Mars Surface Hydration Seen by MEx/OMEGA Through the 3 μm Feature.

The near infrared Mars surface global mapping done by OMEGA gives the first opportunity to study the global and detailed characteristics of the 3 μm hydration absorption band on Mars surface. This feature is indistinctly due to bending and stretching vibrations of water adsorbed on the surface of minerals or confined inside them, of water structurally bound to minerals, and of hydroxyl groups. Specific data reduction scheme has been developed to remove the thermal part of OMEGA spectra and to assess quantitatively the 3 μm band depth. All data at nominal calibration level are processed, which corresponds to about 30% of the Martian surface. They show 1) the presence of this absorption feature overall the Martian surface, which could be explained by the presence of adsorbed water as well as of rinds or coating resulting from weathering; 2) a possible increase of hydration with albedo that is discussed; 3) an increased hydration in terrains enriched in phyllosilicates, sulfates or hydroxides; 4) a tentatively decrease of the hydration with elevation for low albedo regions; 5) the seasonal decrease of hydration between spring and summer for mid latitudes regions. This winter hydration enrichment may be explained by the presence of frost followed by equilibration with the atmosphere, and by the summer temperature increase reducing the adsorptive capacity of the regolith. An attempt of completing these results by the reduction of data from lower calibration levels will be also presented.