HiRISE Images of the Sublimation of Mars Northern Seasonal Polar Cap

The High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter (MRO) has completed a campaign to image the sublimation of Mars" northern seasonal polar cap. The images show a number of interesting phenomena associated with the dunes in the north polar erg, including slope streaks, circular defrosting features, and evidence for explosive gas release. Time-lapse sequences show these features evolving through the spring as the seasonal frost disappears. Slope streaks appear to be mobilized by the vigorous activity associated with sublimation of the CO2 ice. Streaks are observed to lengthen as the season progresses, and mass wasting from the crests of the dunes is implicated. Oblong features associated with defrosting show alternating bright and dark rings. They are found at the crests of dunes, and at the interface of the bottom of the dunes with the substrate. The contrast is most pronounced early in the spring and gradually fades as more and more ice is sublimated. This is a surficial albedo effect, not due to underlying surface morphology. The degree of cementation of the substrate below the ice layer is important and sublimation features which show mobilization of material are found on dunes, not on the surface beneath the dunes. Evidence for translucent CO2 ice similar to the seasonal ice found at southern high latitudes has been found at northern sites suggested by Piqueux and Christensen [1]. Dark and bright streaks are observed with orientations determined by the prevailing wind. We have not observed the araneiform terrain with its spider- like channels found in many locations in the south polar region. Frost-filled polygons are observed however. This work was partially supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. [1] Piqueux, S., P. Christensen, JGR 113 E06005 (2008).