Early infrared observations of the Martian surface have revealed the presence of a deep absorption between 2.6 and 3.5 µm usually referred to as the 3-µm band. Two non-exclusive explanations have been formulated to account for this 3-µm band. The first possibility is that it relates to the presence of some amount of adsorbed atmospheric water, water that would be exchangeable and could play a role in present day Mars water cycle. The alternative is that this absorption is due to an -OH or H2O bearing mineral component within the Martian dust, which would imply that water is involved at some stage of the dust production mechanism. Several lines of evidences seem to converge toward the second hypothesis. First, after 10 years of mapping of the martian surface with OMEGA (MEX), the lack of variation of the 3-µm feature with surface relative humidity suggests that the surface hydration does not exchange with the atmosphere. This is in agreement with laboratory adsorption experiments under Mars conditions.
More recently the Mars Science Laboratory with unprecedented payload has provided some crucial constraints on the nature of surface hydration. The ChemCam instrument onboard MSL has detected hydrogen in dust and soil, and revealed the lack of variation of H concentration with surface humidity, including nighttime measurements. Also, evolved gas analysis by SAM of Gale crater soils shows that water is released at relatively high-temperature and suggest that -OH or H2O bearing phases are trapped in the amorphous component of soils.
Finally, the exceptional "black beauty" meteorite might also provide clues to the nature of Mars's surface hydration. This Martian breccia shows elevated amount of water with respect to any other Martian meteorite. By studying the spectral properties of NWA 7533, we were able to show the presence of a 3-µm band and a small red-slope; this Martian breccia seems to contain a fine-oxidized-H-bearing component, possibly similar to that present in the Martian dust.