Rock size-frequency distributions on Mars and implications for Mars Exploration Rover landing safety and operations

[1] The cumulative fractional area covered by rocks versus diameter measured at the Pathfinder site was predicted by a rock distribution model that follows simple exponential functions that approach the total measured rock abundance (19%), with a steep decrease in rocks with increasing diameter. The distribution of rocks > 1.5 m diameter visible in rare boulder fields also follows this steep decrease with increasing diameter. The effective thermal inertia of rock populations calculated from a simple empirical model of the effective inertia of rocks versus diameter shows that most natural rock populations have cumulative effective thermal inertias of 1700 - 2100 J m(-2) s(-0.5) K-1 and are consistent with the model rock distributions applied to total rock abundance estimates. The Mars Exploration Rover ( MER) airbags have been successfully tested against extreme rock distributions with a higher percentage of potentially hazardous triangular buried rocks than observed at the Pathfinder and Viking landing sites. The probability of the lander impacting a > 1 m diameter rock in the first 2 bounces is < 3% and < 5% for the Meridiani and Gusev landing sites, respectively, and is < 0.14% and < 0.03% for rocks > 1.5 m and > 2 m diameter, respectively. Finally, the model rock size-frequency distributions indicate that rocks > 0.1 m and > 0.3 m in diameter, large enough to place contact sensor instruments against and abrade, respectively, should be plentiful within a single sol's drive at the Meridiani and Gusev landing sites.