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Context. Phyllosilicates were definitely identified on Mars by the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) instrument onboard the Mars Express spacecraft. The identification, characterization, and mapping of deposits of these minerals hold clues to the potential past habitability. They also constitute a key element in planning for future landing sites.

The OMEGA imaging spectrometer aboard Mars Express has been used to study the evolution of the CO abundance over the Hellas basin, as a function of the seasonal cycle. In spite of significant uncertainties, due to the low intensity of the (2-0) CO band at 2.3 μm, these data allow to search for spatial variations over Hellas, where the surface pressure and the CO band depth are maximum. The OMEGA data suggest an enhancement over Hellas at Ls = 130-150 deg (end of southern winter), by a factor of about 2 with regard to Ls = 330-350 deg (end of northern summer). This behavior is to be compared with the evolution of argon, another non-condensible species which also shows strong variations with Ls at southern latitudes (Sprague et al. 2004). This result appears also consistent with the GCM predictions which indicate an enrichment by a factor of about 2 of non-condensible species over Hellas during southern winter (Forget et al. 2006).

We report on daytime limb observations of Mars upper atmosphere acquired by the OMEGA instrument on board the European spacecraft Mars Express. The strong emission observed at 4.3 μm is interpreted as due to CO2fluorescence of solar radiation and is detected at a tangent altitude in between 60 and 110 km. The main value of OMEGA observations is that they provide simultaneously spectral information and good spatial sampling of the CO2emission. In this study we analyzed 98 dayside limb observations spanning over more than 3 Martian years, with a very good latitudinal and longitudinal coverage. Thanks to the precise altitude sounding capabilities of OMEGA, we extracted vertical profiles of the non‐local thermodynamic equilibrium (non‐LTE) emission at each wavelength and we studied their dependence on several geophysical parameters, such as the solar illumination and the tangent altitude. The dependence of the non‐LTE emission on solar zenith angle and altitude follows a similar behavior to that predicted by the non‐LTE model. According to our non‐LTE model, the tangent altitude of the peak of the CO2emission varies with the thermal structure, but the pressure level where the peak of the emission is found remains constant at ∼0.03 ± 0.01 Pa, . This non‐LTE model prediction has been corroborated by comparing SPICAM and OMEGA observations. We have shown that the seasonal variations of the altitude of constant pressure levels in SPICAM stellar occultation retrievals correlate well with the variations of the OMEGA peak emission altitudes, although the exact pressure level cannot be defined with the spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAM) nighttime data. Thus, observed changes in the altitude of the peak emission provide us information on the altitude of the 0.03 Pa pressure level. Since the pressure at a given altitude is dictated by the thermal structure below, the tangent altitude of the peak emission represents then an important piece of information of the atmosphere, of great value for validating general circulation models. We thus compared the altitude of OMEGA peak emission with the altitude of the 0.03 Pa level predicted by the Laboratoire de météorologie dynamique (LMD)‐Mars global circulation model and found that the peak emission altitudes from OMEGA present a much larger variability than the tangent altitude of the 0.03 Pa level predicted by the general circulation model. This variability could be possibly due to unresolved atmospheric waves. Further studies using this strong CO2limb emission data are proposed. We analyzed 3 Martian years of CO2fluorescence emission in Mars' upper atmosphereObservations are mainly affected by solar illumination conditions and the altitude of the emissionAccording to non‐LTE model, the peak of the emission occurs at a constant pressure level of 0.03 Pa

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

We used the OMEGA imaging spectrometer aboard Mars Express to study the evolution of the water vapor abundance over the Hellas basin, as a function of the seasonal cycle. The H2O?column density is found to range from very low values (between southern fall and winter) up to more than 15?pr-?m during southern spring and summer. The general behavior is consistent with the expected seasonal cycle of water vapor on Mars, as previously observed by?TES and modeled. In particular, the maximum water vapor content is observed around the southern solstice, and is significantly less than its northern couterpart. However, there is a noticeable discrepancy around the northern spring equinox (Ls= 330?60?), where the observed H2O?column densities are significantly lower than the values predicted by the?GCM. Our data show an abrupt enhancement of the water vapor column density (from?3 to 16?pr-?m) on a timescale of 3?days, for Ls= 251?254?. Such an increase, not predicted by the?GCM, was also occasionally observed by TES over Hellas during previous martian years at the same season; however, its origin remains to be understood.?

The wide spectral coverage and extensive spatial, temporal, and phase-angle mapping capabilities of the Visual Infrared Mapping Spectrometer (VIMS) onboard the Cassini-Huygens Orbiter are producing fundamental new insights into the nature of the atmospheres of Saturn and Titan. For both bodies, VIMS maps over time and solar phase angles provide information for a multitude of atmospheric constituents and aerosol layers, providing new insights into atmospheric structure and dynamical and chemical processes. For Saturn, salient early results include evidence for phosphine depletion in relatively dark and less cloudy belts at temperate and mid-latitudes compared to the relatively bright and cloudier Equatorial Region, consistent with traditional theories of belts being regions of relative downwelling. Additional Saturn results include (1) the mapping of enhanced trace gas absorptions at the south pole, and (2) the first high phase-angle, high-spatial-resolution imagery of CH4fluorescence. An additional fundamental new result is the first nighttime near-infrared mapping of Saturn, clearly showing discrete meteorological features relatively deep in the atmosphere beneath the planet’s sunlit haze and cloud layers, thus revealing a new dynamical regime at depth where vertical dynamics is relatively more important than zonal dynamics in determining cloud morphology. Zonal wind measurements at deeper levels than previously available are achieved by tracking these features over multiple days, thereby providing measurements of zonal wind shears within Saturn’s troposphere when compared to cloudtop movements measured in reflected sunlight. For Titan, initial results include (1) the first detection and mapping of thermal emission spectra of CO, CO2, and CH3D on Titan’s nightside limb, (2) the mapping of CH4fluorescence over the dayside bright limb, extending to ∼ ∼750 km altitude, (3) wind measurements of ∼ ∼0.5 ms−1, favoring prograde, from the movement of a persistent (multiple months) south polar cloud near 88° S latitude, and (4) the imaging of two transient mid-southern-latitude cloud features.

We have studied by time of flight mass spectrometry the ion desorption from icy films stimulated by the irradiation of MeV/amu Kr ions. We have determined the mass of the desorbed species, and detected clusters of the form H(H2O)n+, O(H2O)n- and OH(H2O)n-, with n higher than 20. We have measured the variation of the desorption yields with the energy of the primary beam, for both the positively and negatively charged ions. The production of ions with high masses induced by the cosmic ray bombardment of icy grains may play an important role in cosmic chemistry.

Particle-grain interactions at implantation energies occur in a large variety of astrophysical sites. The expected physical, chemical and isotopic effects are discussed, as deduced from the analyses of (i) lunar grains implanted with the solar wind ions, and (ii) terrestrial grains implanted in the lab with H, C, N,···, ions. Special emphasis is placed on the amorphisation and the erosion of the grains, and on the synthesis of radicals and molecules within the implanted grains. Implications in planetology and astrophysics are presented.

Global mapping by the visible/near‐infrared OMEGA spectrometer gives the first opportunity to study in detail the characteristics of the 3 μm hydration absorption on the surface of Mars. This feature is caused by bending and stretching vibrations of adsorbed or structural H2O 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 μm 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 μm. 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.

We present a model of radiative transfer through atmospheric particles based on Monte Carlo methods. This model can be used to analyze and remove the contribution of aerosols in remote sensing observations. We have developed a method to quantify the contribution of atmospheric dust in near‐IR spectra of the Martian surface obtained by the OMEGA imaging spectrometer on board Mars Express. Using observations in the nadir pointing mode with significant differences in solar incidence angles, we can infer the optical depth of atmospheric dust, and we can retrieve the surface reflectance spectra free of aerosol contribution. Martian airborne dust properties are discussed and constrained from previous studies and OMEGA data. We have tested our method on a region at 90°E and 77°N extensively covered by OMEGA, where significant variations of the albedo of ice patches in the visible have been reported. The consistency between reflectance spectra of ice‐covered and ice‐free regions recovered at different incidence angles validates our approach. The optical depth of aerosols varies by a factor 3 in this region during the summer of Martian year 27. The observed brightening of ice patches does not result from frost deposition but from a decrease in the dust contamination of surface ice and (to a lower extent) from a decrease in the optical thickness of atmospheric dust. Our Monte Carlo–based model can be applied to recover the spectral reflectance characteristics of the surface from OMEGA spectral imaging data when the optical thickness of aerosols can be evaluated. It could prove useful for processing image cubes from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on board the Mars Reconnaissance Orbiter (MRO).

The OMEGA visible/near‐infrared imaging spectrometer on board Mars Express has observed the southern seasonal cap in late 2004 and 2005 and then in the summer of 2006. These observations extended from the period of maximum extension, close to the southern winter solstice, to the end of the recession at Ls325°. The spectral range and spectral resolution of OMEGA make it possible to monitor the extent and effective grain size of CO2ice and H2O ice on the ground, the level of contamination of CO2ice and H2O ice by dust, and the column density of μm‐sized ice grains in the atmosphere. The CO2seasonal cap is very clean and clear in early southern winter. Contamination by H2O ice spreads eastward from the Hellas basin until the southern spring equinox. During southern spring and summer, there is a very complex evolution in terms of effective grain size of CO2ice and contamination by dust or H2O ice. H2O ice does not play a significant role close to the southern summer solstice. Contamination of CO2ice by H2O ice is only observed close to the end of the recession, as well as the few H2O ice patches already reported by Bibring et al. (2004a). These observations have been compared to the results of a general circulation model, with good qualitative agreement on the distribution of H2O ice on the surface and in the atmosphere. Resolving the remaining discrepancies will improve our understanding of the water cycle on Mars.

After 2 years of operation the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) imaging spectrometer on board Mars Express has acquired data coverage of the Martian surface with spatial resolution varying between 300 m and 4.8 km, depending on the pericenter altitude of the spacecraft's elliptical orbit. We report the global surface distributions of some minerals using the OMEGA observations in the visible and near infrared (VNIR) wavelength domains (0.35–2.5 μm). Global maps of ferric phases, mafic minerals (pyroxenes and olivines), and hydrated minerals have been derived from spectral parameters. The limits of detection in terms of abundance for some minerals of different grain size distributions are given. The distribution of pyroxenes is in general agreement with the mineral maps of previous telescopic and space observations. The Fe3+absorption feature in the visible wavelength region is present everywhere on the surface. The spectra of the bright regions compare with anhydrous nanophase ferric oxides. Terrains with water‐bearing minerals cover a very small fraction of the Martian surface. Olivine (Mg‐rich compositions) is detected in more extensive regions of the pyroxene‐rich zones than previously reported. Olivine with higher iron content and/or larger grain size (>100 μm) is only detected in isolated areas. The mineralogy of the northern low‐albedo regions is discussed in the light of these mineral maps. Chemical alteration or oxidation during extrusion producing a coating or varnish of anhydrous ferric phases over a dark basaltic surface best accounts for the VNIR spectral properties of these regions, although a glassy composition resulting from impact is also considered.

The poles of Mars are known to have recorded recent (<107years) climatic changes. While the south polar region appears to have preserved its million‐year‐old environment from major resurfacing events, except for the small portion containing the CO2residual cap, the discovery of residual water ice units in areas adjacent to the cap provides compelling evidence for recent glaciological activity. The mapping and characterization of these H2O‐rich terrains by Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) on board Mars Express, which have supplemented earlier findings by Mars Odyssey and Mars Global Surveyor, have raised a number of questions related to their origin. We propose that these water ice deposits are the relics of Mars' orbit precession cycle and that they were laid down when perihelion was synchronized with northern summer, i.e., more than 10,000 years ago. We favor precession over other possible explanations because (1) as shown by our General Circulation Model (GCM) and previous studies, current climate is not conducive to the accumulation of water at the south pole due to an unfavorable volatile transport and insolation configuration, (2) the residual CO2ice cap, which is known to cold trap water molecules on its surface and which probably controls the current extent of the water ice units, is geologically younger, (3) our GCM shows that 21,500 years ago, when perihelion occurred during northern spring, water ice at the north pole was no longer stable and accumulated instead near the south pole with rates as high as 1 mm yr−1. This could have led to the formation of a meters‐thick circumpolar water ice mantle. As perihelion slowly shifted back to the current value, southern summer insolation intensified and the water ice layer became unstable. The layer recessed poleward until the residual CO2ice cover eventually formed on top of it and protected water ice from further sublimation. In this polar accumulation process, water ice clouds play a critical role since they regulate the exchange of water between hemispheres. The so‐called “Clancy effect,” which sequesters water in the spring/summer hemisphere coinciding with aphelion due to cloud sedimentation, is demonstrated to be comparable in magnitude to the circulation bias forced by the north‐to‐south topographic dichotomy. However, we predict that the response of Mars' water cycle to the precession cycle should be asymmetric between hemispheres not only because of the topographic bias in circulation but also because of an asymmetry in the dust cycle. We predict that under a “reversed perihelion” climate, dust activity during northern summer is less pronounced than during southern summer in the opposite perihelion configuration (i.e., today's regime). When averaged over a precession cycle, this reduced potential for dust lifting will force a significantly colder summer in the north and, by virtue of the Clancy effect, will curtail the ability of the northern hemisphere to transfer volatiles to the south. This process may have helped create the observed morphological differences in the layered deposits between the poles and could help explain the large disparity in their resurfacing ages.

OMEGA/Mars Express has discovered large outcrops rich in phyllosilicates in the region of Mawrth Vallis, Mars (around 20°W, 25°N). The region is located in Noachian highly cratered terrains, close to the limit of the Martian dichotomy, where the outflow channel Mawrth Vallis cuts the highlands. We have examined this region using OMEGA spectra of the surface from 0.9 μm to 2.6 μm, with spatial sampling from 500 m to 3 km, offering a full coverage of the region. OMEGA spectra show two broad bands centered at 1 μm and 2.2 μm, revealing the presence of clinopyroxene on dark surfaces. Phyllosilicates have been identified by absorption bands at 1.4 μm, 1.9 μm, and 2.2 or 2.3 μm. Comparison with laboratory spectra reveals similarities with Al‐OH smectites such as montmorillonites, or Fe‐ or Mg‐OH smectites such as nontronite. A precise location of the phyllosilicate‐rich areas on visible HRSC images indicates that they are placed exclusively on bright outcrops, mostly on the plateaus, dated to the Noachian period. On HRSC and MOC images the phyllosilicate‐rich outcrops reveal strong erosional features such as numerous residual buttes composed of layers a few meters thick. The phyllosilicate‐rich unit corresponds to a geological unit more than 100 m thick, over a horizontal extension approximately of 300 km × 400 km. This unit implies a large volume of altered rocks, either in situ or after transport and deposition, in Noachian terrains, revealing a different climatic and geologic environment from the present one.

Nili Fossae is a region with fresh exposures of old material. OMEGA spectral data have revealed a strong diversity of minerals in the Nili Fossae region, including mafic minerals and phyllosilicates. Phyllosilicates are found to be primarily Fe‐rich smectites (a type of clay minerals). These phyllosilicates are observed only on the Noachian crust, in four types of locations: Three are without any signatures other than phyllosilicates, on (1) smooth layered deposits, (2) massive exhumed material, and (3) crater ejecta, and one type of location is associated with olivine (4) over an altered mafic unit. This diversity suggests a widespread liquid water activity and a variety of alteration processes. The formation of phyllosilicates underwent considerable reduction, if not cessation, by the time of formation of Syrtis Major lava flows in the Hesperian period. Phyllosilicates are spatially and chronologically disconnected from the fluvial and depositional landforms observed in the Nili Fossae region. Observations suggest that these landforms formed after the extensive formation of phyllosilicates during short periods of time and/or temperatures close to freezing.

The Nili Fossae region located on the northwestern quadrant of the Isidis Basin, Mars, displays superb exposures of bedrock outcrops that reveal mineralogy and composition of the crust. Previous work has shown that this region exhibits the largest exposures of olivine‐dominated rock units on Mars. Visible‐infrared imaging spectrometer data acquired by OMEGA were calibrated to surface reflectance and analyzed to determine surface mineralogy. The dominant minerals identified are iron‐bearing mafic silicates (low‐ and high‐calcium pyroxene, olivine) and water‐bearing phyllosilicate (iron‐rich smectite clay). The strength and position of mineral absorption features were used to produce mineral indicator maps for the dominant species, and the maps were integrated with high‐resolution imaging from the THEMIS, MOC, and HRSC instruments, and MOLA topography. We show that olivine and phyllosilicate occur in spatially distinct outcrops; the olivine‐bearing rock unit is a meters‐ to tens of meters‐thick cap unit resting on phyllosilicate‐bearing bedrock, and the phyllosilicate units predate the Isidis basin‐forming event. On the basis of superposition, crosscutting, and geomorphic relationships, we interpret the emplacement of the olivine‐bearing units as having been contemporaneous with the Isidis impact event. By analogy with the Orientale basin on the Moon, we propose that the olivine‐bearing unit represents the surface exposure of the impact melt from the Isidis impact event. These results demonstrate that large regions of crust had been altered in the presence of water prior to the date of the Isidis basin‐forming event in the Late Noachian (≈3.96 Ga).

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) is the most recent spectrometer to arrive at Mars. The instrument is a hyperspectral imager covering visible to near‐infrared wavelengths (0.37–3.92 μm at 6.55 nm/channel). Summary products based on multispectral parameters will be derived from reflectances in key wavelengths for every CRISM observation. There are 44 summary products formulated to capture spectral features related to both surface mineralogy and atmospheric gases and aerosols. The intent is to use the CRISM summary products as an analysis tool to characterize composition as well as a targeting tool to identify areas of mineralogic interest to observe at higher spectral and spatial resolution. This paper presents the basis for the summary products and examines the validity of the above approach using data from the Mars Express OMEGA instrument, a visible/near‐infrared imaging spectrometer with spatial and spectral coverage similar to that of CRISM. Our study shows that the summary products vary in utility, but succeed in capturing the known diversity of the Martian surface and variability of the Martian atmosphere, and successfully highlight locations with strong spectral signatures. Thus the CRISM summary products will be useful in both operations and science applications. Caveats and limitations related to the summary products and their interpretation are presented to assist with their application by the community at large.

This paper focuses on the spectral characteristics of lobate ejecta across Syrtis Major volcano, a dark region of Mars presenting a mafic composition as revealed from the Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) instrument. Two spectrally distinct crater types are identified. Type I is enriched in high‐calcium pyroxene (HCP) relative to the volcanic rocks. Type II is similar to the lava flow mineralogy driven by HCP, compared to the Noachian crust dominated by low‐calcium pyroxene (LCP). Type I craters are systematically younger than type II. The type II mineralogy has been likely affected by a long‐term weathering in a cold environment for the last 2 Ga. The axisymmetric spectral signatures of type I ejecta appear dominated by rock‐forming minerals rather than by soils and reflect the composition of excavated materials offering a window for exploring the subsurface. A progressive change in the slope of the spectra around 1.5–2 μm is observed across the ejecta layers, pointing at a change in the HCP/LCP ratio. The deconvolution of the spectra by the Modified Gaussian Model unravels a maximum of HCP/LCP band strength ratio located between 1.2 and 3 crater radii for all type I craters. Using Z‐modeling, this observation translates into a maximum in the HCP/LCP abundances at a few hundreds of meters depth and suggests a homogeneous subsurface structure of the volcanic edifice. Below this horizon the HCP/LCP decrease may reveal a more ancient lava composition or the signature of the underlying Noachian crust.