HR: 0800h
AN: P21A-0131    [Abstracts]
TI: The Distribution of Rocks on the Gusev Plains and on Husband Hill
AU: * Grant, J A
EM: grantj@si.edu
AF: Smithsonian Institution, National Air and Space Museum, Center for Earth and Planetary Studies, 6th and Independence SW, Washington, DC 20560 United States
AU: Koestler, D L
P21A-0131 AF: East Stroudsburg University, 200 Prospect Street, East Stroudsburg, PA 18301 United States
AB: The size and number of rocks were measured at more than 20 locations over greater than 3 km of the traverse covered by the Spirit Mars Exploration Rover across the Gusev plains and Husband Hill. Rocks distributions were calculated using images collected at the end of rover drives and generally covered a distance of 8-20 meters from the rover and an azimuth range of 20 to 40 degrees. Corresponding count areas were typically 80 to 100 square meters and all visible rocks larger than 15 cm (long axis) were included. Multiple measurements made on single rocks and for some individual counts provide confidence in the reproducibility of the results. Individual rock counts were grouped based on their location relative to small impact structures that mark the plains or whether they were located on Husband Hill. Counts made on the plains are fairly similar and independent of proximity to most craters larger than 100 m in diameter. The average long axis of the rocks in all counts on the plains and around craters was close to 25 cm with a standard deviation of 10 cm. The average number of rocks per square meter was slightly more variable and ranged between 0.15 and 0.4 for counts on the more distal ejecta and plains, respectively, to 0.5 for counts made closer to crater rims. Average largest rocks increased from about 50 cm on the plains and distal ejecta to 70 cm nearer most crater rims. Exceptions to these results may include counts from the more exposed near-rim of the relatively pristine Bonneville crater and the flanks of Husband Hill. Along Bonneville's rim, the number of rocks per square meter and the largest rock is more than double the average on lower relief plains and more distal ejecta. Initial counts on Husband Hill reveal an average rock size of about 30 cm, but with a standard deviation of more than 20 cm. The average number of rocks per square meter is 0.4 and comparable to that on the plains, but the long axis of the largest rocks average 1 m and individual rocks are up to 1.8 m across. Broad similarities in the distribution of rocks on most low relief plains surfaces likely reflect the dominant role of impact fragmentation in modifying the basaltic surface. Hence, small differences between rock sizes, sorting, and number from location to location may relate less to proximity to nearby craters than to redistribution of only 10's of cm of eolian drift that variable buries or exposes rocks on some surfaces. By contrast, on more elevated relief, such as along the rim of Bonneville crater, more numerous and large rocks remain visible. On Husband Hill, larger rock sizes and standard deviations could reflect continued exposure of bedrock in some locations and accumulation of drift and locally derived impact debris in topographic alcoves and on low relief surfaces. If borne out by additional counts, this distribution suggests the overall morphology of the hill reflects initial formation process more than subsequent shaping by small impacts.
DE: 5415 Erosion and weathering
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5455 Origin and evolution
DE: 5460 Physical properties of materials
DE: 5470 Surface materials and properties
SC: Planetary Sciences [P]
MN: Fall Meeting 2005