Global mineralogical mapping of Mars by the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument on the European Space Agency's Mars Express spacecraft provides new information on Mars' geological and climatic history. Phyllosilicates formed by aqueous alteration very early in the planet's history (the “phyllocian” era) are found in the oldest terrains; sulfates were formed in a second era (the “theiikian” era) in an acidic environment. Beginning about 3.5 billion years ago, the last era (the “siderikian”) is dominated by the formation of anhydrous ferric oxides in a slow superficial weathering, without liquid water playing a major role across the planet.

[1] 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 mm hydration feature to absolute water content to OMEGA spectra acquired during the first $1200 orbits of the Mars Express mission to estimate the H 2 O content of the Martian surface. Three methods were used to examine the strength of the 3 mm absorption: integrated band depth, apparent absorbance, and the effective single-particle absorptionthickness (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.% H 2 O 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 H 2 O 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 H 2 O 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.

[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 H 2 O and/or OH-bound 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 H 2 O 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.

Here we discuss one of the current reservoirs of water on Mars, the regolith and rocks exposed at the surface. This reservoir is characterized by the presence of H 2 O-and OH-bearing phases that produce a broad absorption at a wavelength of~3 μm in near-infrared (NIR) reflectance spectra. This absorption is present in every ice-free spectrum of the Martian surface obtained thus far by orbital NIR spectrometers. We present a quantitative analysis of the global distribution of the 3 μm absorption using the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) imaging spectrometer that has been mapping the surface of Mars at kilometer scale for more than 10 years. Based on laboratory reflectance spectra of a wide range of hydrous minerals and phases, we estimate a model-dependent water content of 4 ± 1 wt % in the equatorial and midlatitudes. Surface hydration increases with latitude, with an asymmetry in water content between the Northern and Southern Hemispheres. The surface hydration is compared to various parameters (albedo, dust, geological units, time, relative humidity, atmospheric water pressure, and in situ measurements performed by Phoenix and Curiosity) to constrain the nature of the reservoir. We conclude that the nature of the surface hydration of the Martian low latitudes is not adsorbed water but rather more tightly bound water molecules and hydroxyl groups in the structure of the materials of the near-top surface. A frost-related process best explains the implementation of water into and onto the first microns of the high-latitude Martian regolith.

[1] Mapping of the aphelion clouds over the Tharsis plateau and retrieval of their particle size and visible opacity are made possible by the OMEGA imaging spectrometer aboard Mars Express. Observations cover the period from MY26 L s = 330 to MY29 L s = 180and are acquired at various local times, ranging from 8 AM to 6 PM. Cloud maps of the Tharsis region constructed using the 3.1 mm ice absorption band reveal the seasonal and diurnal evolution of aphelion clouds. Four distinct types of clouds are identified: morning hazes, topographically controlled hazes, cumulus clouds and thick hazes. The location and time of occurrence of these clouds are analyzed and their respective formation process is discussed. An inverse method for retrieving cloud particle size and opacity is then developed and can only be applied to thick hazes. The relative error of these measurements is less than 30% for cloud particle size and 20% for opacity. Two groups of particles can be distinguished. The first group is found over flat plains and is composed of relatively small particles, ranging in size from 2 to 3.5 mm. The second group is characterized by particle sizes of $5 mm which appear to be quite constant over L s and local time. It is found west of Ascraeus and Pavonis Mons, and near Lunae Planum. These regions are preferentially exposed to anabatic winds, which may control the formation of these particles and explain their distinct properties. The water ice column is equal to 2.9 pr.mm on average, and can reach 5.2 pr.mm in the thickest clouds of Tharsis.Citation: Madeleine, J. -B., et al. (2012), Aphelion water-ice cloud mapping and property retrieval using the OMEGA imaging spectrometer onboard Mars Express,