Rock Populations Along the Spirit Rover Traverse: Implications for the Timing and Amount of Gradation in Gusev Crater, Mars

The rock population in Gusev Crater was analyzed at 36 locations along ~4 km of the Spirit rover's traverse, including 27 across the plains and 9 from Husband Hill. Rocks located between 8 20 m from the rover with a long axis greater than 15 cm in diameter were measured using Pancam images taken at the end of rover drives. The Pancam images typically covered an azimuth wedge of approximately 45 degrees, resulting in an 80-100 m2 sampling area. Rock measurements made on the plains prior to reaching Husband Hill were associated with geomorphic surfaces dominated by crater rims, ejecta, or plains surfaces. The average number of rocks per square meter was 0.2 on ejecta, 0.3-0.5 on the plains and Husband Hill, and just over 0.5 on crater rims. The average axis of the measured rocks on all surfaces ranges between 22-29 cm (associated standard deviations were 8-18 cm). The size-frequency distribution of rocks on all surfaces display an exponential increase in number with decreasing size, but bedrock outcrops on Husband Hill result in a greater number of rocks larger than 50 cm in diameter relative to observations on the plains surfaces. The size-frequency distribution plot can be modified by cubing the rock diameters and multiplying by an assumed density (3000 kg/m3) to serve as a proxy for rock mass. The resulting mass-frequency distribution plot yields slopes of 0.9-1.2 for the plains and Husband Hill surfaces in Gusev, which are broadly consistent with those expected for surfaces modified by multiple impact fragmentation processes. This result, in concert with an absence of sorting or other evidence of modification by alternate processes (other than tens of centimeters of aeolian redistribution of fine sediments), confirms the dominant role of impact cratering in the modification of these surfaces since the Hesperian. Moreover, data from the Mini-TES instrument on the Spirit rover indicate some rocks on Husband Hill smaller than 50 cm in diameter are compositionally distinct from the rocks that surround them, and are referred to as exotics. The size-frequency distribution of these exotics suggests that they are impact ejecta and some could have been derived from local impacts on the hill or on the surrounding plains. Other exotics, however, do not match known hill outcrop compositions or the composition of the plains. Hence, at least some of these exotics may represent ejecta whose arrival on Husband Hill predates the emplacement of the plains. If correct, the survival of these exotics suggests that Husband Hill has been subject to no more than meters of erosion since the early Hesperian and perhaps even late Noachian, and highlights the limited erosion occurring in Gusev Crater over much of Martian history.