Structure of the basal unit of the North Polar Plateau of Mars, from MARSIS

We use 36 MARSIS orbits to extract reflector depths for laterally extensive internal surfaces in the North Polar Plateau and compare the interpolated surface at the bottom of the ice to a prediction of that surface using MOLA data. We find that the prediction and data are in good agreement in terms of regional slope, and may even agree on the finer-scale saddle feature expected based on MOLA. We conclude that the underlying plains are not deflected anywhere beneath the ice deposits, and infer a possibly atypical composition or texture at the top of the Basal Unit in the 270° longitude direction, perhaps indicative of a paleo-dunefield.