Size-Frequency Distribution of Rock Clasts at the Phoenix Landing Site

Rock populations on the plains surrounding the Phoenix landing site were analyzed using a combination of ground-based and orbital data. We determined the size-frequency distribution of rocks larger than 1.5 meters in diameter using images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE). Surface images taken by the Phoenix Lander Surface Stereo Imager camera were used to characterize the size-frequency distribution of rocks as small as 2 centimeters. Comparison of the size-frequency distribution of rocks for the Phoenix landing site with model curves shows that the rock population is characterized by significantly more pebble-sized rocks (>10 centimeters) than simple crushing models predict. Additionally, comparison with rock counts from Mars Exploration Rover Spirit rover images show that the Phoenix landing site is depleted in rocks relative to the Gusev plains. The depletion of rocks of all sizes at the Phoenix landing site is consistent with the proposed hypothesis that rocks were removed from the surface during fluidized ejecta emplacement by nearby Heimdall crater. We also characterized rock populations on a detailed scale within eight meters of the Phoenix Lander. Results indicate that more rocks are located in polygon troughs than in polygon interiors, although the biggest rocks are found within polygon interiors. Nearest neighbor statistics show that rocks with diameters between 2cm and 30cm on polygon interiors are clumped or less uniformly dispersed, whereas rocks in polygon troughs are uniformly spaced. The differences observed between rock distributions within the polygon interior and polygon trough units suggest that polygons are actively redistributing rocks in a manner consistent with thermal-contraction-based cryoturbation.