HR: 0800h
AN: P21A-0202    [Abstracts]
TI: The Size-Frequency and Areal Distribution of Rock Clasts at the Spirit Landing Site, Gusev Crater, Mars
AU: * Ward, J G
EM: jgward@wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, Campus Box 1169, One Brookings Drive, Saint Louis, MO 63130 United States
AU: Arvidson, R E
EM: arvidson@wunder.wustl.edu
AF: Washington University, Department of Earth and Planetary Sciences, Campus Box 1169, One Brookings Drive, Saint Louis, MO 63130 United States
AB: Data returned by the Mars Exploration Rover, Spirit, have revealed important information about the processes that have shaped landscapes in Gusev crater. In this work, we add to an understanding of surface processes by examining the size-frequency distributions and spacings of rock clasts along the Spirit traverses from its landing site to the base of the Columbia Hills (an $\sim$3.5 km distance via wheel odometry). Panoramic and Navigation Camera images reveal a surface that has been modified by volcanic, impact, and eolian processes. Size-frequency measurements of clasts between $\sim$0.1 and 10 cm reveal geometric mean clast sizes that increase along the approach to the $\sim$180 m diameter Bonneville crater. Clasts on the intercrater plains are finer-grained and better sorted than clasts found on the rim of Bonneville and the rims of hollows (presumed secondary impact craters). Further, statistical analysis relative to randomly dispersed clasts indicates that clasts on the intercrater plains are evenly spaced. We suggest that these clasts (geometric mean: 1.8 cm, geometric standard deviation: 1.6) were evenly dispersed by wind to the point where surface roughness became too low to initiate further clast motion. In fact, the clast threshold velocities for movement are too high under current atmospheric conditions. This implies that the clast evening process probably occurred when the atmosphere was denser in the Martian past. In contrast, our analyses indicate that the evenly spaced clasts on Earth that we have inventoried could have been moved under current terrestrial atmospheric conditions.
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting