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Wide distribution and glacial origin of polar gypsum on Mars.

Wednesday, February 22, 2012

The North Polar Cap of Mars is associated with different kinds of superficial sediments, including the Circumpolar Dune Field and sedimentary veneers scattered over the ice cap. In order to resolve the mineralogical composition of these sediments, we processed OMEGA and CRISM hyperspectral data with an original method based on spectral derivation (Huguenin and Jones, 1986). We find that gypsum is present in all areas where undefined hydrated minerals had been previously detected (Poulet et al., 2008; Horgan et al., 2009; Calvin et al., 2010), including the superficial sedimentary veneers found on the North Polar Cap and the whole Circumpolar Dune Field. Integrated morphological and structural analyses reveal that these gypsum crystals derive directly from the interior of the ice cap (Massé et al., 2010). The source of 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 exhumed, on south-facing slopes of spiral troughs and arcuate scarps, by ice ablation induced 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 source of the polar dunes are sand-sized particles that were previously contained in the sediment-rich BU (Basal Unit), corresponding to the lower part of the ice cap. These particles were exhumed from the BU, by regressive ablation of the ice at marginal scarps that border the North Polar Cap, or by vertical ablation of the ice on Olympia Planum. From a reconstruction of wind flow lines over and around the ice cap, we infer that katabatic winds descending from the polar high and rotating around the North Polar Cap are responsible for the exhumation of this gypsum-bearing sand and for its redistribution in the Circumpolar Dune Field. The intensity of gypsum diagnostic spectral absorption bands decreases along wind flow lines in the Circumpolar Dune Field, which we interpret as indicating that the size of the gypsum particles decreases as the distance from their source increases (Langford, 2003; Grhefat et al., 2007). Comparisons with sulfates found in terrestrial glaciers (Ohno et al., 2006; Iizuka et al., 2006 and 2008) suggest that gypsum crystals that were trapped in the North Polar Cap before their exhumation had formed initially by weathering of dust particles either in the atmosphere and/or in the ice cap. These results can provide some key elements for the understanding of the equatorial sulfates formation.

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