Size-frequency distributions of rocks on the northern plains of Mars with special reference to Phoenix landing surfaces

The size-frequency distributions of rocks > 1.5 m diameter fully resolvable in High Resolution Imaging Science Experiment (HiRISE) images of the northern plains follow exponential models developed from lander measurements of smaller rocks and are continuous with rock distributions measured at the landing sites. Dark pixels at the resolution limit of Mars Orbiter Camera thought to be boulders are shown to be mostly dark shadows of clustered smaller rocks in HiRISE images. An automated rock detector algorithm that fits ellipses to shadows and cylinders to the rocks, accurately measured (within 1-2 pixels) rock diameter and height (by comparison to spacecraft of known size) of similar to 10 million rocks over > 1500 km(2) of the northern plains. Rock distributions in these counts parallel models for cumulative fractional area covered by 30-90% rocks in dense rock fields around craters, 10-30% rock coverage in less dense rock fields, and 0-10% rock coverage in background terrain away from craters. Above similar to 1.5 m diameter, HiRISE resolves the same population of rocks seen in lander images, and thus size- frequency distributions can be extrapolated along model curves to estimate the number of rocks at smaller diameters. Extrapolating sparse rock distributions in the Phoenix landing ellipse indicate < 1% chance of encountering a potentially hazardous rock during landing or that could impede the opening of the solar arrays. Extrapolations further suggest rocks large enough to depress the ground ice table and small enough to be picked up or pushed by the robotic arm should be present within reach for study after landing.