Distribution of rocks on the Gusev Plains and on Husband Hill, Mars

The population of rocks larger than 15 cm in diameter was measured at 36 locations imaged by the Spirit rover over ∼ 4 km of the traverse across the Gusev plains and Husband Hill in Gusev crater on Mars. The rock population observed on plains surfaces is consistent with impact fragmentation of rubbly/fractured volcanics and reveals little evidence for modification by secondary processes or weathering. Interpretation of counts from Husband Hill suggest an influence by bedrock for rocks larger than 0.5 m across, but the distribution of smaller rocks is consistent with ejecta possessing contributions from pre-plains impact events. Results indicate plains surfaces experienced only tens of centimeters of eolian erosion/deposition since the Hesperian, whereas meters rather than tens of meters of erosion modified Husband Hill since the early Hesperian.