Hydration state of the Martian surface as seen by Mars Express OMEGA: 1. Analysis of the 3 mm hydration feature

[1] Global mapping by the visible/near-infrared OMEGA spectrometer gives the first opportunity to study in detail the characteristics of the 3 mm hydration absorption on the surface of Mars. This feature is caused by bending and stretching vibrations of adsorbed or structural H2O and/or OHbound to minerals. A specific data reduction scheme has been developed to remove the contribution of thermally emitted radiance from OMEGA spectra. With the derived albedo spectra, variations in strength and shape of the 3 mm feature can be accurately assessed. Calibration issues are discussed and data only within the nominal calibration level of the instrument are analyzed, which corresponds to a surface coverage of 30%. All OMEGA spectra exhibit the presence of this absorption feature, which can be explained by the presence of adsorbed water as well as by alteration rinds or coatings resulting from weathering. Strong variations of the band strength are observed. Correlations between hydration, albedo, and elevation are examined. Terrains enriched in phyllosilicates, sulfates, or hydroxides exhibit an increased hydration signature as well as a weaker combination overtone H2O absorption at 1.9 mm. Careful analysis also reveals seasonal variations in surface hydration, with soils in the northern midlatitudes decreasing in hydration between northern spring and summer. This hydration change is best explained by the presence of winter frost followed by equilibration of frost-free soil with the atmosphere and by increased surface temperatures reducing the adsorptive capacity of the regolith.