Stephane Erard

Perennial water ice identified in the south polar cap of Mars.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

The surface of Syrtis Major: Composition of the volcanic substrate and mixing with altered dust and soil

Syrtis Major is an old, low relief volcanic plateau near the equatorial regions of Mars. It is a persistent low-albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and/or locally derived surface material and debris. Spatially resolved variations in surface spectral properties, and therefore composition, are investigated with data from the Imaging Spectrometer for Mars (ISM) instrument. ISM obtained 128 wavelength channel spectra from 0.76 to 3.16 μm for contiguous pixels approximately 22 × 22 km in size across much of the plateau. The value and spatial distribution of four primary spectral variables (albedo, continuum slope, wavelength of the ferric-ferrous band minimum, area of the ferric-ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these maps shows that although there is a high degree of overall spectral variability on the plateau, the key indicators of mafic mineralogy are relatively homogeneous. Detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite-bearing basalts and the pyroxene composition in the basalts is estimated to be 0.275± 0.075 Ca/(Ca+Fe+Mg) and 0.3± 0.1 Fe/(Fe+Ca+Mg). Additional mineral components may include olivine, feldspar, and glass. Most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and/or locally derived surface material and debris with highly altered dust and soil. In western Syrtis Major the altered material is a transient component on the surface or occurs in large spatially coherent patches (e.g., crater rims). In eastern Syrtis Major it is apparent that the dust components are firmly fixed to the basaltic substrate as a stable oxide rind or coating.

South-polar features on Venus similar to those near the north pole

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright ‘dipole’ feature surrounded by a cold ‘collar’ at its north pole. The polar dipole is a ‘double-eye’ feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus’ south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

In situ compositions of Martian volcanics: Implications for the mantle

The primary objective of this analysis is to determine the mineralogic composition of relatively pristine terrains exposed on the surface of Mars. This analysis is conducted using imaging spectrometer data acquired in 1989 by the Imaging Spectrometer for Mars (ISM) instrument that was onboard the Phobos-II spacecraft. The ISM instrument acquired some of the highest spectral (64 channels between 0.77 and 3.14 μm) and spatial resolution (≈22 km/pixel) observations to date of Mars, focused on the equatorial region. Relatively unaltered surfaces were identified in these data on the basis of their spectral properties (low albedo, weak spectral slope, absence of ferric absorptions), and were found in volcanic terrains of Syrtis Major, Valles Marineris, Ophir Planum, and Sinus Meridiani/Oxia Palus. Spectra representative of the diversity among terrains were extracted for detailed analysis using the Modified Gaussian Model to determine the primary mafic mineralogy, It was determined that the mafic mineralogy of the regions investigated is dominated by low-calcium and high-calcium pyroxene (LCP, HCP), and that there was homogeneity within a given region but some heterogeneity between regions in the relative abundance of LCP to HCP. The two-pyroxene mineralogy is consistent with the mineralogies of the basaltic SNC meteorites believed to have originated on Mars. Two-pyroxene volcanic mineralogies are indicative of large degrees of partial melting (e.g., komatiites) or a mantle source depleted in aluminum. Integration of these findings with previous studies of Mars and the SNC meteorites indicates that (1) the SNC mineralogies are representative of large volcanic regions of Mars with surface ages as old as 2–3 Ga, (2) the martian mantle was depleted in aluminum at least as long ago as the oldest terrain analyzed (Ophir Planum), and (3) there has been little evolution in the composition of mantle source regions as the 180 Ma SNC mineralogies are comparable to those of the oldest terrains analyzed.

A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90–120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 µm, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at ∼115 km and varies with solar zenith angle over a range of ∼10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km ± 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

The 2.4-45 mu m spectrum of Mars observed with the Infrared Space Observatory

The spectrum of Mars at 2.4-45 mum has been observed on July 31, 1997 (L-s = 157 degrees) by the Short-Wavelength Spectrometer of the Infrared Space Observatory. The data consist of a high signal to noise, complete grating spectrum (resolving power R similar to 1500-2500) and portions of the 20-45 mum spectrum observed in Fabry-Perot mode (R similar to 31000). The data show the infrared bands of known martian atmospheric species (CO2, H2O, and CO) with an unprecedented amount of details. The vertical distribution of H2(O) is determined, showing saturation near 10 km. Evidence for scattering in the saturated CO2 band at 2.7 mum and for fluorescence emission in the CO2 4.3 pm band is obtained. No detection of new atmospheric species is achieved, but upper limits are obtained for CH4 and H2CO. In the solar reflected part of the spectrum, which dominates at lambda less than or equal to 4.2 mum, the surface reflectance clearly shows the hydration band with maximum absorption at 2.9 mum, from which a 2.0 -2.7% (by weight) water content in the martian uppermost layer is estimated. A decrease of reflectance from 3.8 to 5 mum is also seen. This behaviour is consistent with basalts and palagonite, but not hematite. Ln the thermal part, mineralogic signatures at 5-12 mum are globally consistent with a basaltic composition. Specific minima an also detected at 5.7, 6.3 (tentative), 7.2 and 11.1 mum. Reexamination of earlier datasets indicates that the latter two have been observed before, although generally not discussed. The presence of additional absorptions at 26.5, 31 and 33.5 mum is also indirectly suggested. Carbonate minerals are tentatively detected from this ensemble of features, though no single carbonate species can be unambiguously identified

Discrimination between maturity and composition of lunar soils from integrated Clementine UV‐visible/near‐infrared data: Application to the Aristarchus Plateau

The reflectance spectrum of a lunar soil is mainly dominated by the composition and the degree of exposure to space weathering processes such as micrometeorite bombardment and solar wind implantation. The spectral alteration effects of space weathering should be removed for accurately investigating the composition of the lunar surface using remote sensing data. In this paper we show that the integration of the Clementine UV-visible (UVVIS) and near-infrared (NIR) channels provides an improved evaluation of the spectral alteration. The depth of the mafic absorption feature at 0.95 μm is also better defined by combining the UVVIS and NIR data. Laboratory spectra of lunar soil samples indicate that the continuum slope derived from the 1500/750 nm ratio is closely related to the concentration of fine-grained submicroscopic iron (Is). The continuum slope therefore provides an evaluation of the spectral alteration of the surface, which can be subtracted from the 1 or 2 μm absorption band depths to retrieve compositional information. This method has been applied to the Aristarchus plateau, which exhibits a broad range of mineralogical composition and maturity. A nine-channel multispectral mosaic of 680 Clementine images of the Aristarchus plateau has been processed. Eight telescopic spectra have been used to check the validity of the reduction process for the near infrared bands. The 1 μm absorption band, once corrected for spectral alteration, provides an evaluation of the initial FeO content in mafic silicates (mafic iron). Lunar soil samples show that it is possible to quantitatively map mafic iron with this technique. Our results are in good agreement with those obtained using the algorithm of Lucey et al. [1995,1998a], which is based on UVVIS bands alone. The mafic iron content and total iron content which can be derived from the combined UVVIS and NIR data sets are less sensitive to local slopes than that derived from Lucey et al.s method. This new method could therefore be useful for investigating areas at middle to high latitudes. Removing spectral alteration from the 2000/1500 nm ratio also makes possible a better discrimination between olivine and pyroxene within identified mare basalts on the Aristarchus plateau.

First detection of hydroxyl in the atmosphere of Venus

Context. Airglow emissions, such as previously observed from NO and O2(a−X )( 0−0) on Venus, provide insight into the chemical and dynamical processes that control the composition and energy balance in the upper atmospheres of planets. The OH airglow emission has been observed previously only in the Earth’s atmosphere where it has been used to infer atomic oxygen abundances. The O2(a − X )( 0−1) airglow emission also has only been observed in the Earth’s atmosphere, and neither laboratory nor theoretical studies have reached a consensus on its transition probability. Aims. We report measurements of night-side airglow emission in the atmosphere of Venus in the OH (2−0), OH (1−0), O2(a − X )( 0−1), and O2(a − X )( 0−0) bands. This is the first detection of the first three of these airglow emissions on another planet. These observations provide the most direct observational constraints to date on H, OH, and O3, key species in the chemistry of Venus’ upper atmosphere. Methods. Airglow emission detected at wavelengths of 1.40−1.49 and 2.6−3.14 µm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the Venus Express spacecraft is attributed to the OH (2−0) and (1−0) transitions, respectively, and compared to calculations from a photochemical model. Simultaneous limb observations of airglow emission in the O2(a − X )( 0−0) and (0−1) bands at 1.27 and 1.58 µm, respectively, were used to derive the ratio of the transition probabilities for these bands. Results. The integrated emission rates for the OH (2−0) and (1−0) bands were measured to be 100 ± 40 and 880 ± 90 kR respectively, both peaking at an altitude of 96 ± 2 km near midnight local time for the considered orbit. The measured ratio of the O2(a −X )( 0−0) and (0−1) bands is 78 ± 8. Conclusions. Photochemical model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth. The observed ratio of the intensities of the O2(a − X )( 0−0) and (0−1) bands implies the ratio of their transition probabilities is 63 ± 6.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

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.

Three-dimensional direct simulation Monte-Carlo modeling of the coma of comet 67P/Churyumov-Gerasimenko observed by the VIRTIS and ROSINA instruments on board Rosetta

Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H2O and CO2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H2O and CO2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H2O and CO2 coma of comet 67P. Results. We obtain activity distribution of H2O and CO2 showing a dominant source of H2O in the Hapi region, while more CO2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINA-DFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H2O and CO2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H2O and CO2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P’s coma. This model can be used for further data analyses, such as dust modeling, in a future work.