Martian polar and circum-polar sulfate-bearing deposits: Sublimation tills derived from the north polar cap

International audiencePrevious spectroscopic studies have shown the presence of hydrated minerals in various kinds of sedimentary accumulations covering and encircling the Martian North Polar Cap. More specifically, gypsum, a hydrated calcium sulfate, has been detected on Olympia Planum, a restricted part of the Circumpolar Dune Field. To further constrain the geographical distribution and the process of formation and accumulation of these hydrated minerals, we performed an integrated morphological, structural and compositional analysis of a key area where hydrated minerals were detected and where the main polar landforms are present. By the development of a spectral processing method based on spectral derivation and by the acquisition of laboratory spectra of gypsum-ice mixtures we find that gypsum-bearing sediment is not restricted to the Olympia Planum dunes but is also present in all kinds of superficial sediment covering the surface of the North Polar Cap and the Circumpolar Dune Field. Spectral signatures consistent with perchlorates are also detected on these deposits. The interpretation of landforms reveals that this gypsum-bearing sediment was released from the ice cap by sublimation. We thus infer that gypsum crystals that are now present in the Circum-Polar Dune Field derive from the interior of the North Polar Cap. Gypsum crystals that were initially trapped in the ice cap have been released by sublimation of the ice and have accumulated in the form of ablation tills at the surface of the ice cap. These gypsum-bearing sublimation tills are reworked by winds and are transported towards the Circum-Polar Dune Field. Comparison with sulfates found in terrestrial glaciers suggests that gypsum crystals in the Martian North Polar Cap have formed by weathering of dust particles, either in the atmosphere prior to their deposition during the formation of the ice cap, and/or in the ice cap after their deposition