{o}ssbauer Spectrometer data with depth into rocks is an effective indicator of the extent of aqueous alteration. On the plains within Gusev crater, three rocks were brushed and abraded by the RAT. The interiors of these rocks ($\sim$2 to 5 mm from the surface) are nearly indistinguishable in elemental chemistry and iron mineralogy, indicating a common source region. As evidenced by the consistency of brushed and abraded measurements, the relatively small interior ferric-to-total iron ratio (0.10 to 0.20), and the low bromine content ($<$50 PPM), two of these rocks are minimally altered basalts with a thin ($<$500 microns) surface coating of dust/soil. The third rock, referred to as ``Mazatzal,'' has a brushed surface with moderate amounts of Br ($>$100 PPM), ferric-to-total iron ratios of $\sim$0.4 and greater, and indications that interior magnetite may have weathered into other oxide phases such as hematite. ``Mazatzal'' is the most extensively weathered of the abraded Gusev plains rocks. The rocks abraded and analyzed in the Columbia Hills, however, are far more weathered than even ``Mazatzal.'' A rock referred to as ``Clovis'' was abraded to a depth of 8.9 mm, and its interior remains dominated by ferric iron. The surface has minimal differences in elemental chemistry relative to the interior, suggesting an alteration thickness in excess of the RAT depth. The high surface concentration of bromine ($\sim$1000 PPM) is indicative of aqueous interaction, and its variability over centimeter-scales suggests redistribution with minor amounts of water. On the basis of elemental differences observed after application of the RAT and the ferrous iron signature in the M\"{o}ssbauer data, the other fully abraded rocks in the Columbia Hills exhibit lower levels of alteration in comparison to ``Clovis.'' At the Opportunity landing site, the RAT enabled interior analyses of outcrop material as well as rock coatings. In one example of a coating, the surface bromine concentration was approximately 800 PPM and decreased as the layer was brushed and abraded. Associated decreases of potassium, sodium, and chlorine in the abraded target suggest a redistribution of salts through aqueous transport at some point after emplacement of the underlying rock.
DE: 5415 Erosion and weathering
DE: 6225 Mars
DE: 3625 Descriptive mineralogy
DE: 3655 Major element composition
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting