HR: 11:00h
AN: P32A-03    [Abstracts]
TI: Spirit's Traverse to the Columbia Hills: Systematic Variations in Clast Morphometry and Texture of Pebble to Cobble Sized Clasts, With Implications for Geological Processes and History
AU: * Grant, F D
EM: frederick.grant@asu.edu
AF: Arizona State University, Department of Geological Sciences Astrobiology Program PO Box 871404, Tempe, Az 85287-1404 United States
AU: Farmer, J D
EM: jack.farmer@asu.edu
AF: Arizona State University, Department of Geological Sciences Astrobiology Program PO Box 871404, Tempe, Az 85287-1404 United States
AU: Team, M
AF: Jet Propulsion Labratory, California Institute of Technology 4800 Oak Grove Drive, Pasadena, Ca 91109-8099 United States
AB: During the course of Spirit's traverse from the Columbia Memorial Station to the Columbia Hills a systematic set of PanCam observations called the clast survey were taken to look for evidence of fluvial activity affecting the morphology of pebble to cobble sized material. These PanCam observations employed a single frame, blue filter shot at 4 bits/pixel looking just above the deck in front of the rover at an angle centering the frame at -72 degrees. These images were taken at 42 sites during the course of the traverse from the landing site to the base of the Columbia Hills. This traverse encountered approximately 6 different geological units that were inferred from orbital data including thermal inertia estimates from Odyssey's THEMIS instrument, as well as geomorphic features observed by the Mars Global Surveyor's Mars Orbital Camera (MOC). Clast survey observations enabled quantification of changes in the size, roundness, sphericity, sorting, density (clasts/meter2), dispersion (nearest neighbor distances) and vesicularity of clasts over the course of Spirit's traverse across the plains to the base of the Columbia Hills. The overall goal was to look for trends in the above parameters that could allow an objective discrimination between basic erosional/depositional processes, including impact, fluvial, debris flow, glacial, and aeolian. To assist the interpretation of this data set, a variety of potential terrestrial analogs were investigated using the same clast survey parameters that were employed during Spirit's traverse. Each terrestrial analog was selected to represent an end member geologic process that could have shaped local clast distribution and morphology. These data sets were analyzed using SAS/STAT statistical software, employing Principle Component Analysis (PCA) to reduce the dimensionality of the data set, focus attention on the relationships between independent variables, and to identify factors that, taken together, could provide an objective basis for discriminating between geological processes. During the course of the traverse, significant changes were observed in clast size when moving from the continuous ejecta blanket of Bonneville crater (high thermal inertia) onto the intercrater plains (low thermal inertia). However, this trend was not apparent when crossing the continuous ejecta of two smaller craters, Lahontan and Missoula. In fact, clast sizes for these two craters compared more closely to the smooth intercrater plains unit previously mapped from orbit. Over the traverse, significant variations were observed in the distribution of vesicular clasts and in clast density. Changes in vesicularity are interpreted as relfecting local changes in the distribution and impact excavation depths of buried lava flow surfaces. Observed trends in clast size correlated well with thermal inertia values, as estimated from orbital (THEMIS) data. Over the course of the traverse, clast roundness and sorting remained remarkably consistent, with mean estimates falling between sub-angular to subrounded, and poorly sorted. These observations do not support previous suggestions of water-based depositional systems (fluvial, debris flow, or glacial processes) at the Spirit landing site, based on orbital data. Instead, observed trends are consistent with a heavily cratered, wind modified ejecta surface, developed above a flow-dominated basaltic volcanic sequence.
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2005 Joint Assembly