SPIRIT'S TRAVERSE TO THE COLUMBIA HILLS: SYSTEMATIC VARIATIONS IN CLAST MORPHOLOGY OF PEBBLE TO COBBLE SIZED CLASTS, WITH IMPLICATIONS FOR GEOLOGICAL PROCESSES AND HISTORY

During the course of Spirit's traverse from the Columbia Memorial Station to the Columbia Hills a systematic set of Panoramic camera (PanCam) observations referred to as clast surveys 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 -72º. These images were taken at the beginning or end of drive segments at 42 sites during the course of the traverse from the landing site to the base of the Columbia Hills. During the course of this traverse, Spirit encountered six geologic units that had been mapped remotely from by THEMIS. Clast survey observations enabled quantification of changes in the size, roundness, shape, sorting, density, dispersion, and vesicularity of pebble to cobble sized clasts in these putative geologic units. The two overall goals were to a) look for trends in the above parameters that could allow an objective discrimination between basic erosional/depositional processes, including impact, fluvial, debris flow, and aeolian and b) determine if the units mapped remotely could be ground-truthed as distinct geologic units. Over the course of the traverse, clast roundness and sorting remained remarkably consistent, with mean estimates falling between sub-angular to sub-rounded, as well as being moderately well sorted. There was variation in the clast size, dispersion, and clast density. Trends in clast size correlated well with thermal inertia values, as estimated from orbital (THEMIS) data. The observed trends do not support previous suggestions of water-based depositional systems (fluvial, debris flow, or glacial processes) at the Spirit landing site. Instead, observed trends are consistent with a heavily cratered, wind modified ejecta surface, developed above a flow-dominated basaltic volcanic sequence