HR: 0800h
AN: P31B-0429    [Abstracts]
TI: Degradation of Victoria Crater, Meridiani Planum, Mars
AU: * Grant, J A
EM: grantj@si.edu
AF: Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC 20560, United States
AU: Wilson, S A
EM: wilsons@si.edu
AF: Center for Earth and Planetary Studies, Smithsonian Institution, Washington, DC 20560, United States
AU: Cohen, B A
EM: bcohen@unm.edu
AF: Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, United States
AU: Golombek, M P
EM: mgolombek@jpl.nasa.gov
AF: Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, United States
AU: Geissler, P E
EM: pgeissler@usgs.gov
AF: US Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001, United States
AU: Sullivan, R J
EM: rjs33@cornell.edu
AF: Dept. of Astronomy, Cornell University, Ithaca, NY 14853, United States
AB: Victoria crater (2.05N, 354.51E) is ~750 m in diameter and the largest crater on Mars observed in situ. The Mars Exploration Rover Opportunity traversed NW to SE across a broad annulus dominated by dark sand that at least partially surrounds the crater before navigating the northern crater rim. Rover observations of the crater and ejecta deposits are complemented by images with 26-52 cm/pixel scales from the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter and enable assessment of degradation state. The present depth/diameter ratio for Victoria is 0.1, less than the 0.2 expected for a pristine primary impact structure. Together with the eroded, serrated rim, this implies an originally smaller crater diameter and/or considerable infilling consistent with occurrence of a large dune field and few exposed rocks on the crater floor. The height and width of the raised rim is generally 4-5 m and 150-225 m, respectively, less than the 30 m and 500-600 m, respectively, expected for a pristine 750 m diameter crater. Ejecta thicknesses around the rim were derived using rover-based and HiRISE images and yield consistent estimates averaging ~3 m. The serrated rim plan creates a series of promontories extending up to 50 m into the crater and generally fronted by 30-60 degree slopes that are locally vertical and are separated by bays whose floors typically slope 15-25 degrees. A crater originally on order of 600-650 m in diameter and subsequently enlarged by mass wasting and aeolian erosion may yield a structure resembling Victoria today. The steep expression of the promontories and local outcroppings of rocks in the ejecta blanket points to some ongoing mass wasting, but the relative paucity of associated flanking talus indicates derived blocks of sulfate sandstone are not resistant to saltating sand and are rapidly broken down by the wind or are completely covered/filled in by aeolian drift. At Cape St. Vincent, the promontory appears undercut by aeolian erosion; thereby leading to mass wasting and accumulation of the large angular blocks observed on the slopes below By contrast, the shallow slope in the bays implies mass wasting is currently limited. Most likely, early mass wasting and aeolian stripping of the walls was gradually superseded by aeolian activity that proceeded most quickly along structural weaknesses and resulted in formation of the bays. Redistributed wall material contributes to crater infilling and/or was transported out of the crater. Occurrence of dark basaltic sands in the crater and in wind streaks along the north rim indicates additional transport into and out of the crater from other sources. The diminished expression of the rim, planed-off appearance of the ejecta blanket, paucity of exposed blocks, and dark wind streaks outside the crater imply efficient aeolian modification of Victoria crater, likely contributing to down-wasting and sediment transport that may be responsible for thin, local outcrops of finely bedded sediments capping the ejecta. The annulus appears to have resulted from combined erosion and planing of the ejecta, leading to evolution of a resistant lag of hematite concretions along with infilling and covering by the basaltic sand. There is no evidence for water-related erosional contributions to the present form of Victoria crater.
DE: 5415 Erosion and weathering
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting