Boulder movement at high northern latitudes of Mars

We examine a narrow latitudinal band (60°N-70°N) on Mars to place constraints on the seasonally averaged velocity Q of boulder movements over patterned ground. These latitudes comprise a region of the northern lowlands where patterned ground covers nearly every surface. Here boulders meters in diameter are consistently found to be concentrated at or near the cracks that define the polygonal networks, indicating a mobilization process. Because impact craters are the source for many boulders, we can use craters and their degradation to estimate the time scales for boulder movement. We study and catalog 1018 degraded impact craters (100 m < D < 1 km) in 55 High Resolution Imaging Science Experiment (HiRISE) images. We find that crater degradation occurs on a time scale <∼1 Ma, which is too recent for melting-related mechanisms in a past, warmer epoch. Clustering of boulders occurs at a time scale of a few Ma or shorter, which means that boulders on 5-20 m diameter polygons move at seasonal velocities Q ∼1-10 m/yr or faster.