Viking Orbiter 2 images of the north polar region reveal an enormous sand sea (erg) covering an area of >5×105 km2 around the perennial ice cap. All dunes are either transverse or barchan. The various dune morphologies and modification of primary dune types reflect a wind regime having more than one wind direction. In the summer, two major wind directions prevail: (1) off-pole winds that become easterly due to coriolis forces and (2) on-pole winds that become westerly. During the winter and/or spring, only the on-pole winds exist. Strong winds (>75 m/s) are required for sand accumulation to form the thick transverse dunes. The strongest winds in the north polar region are thought to exist during summer over the transverse dune field between 110°W and 220°W; this area is a relatively warm belt (temperature >230 K) between two ice zones (temperature <220 K). A frequent cyclogenetic process in this area may cause sand storms. The dunes seem to be currently active. The lack of well-developed longitudinal dunes implies that the dune field is young. The relationship of the present dune field to the perennial ice indicates that the dunes began to form after the formation of the present ice cap.
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