Visible‐near infrared reflectance spectra acquired by the Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) spectrometer are used to estimate the absolute water content within the uppermost fraction of the Martian regolith. This upper surface layer represents the boundary between the regolith and atmosphere; thus the amount of water stored in these two reservoirs and at this boundary is expected to vary spatially and temporally with changing equilibrium conditions. We have applied models derived from laboratory experiments relating the strength of the 3 μm hydration feature to absolute water content to OMEGA spectra acquired during the first ∼1200 orbits of the Mars Express mission to estimate the H₂O content of the Martian surface. Three methods were used to examine the strength of the 3 μm absorption: integrated band depth, apparent absorbance, and the effective single‐particle absorption‐thickness (ESPAT) parameter. Integrated band depth and apparent absorbance values are correlated to albedo when derived from reflectance spectra, implying that bright regions are more hydrated than dark regions. The ESPAT parameter, however, relies on single scattering albedo instead of reflectance and is capable of estimating absolute water content within ±1 wt.% H₂O for a wide range of albedo values, compositions, and particle sizes. Applying this model to the OMEGA data reveals that bright and dark regions commonly have similar water contents in equatorial regions and the largest spatial variations in H₂O occur as a function of latitude. Equatorial regions exhibit water contents in the range of ∼2–5 wt.%, whereas latitudes higher than ∼45°N are characterized by a continuous increase in H₂O with latitude from ∼5–15 wt.%. Phyllosilicate and sulfate bearing terrains are more hydrated than average bright and dark regions and their locations are in widely separated areas of Noachian‐aged material, suggesting chemical alteration by water‐rock interaction may have been spatially extensive in the early history of Mars.

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