Department of Earth Sciences

The north polar ice cap of Mars changes its face during the course of the year: a permanent cap composed mainly of water ice is observed in the summer season, as shown in this HRSC image. During the winter season, temperatures fall below -125 degrees Celsius, cold enough for carbon dioxide to precipitate and build a thin (1-2 meters) seasonal polar cap of carbon dioxide ice. At this time of the year, the ice cap is often covered by thick carbon dioxide clouds and thus difficult to observe with cameras from orbit.

The darker troughs in between the shining white water ice deposits are part of an impressive system of depressions that spiral outwards from the pole center in a counterclockwise direction and cut through the thick stack of layered deposits that make up the north polar cap. These layered deposits are composed of ice intermixed with dust and record the evolution of the Martian climate over the last Millions of years.

The clouds on this HRSC image have been interpreted as small local dust storms aligning perpendicular to the troughs. Local dust storms were also frequently observed in THEMIS and HiRISE data, especially at the equator-facing walls of the polar troughs. This kind of dust mobilization could facilitate scarp erosion and retreat. Both sublimation and katabatic wind erosion seem to be important active eolian processes responsible for the long-term modification of the trough walls.

Wind erosion could be the driving force for the formation of the spiral troughs, as suggested by the analysis of radar data. One theory is that the spiral troughs are cyclic steps formed by katabatic winds blowing over ice. A katabatic wind denotes a downslope gravity flow of cold, dense air. It often initiates on higher-elevation areas with cold dry air, e.g. glacial or snow-covered plateaus, and flows down to lower-elevation areas such as valleys hosting warm, less dense air. In the case of the Mars polar cap, air movement is occurring radially away from the pole, and additionally controlled by Coriolis forces that lead to a spiral path of the flow.