P. Drossart

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ−1), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Hyperspectral imaging of convective CO2 ice clouds in the equatorial mesosphere of Mars

[1]xa0A unique feature of the Martian climate is the possibility for carbon dioxide, the main atmospheric constituent, to condense as ice. CO2 ice is usually detected as frost but is also known to exist as clouds. This paper presents the first unambiguous observation of CO2 ice clouds on Mars. These images were obtained by the visible and near-infrared imaging spectrometer OMEGA on board Mars Express. The data set encompasses 19 different occurrences. Compositional identification is based on the detection of a diagnostic spectral feature around 4.26 μm which is produced by resonant scattering of solar photons by mesospheric CO2 ice particles in a spectral interval otherwise dominated by saturated gaseous absorption. Observed clouds exhibit a strong seasonal and geographic dependence, concentrating in the near-equatorial regions during two periods before and after northern summer solstice (Ls 45° and 135°). Radiative transfer modeling indicates that the 4.26 μm feature is very sensitive to cloud altitude, opacity, and particle size, thereby explaining the variety of spectra associated with the cloud images. On two orbits, the simultaneous detection of clouds with their shadow provides straightforward and robust estimates of cloud properties. These images confirm the conclusions established from modeling: clouds are thick, with normal opacities greater than 0.2 in the near infrared, and are lofted in the mesosphere above 80 km. The mean radius of CO2 ice crystals is found to exceed 1 μm, an unexpected value considering this altitude range. This finding implies the existence of high-altitude atmospheric updrafts which are strong enough to counteract the rapid gravitational fall of particles. This statement is consistent with the cumuliform morphology of the clouds which may be linked to a moist convective origin generated by the latent heat released during CO2 condensation.

The Surface Composition and Temperature of Asteroid 21 Lutetia As Observed by Rosetta/VIRTIS

A spacecraft flyby of an asteroid reveals a high-density body that is more like a planetesimal than a rubble pile. The Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS) on Rosetta obtained hyperspectral images, spectral reflectance maps, and temperature maps of the asteroid 21 Lutetia. No absorption features, of either silicates or hydrated minerals, have been detected across the observed area in the spectral range from 0.4 to 3.5 micrometers. The surface temperature reaches a maximum value of 245 kelvin and correlates well with topographic features. The thermal inertia is in the range from 20 to 30 joules meter−2 kelvin−1 second−0.5, comparable to a lunarlike powdery regolith. Spectral signatures of surface alteration, resulting from space weathering, seem to be missing. Lutetia is likely a remnant of the primordial planetesimal population, unaltered by differentiation processes and composed of chondritic materials of enstatitic or carbonaceous origin, dominated by iron-poor minerals that have not suffered aqueous alteration.

A latitudinal survey of CO, OCS, H2O, and SO2 in the lower atmosphere of Venus: Spectroscopic studies using VIRTIS‐H

[1]xa0The high-resolution channel (R ≃ 2000) of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument (VIRTIS-H) aboard Venus Express has provided numerous spectra of the nightside infrared thermal emission in the 2.3-μm window. Mixing ratios of various minor species in the 30–40 km range could therefore be inferred using this spectral window at higher latitudes accessible to the spacecraft but which cannot be observed from Earth. The previously known enhancement in carbon monoxide (CO) toward high latitudes is confirmed and extended up to 60° with a mixing ratio varying from 24 ± 3 to 31 ± 2 ppmv at 36 km. Measurements of carbonyl sulfide (OCS) also agree with the previously suspected latitudinal variations that are anticorrelated with those of CO, ranging between 2.5 ± 1 and 4 ± 1 ppmv at 33 km. New constraints were also derived on the mean abundance of water vapor (H2O, 31 ± 2 ppmv) and sulfur dioxide (SO2, 130 ± 50 ppmv) in the probed altitude range. CO and OCS variations are interpreted as caused by large-scale vertical motions, an explanation under current testing by various chemical and dynamical modeling. In such a case, these variations may help constrain the chemical time scale of those species in the lower troposphere.

VARIATIONS IN VENUS CLOUD-PARTICLE PROPERTIES: A NEW VIEW OF VENUS'S CLOUD MORPHOLOGY AS OBSERVED BY THE GALILEO NEAR INFRARED MAPPING SPECTROMETER

Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer, we have studied the correlation of 1.74 and 2.30 μm radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2′ and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2′ particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

Venus’s Southern Polar Vortex Reveals Precessing Circulation

Observations with the Venus Express Orbiter reveal complex polar atmospheric dynamics. Initial images of Venus’s south pole by the Venus Express mission have shown the presence of a bright, highly variable vortex, similar to that at the planet’s north pole. Using high-resolution infrared measurements of polar winds from the Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, we show the vortex to have a constantly varying internal structure, with a center of rotation displaced from the geographic south pole by ~3 degrees of latitude and that drifts around the pole with a period of 5 to 10 Earth days. This is indicative of a nonsymmetric and varying precession of the polar atmospheric circulation with respect to the planetary axis.

Latitudinal distribution of carbon monoxide in the deep atmosphere of Venus

Abstract A large number of i.r. spectra of Venus was obtained using the Near-Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft, during the February 1990 encounter. Preliminary results show an apparent increase in the tropospheric CO volume mixing ratio (vmr) in the northern polar region. Other possible explanations of the observations are examined and rejected and an increase of the CO abundance north of 47°N of (35 ± 15)% is inferred. Some possible causes of this enhancement are suggested.

Search for spatial variations of the H2O abundance in the lower atmosphere of Venus from NIMS-Galileo

Abstract The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS), recorded during the Galileo flyby of Venus, are analysed to retrieve the water vapour abundance variations in the lower atmosphere of Venus at night. The 1.18 μm spectral window, which probes altitude levels below 20 km, is used for this purpose. Constraints on the CO2 continuum and far-wing opacity from existing ground-based high-resolution observations are included in the modelling of the NIMS spectra. The NIMS measurements can be fitted with a water vapour mixing ratio of 30 ± 15 ppm, in agreement with analyses of ground-based nightside observations. The water vapour abundance shows no horizontal variations exceeding 20% over a wide latitude range (40°S, 50°N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O2 fluorescence emission at 1.27 μm is observed at a latitude of 40°S, over a spatial area about 100 km wide.

First observations of H2O and CO2 vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations. n nAims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). These species have different volatility and are key species of cometary ices. n nMethods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν3 vibrational bands of H2O and CO2 at 2.67 and 4.27 μm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions. n nResults. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO2 is outgassing from both illuminated and non-illuminated regions, which indicates that CO2 sublimates at a depth that is below the diurnal skin depth. The CO2/H2O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO2/H2O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H2O < 1.9% and CO/CO2< 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 1025 s-1, provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90–100 K are derived from H2O and CO2 averaged spectra.

Retrieval of air temperature profiles in the Venusian mesosphere from VIRTIS-M data: Description and validation of algorithms

[1]xa0We present here methods developed for the retrieval of air temperature profiles in the Venusian mesosphere from the absolute radiances measured by the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the Venus Express satellite. The infrared M channel of the instrument acquires multispectral images between 1000 and 5000 nm. In nighttime measurements, radiance in the range 3800–5000 nm is dominated by the thermal emission and absorption by the clouds and carbon dioxide. Since the latter is the main atmospheric component, it is possible to exploit the strong variability of its opacity in this spectral range, as resolved by the instrument, to reconstruct the vertical air temperature profile as a function of pressure. In this context we decided to adopt the Twomey et al. (1977) relaxation scheme. The resulting code was extensively tested on a set of simulated VIRTIS-M data. Comparison of the known input conditions with the results of analysis code allowed us to evaluate the systematic and random errors affecting the retrievals procedures on a statistical basis. The code returns the vertical air temperature profile with an uncertainty of less than 1 K in the region between 70 and 7 mbar (66 and 77 km above the reference surface) and less than 4 K throughout the entire range 100–0.1 mbar (64–95 km). Finally, we present the first examples of the code applied to actual measured Venusian data, demonstrating its capability to achieve a satisfactory modeling of the observations and provide physically reasonable results.