Wide distribution and glacial origin of polar gypsum on Mars

Kurzfassung

The North Polar Cap of Mars is associated with different kinds of superficial sediments, including the Circumpolar Dune Field, interior dune fields and sedimentary veneers scattered over the ice cap. In order to resolve the mineralogical composition and the regional distribution of these sediments, we processed OMEGA and CRISM hyperspectral data with an original method based on spectral derivation. We find that gypsum is present in all areas where undefined hydrated minerals had been previously detected, including superficial sedimentary veneers covering the North Polar Cap, interior dune fields and the whole Circumpolar Dune Field. Morphological and structural analyses reveal that these gypsum crystals derive directly from the interior of the ice cap. The source of superficial sedimentary veneers is the dust that was previously contained in the upper part of the ice cap, the ice-rich North Polar Layered Deposits (NPLD). This gypsum-bearing dust was released, on south-facing slopes of spiral troughs and arcuate scarps, by ice ablation controlled by katabatic winds. By the analysis of all associations of erosional scarps and dune fields over the North Polar Cap, we also demonstrate that the polar dunes are composed of sand-sized particles that were previously contained in the sediment-rich Basal Unit (BU), corresponding to the lower part of the ice cap. These particles contain gypsum and were released from the BU, by regressive ablation of ice at marginal scarps that border the North Polar Cap and by vertical ablation of ice on Olympia Planum. From a reconstruction of wind streamlines over and around the ice cap, we infer that katabatic winds descending from the polar high and rotating around the North Polar Cap control the release of these gypsum-bearing particles by ice ablation and the redistribution of these particles in the Circumpolar Dune Field. The intensity of gypsum diagnostic spectral absorption bands decreases along wind streamlines in the Circumpolar Dune Field, which indicates that the amount or the size of the gypsum particles decreases downwind, as the distance from their source increases.