Sebastien Lebonnois

Density and temperatures of the upper Martian atmosphere measured by stellar occultations with Mars Express SPICAM

[1] We present one Martian year of observations of the density and temperature in the upper atmosphere of Mars (between 60 and 130 km) obtained by the Mars Express ultraviolet spectrometer Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars (SPICAM). Six hundred sixteen profiles were retrieved using stellar occultations technique at various latitude and longitude. The atmospheric densities exhibit large seasonal fluctuations due to variations in the dust content of the lower atmosphere which controls the temperature and, thus, the atmospheric scale height, below 50 km. In particular, the year observed by SPICAM was affected by an unexpected dust loading around Ls = 130° which induced a sudden increase of density above 60 km. The diurnal cycle could not be analyzed in detail because most data were obtained at nighttime, except for a few occultations observed around noon during northern winter. There, the averaged midday profile is found to slightly differ from the corresponding midnight profile, with the observed differences being consistent with propagating thermal tides and variations in local solar heating. About 6% of the observed mesopause temperatures exhibits temperature below the CO 2 frost point, especially during northern summer in the tropics. Comparison with atmospheric general circulation model predictions shows that the existing models overestimate the temperature around the mesopause (above 80 to 100 km) by up to 30 K, probably because of an underestimation of the atomic oxygen concentration which controls the CO 2 infrared cooling.

Three‐dimensional modeling of ozone on Mars

We present the first three-dimensional model simulations of ozone on Mars. The model couples a state-of-the-art gas-phase photochemical package to the general circulation model developed at Laboratoire de Me´te´orologie Dynamique (LMD). The results do not contradict the classical picture of a global anticorrelation between the ozone (O3) and water vapor columns. However, the quantitative approach shows significant departures from this relationship, related to substantial orbital variations in the O3 vertical distribution. Over the period Ls = 180–330, low-latitude to midlatitude O3 is essentially confined below 20 km, has a weak diurnal cycle, and is largely modulated by topography. During the rest of the year (Ls = 330–180) the model predicts the formation of an O3 layer at 25–70 km altitude, characterized by nighttime densities about one order of magnitude larger than during the day. Throughout the year, high-latitude O3 peaks near the surface and reaches maximum integrated amounts (40 mm-atm) in the winter polar vortex, with considerable (30 to 50%) dynamically induced day-to-day variations. The most stringent comparison to date with O3 observational data reveals contrasted results. A good quantitative agreement is found in the postperihelion period (Ls = 290–10), but the model fails to reproduce O3 columns as large as those measured near aphelion (Ls = 61–67). Current uncertainties in absorption cross sections and gas-phase kinetics data do not seem to provide credible explanations to explain this discrepancy, which may suggest the existence of heterogeneous processes.

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.

Heterogeneous chemistry in the atmosphere of Mars

Hydrogen radicals are produced in the martian atmosphere by the photolysis of water vapour and subsequently initiate catalytic cycles that recycle carbon dioxide from its photolysis product carbon monoxide. These processes provide a qualitative explanation for the stability of the atmosphere of Mars, which contains 95 per cent carbon dioxide. Balancing carbon dioxide production and loss based on our current understanding of the gas-phase chemistry in the martian atmosphere has, however, proven to be difficult. Interactions between gaseous chemical species and ice cloud particles have been shown to be key factors in the loss of polar ozone observed in the Earth’s stratosphere, and may significantly perturb the chemistry of the Earth’s upper troposphere. Water-ice clouds are also commonly observed in the atmosphere of Mars and it has been suggested previously that heterogeneous chemistry could have an important impact on the composition of the martian atmosphere. Here we use a state-of-the-art general circulation model together with new observations of the martian ozone layer to show that model simulations that include chemical reactions occurring on ice clouds lead to much improved quantitative agreement with observed martian ozone levels in comparison with model simulations based on gas-phase chemistry alone. Ozone is readily destroyed by hydrogen radicals and is therefore a sensitive tracer of the chemistry that regulates the atmosphere of Mars. Our results suggest that heterogeneous chemistry on ice clouds plays an important role in controlling the stability and composition of the martian atmosphere.

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.

Vertical distribution of ozone on Mars as measured by SPICAM/Mars Express using stellar occultations

[i] The ultraviolet spectrometer of the SPICAM instrument on board the European Mars Express mission has performed stellar occultations to probe the atmosphere. Vertical profiles of ozone are retrieved from inversion of transmission spectra in the altitude range 20-30 to 70 km. They are analyzed here as functions of latitude and season of the observations. These occultations have been monitored on the night side, from northern spring equinox (L s = 8°) to northern winter solstice (L s = 270°). The profiles show the presence of two ozone layers: (1) one located near the surface, the top of which is visible below 30 km altitude, and (2) one layer located in the altitude range 30 to 60 km, a feature that is highly variable with latitude and season. This layer is first seen after L s = 11°, and the ozone abundance at the peak tends to increase until L s ∼ 40°, when it stabilizes around 6-8 x 10 9 cm -3 . After southern winter solstice (L s ∼ 100°), the peak abundance starts decreasing again, and this ozone layer is no longer detected after L s ∼ 130°. A recent model (Lefevre et al., 2004) predicted the presence of these ozone layers, the altitude one being only present at night. Though the agreement between model and observations is quite good, this nocturnal altitude layer is present in SPICAM data over a less extended period than predicted. Though a possible role of heterogeneous chemistry is not excluded, this difference is probably linked to the seasonal evolution of the vertical distribution of water vapor.

Monitoring atmospheric phenomena on Titan

For the past 8 years (1998-2005), we have used adaptive optics imaging (with VLT/NACO and CFHT/PUEO) to explore Titans atmosphere, which is currently scrutinized in situ by the Cassini-Huygens mission. In the course of our work, we have found variations, such as as seasonal and diurnal effects, as well as temporary features in the southern polar region. The north-south asymmetry is shown to have changed since 2000 in the near-IR and to be currently organized in a brighter northern than southern pole. We study this evolution here. With our data, we also have new significant statistical evidence of diurnal effects in Titans stratosphere, with a brighter (as much as 19%) morning limb appearing in our images in many cases, when the phase effect is expected on the evening side. The southern bright feature is probably a time-limited seasonal and/or meteorological phenomenon, revolving around the south pole (confined in its motion within the 80

Net exchange parameterization of thermal infrared radiative transfer in Venus' atmosphere

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The Venus nighttime atmosphere as observed by the VIRTIS‐M instrument. Average fields from the complete infrared data set

S parallel) and located somewhere in the upper troposphere (18-40 km of altitude). Its behavior and possible nature are discussed here.

Impact of the seasonal variations of composition on the temperature field of Titan's stratosphere

[1] Thermal radiation within Venus atmosphere is analyzed in close details. Prominent features are identified, which are then used to design a parameterization (a highly simplified and yet accurate enough model) to be used in General Circulation Models. The analysis is based on a net exchange formulation, using a set of gaseous and cloud optical data chosen among available referenced data. The accuracy of the proposed parameterization methodology is controlled against Monte Carlo simulations, assuming that the optical data are exact. Then, the accuracy level corresponding to our present optical data choice is discussed by comparison with available observations, concentrating on the most unknown aspects of Venus thermal radiation, namely the deep atmosphere opacity and the cloud composition and structure.