A. Martinez, S. Lebonnois, E. Millour, T. Pierron, E. Moisan, G. Gilli, and F. Lefèvre. Exploring the variability of the Venusian thermosphere with the IPSL Venus GCM. Icarus, 389:115272, January 2023. [ bib | DOI | PDF version | ADS link ]

Recent simulations of the Institut Pierre-Simon Laplace (IPSL) Venus Global Climate Model (VGCM) developed at the Laboratoire de Météorologie Dynamique (LMD) were performed with a model top raised from ~10^-5 (~150 km) to ~10^-8 Pa (180-250 km; upper boundary). The parameterizations of non-LTE CO₂ near infrared heating rates and of non-orographic gravity waves were improved. In addition, a tuning of atomic oxygen production was introduced to improve related effects (heating and cooling) and resulting thermospheric number densities. This work focusses on validating the modelled thermospheric structure using data from the Pioneer Venus, Magellan and Venus Express missions which cover similar and complementary (equator and pole) regions at different periods of solar activity, typically above altitudes of 130 km. This version of the IPSL VGCM shows good agreement with the diurnal evolution of the exospheric temperature at the equator reconstructed from the atomic oxygen scale height of the Pioneer Venus Orbiter Neutral Mass Spectrometer data. The model is also able to reproduce the sensitivity of the exospheric temperature and species density to the EUV flux of the solar high activity period (from 180 to 230 solar flux unit; s.f.u). However, to fit with the PV-ONMS density observations, it was necessary to increase the photodissociation of CO₂ into CO and O above 135 km by a factor of 10. Indeed, our study points to the importance of an additional source of oxygen and carbon monoxide production above 130 km other than CO₂ photolysis to explain the vertical profiles of CO and O number density in the thermosphere. Moreover, the presence of a GW drag at altitudes above 140 km has a significant impact on the nightside temperature value and its distribution.

J. B. Garvin, S. A. Getty, G. N. Arney, N. M. Johnson, E. Kohler, K. O. Schwer, M. Sekerak, A. Bartels, R. S. Saylor, V. E. Elliott, C. S. Goodloe, M. B. Garrison, V. Cottini, N. Izenberg, R. Lorenz, C. A. Malespin, M. Ravine, C. R. Webster, D. H. Atkinson, S. Aslam, S. Atreya, B. J. Bos, W. B. Brinckerhoff, B. Campbell, D. Crisp, J. R. Filiberto, F. Forget, M. Gilmore, N. Gorius, D. Grinspoon, A. E. Hofmann, S. R. Kane, W. Kiefer, S. Lebonnois, P. R. Mahaffy, A. Pavlov, M. Trainer, K. J. Zahnle, and M. Zolotov. Revealing the Mysteries of Venus: The DAVINCI Mission. , 3(5):117, May 2022. [ bib | DOI | arXiv | PDF version | ADS link ]

The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission described herein has been selected for flight to Venus as part of the NASA Discovery Program. DAVINCI will be the first mission to Venus to incorporate science-driven flybys and an instrumented descent sphere into a unified architecture. The anticipated scientific outcome will be a new understanding of the atmosphere, surface, and evolutionary path of Venus as a possibly once-habitable planet and analog to hot terrestrial exoplanets. The primary mission design for DAVINCI as selected features a preferred launch in summer/fall 2029, two flybys in 2030, and descent-sphere atmospheric entry by the end of 2031. The in situ atmospheric descent phase subsequently delivers definitive chemical and isotopic composition of the Venus atmosphere during an atmospheric transect above Alpha Regio. These in situ investigations of the atmosphere and near- infrared (NIR) descent imaging of the surface will complement remote flyby observations of the dynamic atmosphere, cloud deck, and surface NIR emissivity. The overall mission yield will be at least 60 Gbits (compressed) new data about the atmosphere and near surface, as well as the first unique characterization of the deep atmosphere environment and chemistry, including trace gases, key stable isotopes, oxygen fugacity, constraints on local rock compositions, and topography of a tessera.

G. Gilli, T. Navarro, S. Lebonnois, D. Quirino, V. Silva, A. Stolzenbach, F. Lefèvre, and G. Schubert. Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements. Icarus, 366:114432, September 2021. [ bib | DOI | arXiv | PDF version | ADS link ]

An improved high resolution (96 longitude by 96 latitude points) ground- to-thermosphere version of the Institut Pierre-Simon Laplace (IPSL) Venus General Circulation Model (VGCM), including non- orographic gravity waves (GW) parameterization and fine-tuned non-LTE parameters, is presented here. We focus on the validation of the model built from a collection of data mostly from Venus Express (2006-2014) experiments and coordinated ground-based telescope campaigns, in the upper mesosphere/lower thermosphere of Venus (80-150 km). These simulations result in an overall better agreement with temperature observations above 90 km, compared with previous versions of the VGCM. Density of CO₂ and light species, such as CO and O, are also comparable with observations in terms of trend and order of magnitude. Systematic biases in the temperature structure are found between 80 and 100 km approximately (e.g. GCM is 20 to 40 K warmer than measurements) and above 130 km at the terminator (e.g. GCM is up to 50 K colder than observed). Possible candidates for those discrepancies are the uncertainties on the collisional rate coefficients used in the non-LTE parameterization (above 130 km), and assumptions on the CO₂ mixing ratio made for stellar/solar occultation retrievals. Diurnal and latitudinal distribution of dynamical tracers (i.e. CO and O) are also analyzed, in a region poorly constrained by wind measurements and characterized by high variability over daily to weekly timescale. Overall, our simulations indicate that a weak westward retrograde wind is present in the mesosphere, up to about 120 km, producing the CO bulge displacement toward 2 h-3 h in the morning, instead of piling up at the anti-solar point, as for an idealized sub-solar to anti- solar circulation. This retrograde imbalance is suggested to be produced by perturbations of a -0.5ex 5 days Kelvin wave impacting the mesosphere up to 110 km (described in a companion paper Navarro et al., 2021), combined with GW westward acceleration in the lower thermosphere, mostly above 110 km. On the whole, these model developments point to the importance of the inclusion of the lower atmosphere, higher resolution and finely tuned parameterizations in GCM of the Venusian upper atmosphere, in order to shed light on existing observations.

T. Navarro, G. Gilli, G. Schubert, S. Lebonnois, F. Lefèvre, and D. Quirino. Venus' upper atmosphere revealed by a GCM: I. Structure and variability of the circulation. Icarus, 366:114400, September 2021. [ bib | DOI | PDF version | ADS link ]

A numerical simulation of the upper atmosphere of Venus is carried out with an improved version of the Institut Pierre-Simon Laplace (IPSL) full-physics Venus General Circulation Model (GCM). This simulation reveals the organization of the atmospheric circulation at an altitude above 80 km in unprecedented detail. Converging flow towards the antisolar point results in supersonic wind speeds and generates a shock-like feature past the terminator at altitudes above 110 km. This shock-like feature greatly decreases nightside thermospheric wind speeds, favoring atmospheric variability on a hourly timescale in the nightside of the thermosphere. A ~5-day period Kelvin wave originating in the cloud deck is found to substantially impact the Venusian upper atmosphere circulation. As the Kelvin wave impacts the nightside, the poleward meridional circulation is enhanced. Consequently, recombined molecular oxygen is periodically ejected to high latitudes, explaining the characteristics of the various observations of oxygen nightglow at 1 . 27 μm . An analysis of the simulated 1 . 27 μm oxygen nightglow shows that it is not necessarily a good tracer of the upper atmospheric dynamics, since contributions from chemical processes and vertical transport often prevail over horizontal transport. Moreover, dayside atomic oxygen abundances also vary periodically as the Kelvin wave momentarily decreases horizontal wind speeds and enhances atomic oxygen abundances, explaining the observations of EUV oxygen dayglow. Despite the nitrogen chemistry not being currently included in the IPSL Venus GCM, the apparent maximum NO nightglow shifted towards the morning terminator might be explained by the simulated structure of winds.

J. E. Silva, P. Machado, J. Peralta, F. Brasil, S. Lebonnois, and M. Lefèvre. Characterising atmospheric gravity waves on the nightside lower clouds of Venus: a systematic analysis. Astronomy Astrophysics, 649:A34, May 2021. [ bib | DOI | arXiv | PDF version | ADS link ]

We present the detection and characterisation of mesoscale waves on the lower clouds of Venus using images from the Visible Infrared Thermal Imaging Spectrometer onboard the European Venus Express space mission and from the 2 μm camera (IR2) instrument onboard the Japanese space mission Akatsuki. We used image navigation and processing techniques based on contrast enhancement and geometrical projections to characterise morphological properties of the detected waves, such as horizontal wavelength and the relative optical thickness drop between crests and troughs. Additionally, we performed phase velocity and trajectory tracking of wave packets. We combined these observations to derive other properties of the waves such as the vertical wavelength of detected packets. Our observations include 13 months of data from August 2007 to October 2008, and the entire available data set of IR2 from January to November 2016. We characterised almost 300 wave packets across more than 5500 images over a broad region of the globe of Venus. Our results show a wide range of properties and are not only consistent with previous observations but also expand upon them, taking advantage of two instruments that target the same cloud layer of Venus across multiple periods. In general, waves observed on the nightside lower cloud are of a larger scale than the gravity waves reported in the upper cloud. This paper is intended to provide a more in-depth view of atmospheric gravity waves on the lower cloud and enable follow-up works on their influence in the general circulation of Venus.

T. Imamura, J. Mitchell, S. Lebonnois, Y. Kaspi, A. P. Showman, and O. Korablev. Superrotation in Planetary Atmospheres. Space Science Reviews, 216(5):87, July 2020. [ bib | DOI | PDF version | ADS link ]

Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit superrotation. Our understanding of superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus's and Titan's atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus's atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong superrotation. Recent gravity data suggests that these superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this superrotation. Moreover, atmospheric circulation models of tidally locked, strongly irradiated exoplanets have long predicted the existence of equatorial superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of superrotation on these objects.

T. Encrenaz, T. K. Greathouse, E. Marcq, H. Sagawa, T. Widemann, B. Bézard, T. Fouchet, F. Lefèvre, S. Lebonnois, S. K. Atreya, Y. J. Lee, R. Giles, S. Watanabe, W. Shao, X. Zhang, and C. J. Bierson. HDO and SO₂ thermal mapping on Venus. V. Evidence for a long-term anti-correlation. Astronomy Astrophysics, 639:A69, July 2020. [ bib | DOI | PDF version | ADS link ]

Since January 2012, we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). Here, we present new data recorded in February and April 2019 in the 1345 cm-1 (7.4 μm) spectral range, where SO2, CO2, and HDO (used as a proxy for H2O) transitions were observed. The cloud top of Venus was probed at an altitude of about 64 km. As in our previous studies, the volume mixing ratio (vmr) of SO2 was estimated using the SO2/CO2 line depth ratio of weak transitions; the H2O volume mixing ratio was derived from the HDO/CO2 line depth ratio, assuming a D/H ratio of 200 times the Vienna standard mean ocean water. As reported in our previous analyses, the SO2 mixing ratio shows strong variations with time and also over the disk, showing evidence for the formation of SO2 plumes with a lifetime of a few hours; in contrast, the H2O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. We have used the 2019 data in addition to our previous dataset to study the long-term variations of SO2 and H2O. The data reveal a long-term anti-correlation with a correlation coefficient of -0.80; this coefficient becomes -0.90 if the analysis is restricted to the 2014-2019 time period. The statistical analysis of the SO2 plumes as a function of local time confirms our previous result with a minimum around 10:00 and two maxima near the terminators. The dependence of the SO2 vmr with respect to local time shows a higher abundance at the evening terminator with respect to the morning. The dependence of the SO2 vmr with respect to longitude exhibits a broad maximum at 120-200° east longitudes, near the region of Aphrodite Terra. However, this trend has not been observed by other measurements and has yet to be confirmed.

Sebastien Lebonnois. Super-rotating the venusian atmosphere. Science, 368(6489):363-364, April 2020. [ bib | DOI | PDF version | ADS link ]

Among the intriguing mysteries that remain for planetary atmospheres, the phenomenon of super-rotation is still a teasing problem. An atmosphere in super-rotation rotates globally faster than the solid body of the planet. In our solar system, super-rotation is observed on Venus and the largest moon of Saturn, Titan (1). The challenge is to explain how angular momentum can accumulate in the atmosphere and what controls the atmospheric angular momentum budget. On page 405 of this issue, Horinouchi et al. (2) address this by analyzing observation data from the onboard cameras of the Venus-orbiting Akatsuki spacecraft.

S. Lebonnois, G. Schubert, T. Kremic, L. M. Nakley, K. G. Phillips, J. Bellan, and D. Cordier. An experimental study of the mixing of CO₂ and N₂ under conditions found at the surface of Venus. Icarus, 338:113550, March 2020. [ bib | DOI | PDF version | ADS link ]

Based on the only reliable temperature profile available in the deepest ∼10 km layer above Venus' surface (obtained by the VeGa-2 landing probe), the mixing conditions of the main constituents of Venus's atmosphere, CO2 and N2, have been questioned. In this work, we report the results of a series of experiments that were done in the GEER facility at Glenn Research Center to investigate the homogeneity of CO2/N2 gas mixtures at 100 bars and temperatures ranging from ∼296 K to ∼735 K. When the gas mixtures are initially well-mixed, separation of the two gases based on their molecular mass does not occur over the time scales observed; although, small systematic variations in composition remain to be fully interpreted. However, when N2 is injected on top of CO2 (layered fill), the very large density ratio makes it more difficult to mix the two chemical species. Timescales of mixing are of the order of 102 hours over the height of the test vessel (roughly 60 cm), and even longer when the gas mixture is at rest and only molecular diffusion is occurring. At room temperature, close to the critical point of the mixture, large pressure variations are obtained for the layered fill, as N2 slowly mixes into CO2. This can be explained by large density variations induced by the mixing. For conditions relevant to the near-surface atmosphere of Venus, separation of CO2 and N2 based on their molecular mass and due to physical properties of the gas mixture is not demonstrated, but cannot be firmly excluded either. This suggests that if the compositional vertical gradient deduced from the VeGa-2 temperature profile is to be trusted, it would most probably be due to some extrinsic processes (not related to gas properties, e.g. CO2 volcanic inputs) and large mixing time constants.

M. Lefèvre, A. Spiga, and S. Lebonnois. Mesoscale modeling of Venus' bow-shape waves. Icarus, 335:113376, January 2020. [ bib | DOI | arXiv | PDF version | ADS link ]

The Akatsuki instrument LIR measured an unprecedented wave feature at the top of Venusian cloud layer. Stationary bow-shape waves of thousands of kilometers large lasting several Earth days have been observed over the main equatorial mountains. Here we use for the first time a mesoscale model of the Venus's atmosphere with high-resolution topography and fully coupled interactive radiative transfer computations. Mountain waves resolved by the model form large-scale bow shape waves with an amplitude of about 1.5 K and a size up to several decades of latitude similar to the ones measured by the Akatsuki spacecraft. The maximum amplitude of the waves appears in the afternoon due to an increase of the near-surface stability. Propagating vertically the waves encounter two regions of low static stability, the mixed layer between approximately 18 and 30 km and the convective layer between 50 and 55 km. Some part of the wave energy can pass through these regions via wave tunneling. These two layers act as wave filter, especially the deep atmosphere layer. The encounter with these layers generates trapped lee waves propagating horizontally. No stationary waves is resolved at cloud top over the polar regions because of strong circumpolar transient waves, and a thicker deep atmosphere mixed layer that filters most of the mountain waves.

P. Scarica, I. Garate-Lopez, S. Lebonnois, G. Piccioni, D. Grassi, A. Migliorini, and S. Tellmann. Validation of the IPSL Venus GCM Thermal Structure with Venus Express Data. Atmosphere, 10(10):584, September 2019. [ bib | DOI | PDF version | ADS link ]

General circulation models (GCMs) are valuable instruments to understand the most peculiar features in the atmospheres of planets and the mechanisms behind their dynamics. Venus makes no exception and it has been extensively studied thanks to GCMs. Here we validate the current version of the Institut Pierre Simon Laplace (IPSL) Venus GCM, by means of a comparison between the modelled temperature field and that obtained from data by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) and the Venus Express Radio Science Experiment (VeRa) onboard Venus Express. The modelled thermal structure displays an overall good agreement with data, and the cold collar is successfully reproduced at latitudes higher than +/−55°, with an extent and a behavior close to the observed ones. Thermal tides developing in the model appear to be consistent in phase and amplitude with data: diurnal tide dominates at altitudes above 102 Pa pressure level and at high-latitudes, while semidiurnal tide dominates between 102 and 104 Pa, from low to mid-latitudes. The main difference revealed by our analysis is located poleward of 50°, where the model is affected by a second temperature inversion arising at 103 Pa. This second inversion, possibly related to the adopted aerosols distribution, is not observed in data.

N. Sugimoto, M. Abe, Y. Kikuchi, A. Hosono, H. Ando, M. Takagi, I. Garate Lopez, S. Lebonnois, and C. Ao. Observing system simulation experiment for radio occultation measurements of the Venus atmosphere among small satellites. Journal of Japan Society of Civil Engineers, Ser. A2 (Applied Mechanics (AM)), 75(2):I_477-I_486, January 2019. [ bib | DOI | PDF version | ADS link ]

We have developed the Venus AFES (atmospheric GCM (general circulation model) for the Earth Simulator) LETKF (local ensemble transform Kalman filter) data assimilation system (VALEDAS) to make full use of observations. In this study, radio occultation measurements among small satellites are evaluated by the observing system simulation experiment (OSSE) of VALEDAS. Idealized observations are prepared by a French Venus Atmospheric GCM in which the cold collar is realistically reproduced. Reproducibility of the cold collar in VALEDAS is tested by several types of observations. The results show that the cold collar is successfully reproduced by assimilating at least 2 or 3 vertical temperature profiles in the polar region every 4 or 6 hours. Therefore, the radio occultation measurements among three satellites in polar orbits would be promising to improve the polar atmospheric structures at about 40–90 km altitudes.

Y. J. Lee, K.-L. Jessup, S. Perez-Hoyos, D. V. Titov, S. Lebonnois, J. Peralta, T. Horinouchi, T. Imamura, S. Limaye, E. Marcq, M. Takagi, A. Yamazaki, M. Yamada, S. Watanabe, S.-y. Murakami, K. Ogohara, W. M. McClintock, G. Holsclaw, and A. Roman. Long-term Variations of Venuss 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope. Astronomical Journal, 158:126, 2019. [ bib | DOI | arXiv | PDF version | ADS link ]

An unknown absorber near the cloud-top level of Venus generates a broad absorption feature from the ultraviolet (UV) to visible, peaking around 360 nm, and therefore plays a critical role in the solar energy absorption. We present a quantitative study of the variability of the cloud albedo at 365 nm and its impact on Venuss solar heating rates based on an analysis of Venus Express and Akatsuki UV images and Hubble Space Telescope and MESSENGER UV spectral data; in this analysis, the calibration correction factor of the UV images of Venus Express (Venus Monitoring Camera) is updated relative to the Hubble and MESSENGER albedo measurements. Our results indicate that the 365 nm albedo varied by a factor of 2 from 2006 to 2017 over the entire planet, producing a 25%40% change in the low-latitude solar heating rate according to our radiative transfer calculations. Thus, the cloud-top level atmosphere should have experienced considerable solar heating variations over this period. Our global circulation model calculations show that this variable solar heating rate may explain the observed variations of zonal wind from 2006 to 2017. Overlaps in the timescale of the long-term UV albedo and the solar activity variations make it plausible that solar extreme UV intensity and cosmic-ray variations influenced the observed albedo trends. The albedo variations might also be linked with temporal variations of the upper cloud SO₂ gas abundance, which affects the H₂SO₄H₂O aerosol formation.

D. Cordier, D. A. Bonhommeau, S. Port, V. Chevrier, S. Lebonnois, and F. García-Sánchez. The Physical Origin of the Venus Low Atmosphere Chemical Gradient. Astrophysical Journal, 880:82, 2019. [ bib | DOI | arXiv | PDF version | ADS link ]

Venus shares many similarities with the Earth, but concomitantly, some of its features are extremely original. This is especially true for its atmosphere, where high pressures and temperatures are found at the ground level. In these conditions, carbon dioxide, the main component of Venus atmosphere, is a supercritical fluid. The analysis of VeGa-2 probe data has revealed the high instability of the region located in the last few kilometers above the ground level. Recent works have suggested an explanation based on the existence of a vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our goal was then to identify which physical processes could lead to the establishment of this intriguing nitrogen gradient, in the deep atmosphere of Venus. Using an appropriate equation of state for the binary mixture CO₂N₂ under supercritical conditions, and also molecular dynamics simulations, we have investigated the separation processes of N₂ and CO₂ in the Venusian context. Our results show that molecular diffusion is strongly inefficient, and potential phase separation is an unlikely mechanism. We have compared the quantity of CO₂ required to form the proposed gradient with what could be released by a diffuse degassing from a low volcanic activity. The needed fluxes of CO₂ are not so different from what can be measured over some terrestrial volcanic systems, suggesting a similar effect at work on Venus.

T. Encrenaz, T. K. Greathouse, E. Marcq, H. Sagawa, T. Widemann, B. Bézard, T. Fouchet, F. Lefèvre, S. Lebonnois, S. K. Atreya, Y. J. Lee, R. Giles, and S. Watanabe. HDO and SO₂ thermal mapping on Venus. IV. Statistical analysis of the SO₂ plumes. Astronomy Astrophysics, 623:A70, 2019. [ bib | DOI | PDF version | ADS link ]

Since January 2012 we have been monitoring the behavior of sulfur dioxide and water on Venus, using the Texas Echelon Cross-Echelle Spectrograph (TEXES) imaging spectrometer at the NASA InfraRed Telescope Facility (IRTF, Mauna Kea Observatory). We present here the observations obtained between January 2016 and September 2018. As in the case of our previous runs, data were recorded around 1345 cm^-1 (7.4 μm). The molecules SO₂, CO₂, and HDO (used as a proxy for H₂O) were observed, and the cloudtop of Venus was probed at an altitude of about 64 km. The volume mixing ratio of SO₂ was estimated using the SO₂/CO₂ line depth ratios of weak transitions; the H₂O volume mixing ratio was derived from the HDO/CO₂ line depth ratio, assuming a D/H ratio of 200 times the Vienna Standard Mean Ocean Water (VSMOW). As reported in our previous analyses, the SO₂ mixing ratio shows strong variations with time and also over the disk, showing evidence of the formation of SO₂ plumes with a lifetime of a few hours; in contrast, the H₂O abundance is remarkably uniform over the disk and shows moderate variations as a function of time. We performed a statistical analysis of the behavior of the SO₂ plumes, using all TEXES data between 2012 and 2018. They appear mostly located around the equator. Their distribution as a function of local time seems to show a depletion around noon; we do not have enough data to confirm this feature definitely. The distribution of SO₂ plumes as a function of longitude shows no clear feature, apart from a possible depletion around 100E-150E and around 300E-360E. There seems to be a tendency for the H₂O volume mixing ratio to decrease after 2016, and for the SO₂ mixing ratio to increase after 2014. However, we see no clear anti-correlation between the SO₂ and H₂O abundances at the cloudtop, neither on the individual maps nor over the long term. Finally, there is a good agreement between the TEXES results and those obtained in the UV range (SPICAV/Venus Express and UVI/Akatsuki) at a slightly higher altitude. This agreement shows that SO₂ observations obtained in the thermal infrared can be used to extend the local time coverage of the SO₂ measurements obtained in the UV range.

S. Lebonnois, G. Schubert, F. Forget, and A. Spiga. Planetary boundary layer and slope winds on Venus. Icarus, 314:149-158, 2018. [ bib | DOI | PDF version | ADS link ]

Few constraints are available to characterize the deep atmosphere of Venus, though this region is crucial to understand the interactions between surface and atmosphere on Venus. Based on simulations performed with the IPSL Venus Global Climate Model, the possible structure and characteristics of Venus' planetary boundary layer (PBL) are investigated. The vertical profile of the potential temperature in the deepest 10 km above the surface and its diurnal variations are controlled by radiative and dynamical processes. The model predicts a diurnal cycle for the PBL activity, with a stable nocturnal PBL while convective activity develops during daytime. The diurnal convective PBL is strongly correlated with surface solar flux and is maximum around noon and in low latitude regions. It typically reaches less than 2 km above the surface, but its vertical extension is much higher over high elevations, and more precisely over the western flanks of elevated terrains. This correlation is explained by the impact of surface winds, which undergo a diurnal cycle with downward katabatic winds at night and upward anabatic winds during the day along the slopes of high-elevation terrains. The convergence of these daytime anabatic winds induces upward vertical winds, that are responsible for the correlation between height of the convective boundary layer and topography.

I. Garate-Lopez and S. Lebonnois. Latitudinal variation of clouds' structure responsible for Venus' cold collar. Icarus, 314:1-11, 2018. [ bib | DOI | PDF version | ADS link ]

Global Climate Models (GCM) are very useful tools to study theoretically the general dynamics and specific phenomena in planetary atmospheres. In the case of Venus, several GCMs succeeded in reproducing the atmosphere's superrotation and the global temperature field. However, the highly variable polar temperature and the permanent cold collar present at 60^o -80^o latitude have not been reproduced satisfactorily yet.

Here we improve the radiative transfer scheme of the Institut Pierre Simon Laplace Venus GCM in order to numerically simulate the polar thermal features in Venus atmosphere. The main difference with the previous model is that we now take into account the latitudinal variation of the cloud structure. Both solar heating rates and infrared cooling rates have been modified to consider the cloud top's altitude decrease toward the poles and the variation in latitude of the different particle modes' abundances.

A new structure that closely resembles the observed cold collar appears in the average temperature field at 2 ×10⁴ - 4 ×10³ Pa (~ 62 - 66 km) altitude range and 60^o -90^o latitude band. It is not isolated from the pole as in the observation-based maps, but the obtained temperature values (220 K) are in good agreement with observed values. Temperature polar maps across this region show an inner warm region where the polar vortex is observed, but the obtained 230 K average value is colder than the observed mean value and the simulated horizontal structure does not show the fine-scale features present within the vortex.

The comparison with a simulation that does not take into account the latitudinal variation of the cloud structure in the infrared cooling computation, shows that the cloud structure is essential in the cold collar formation. Although our analysis focuses on the improvement of the radiative forcing and the variations it causes in the thermal structure, polar dynamics is definitely affected by this modified environment and a noteworthy upwelling motion is found in the cold collar area.

M. Lefèvre, S. Lebonnois, and A. Spiga. Three-Dimensional Turbulence-Resolving Modeling of the Venusian Cloud Layer and Induced Gravity Waves: Inclusion of Complete Radiative Transfer and Wind Shear. Journal of Geophysical Research (Planets), 123:2773-2789, 2018. [ bib | DOI | PDF version | ADS link ]

Venus' convective cloud layers and associated gravity waves strongly impact the local and global budget of heat, momentum, and chemical species. Here we use for the first time three-dimensional turbulence-resolving dynamical integrations of Venus' atmosphere from the surface to 100-km altitude, coupled with fully interactive radiative transfer computations. We show that this enables to correctly reproduce the vertical position (46- to 55-km altitude) and thickness (9 km) of the main convective cloud layer measured by Venus Express and Akatsuki radio occultations, as well as the intensity of convective plumes (3 m/s) measured by VEGA balloons. Both the radiative forcing in the visible and the large-scale dynamical impact play a role in the variability of the cloud convective activity with local time and latitude. Our model reproduces the diurnal cycle in cloud convection observed by Akatsuki at the low latitudes and the lack thereof observed by Venus Express at the equator. The observed enhancement of cloud convection at high latitudes is simulated by our model, although underestimated compared to observations. We show that the influence of the vertical shear of horizontal superrotating winds must be accounted for in our model to allow for gravity waves of the observed intensity (1 K) and horizontal wavelength (up to 20 km) to be generated through the obstacle effect mechanism. The vertical extent of our model also allows us to predict for the first time a 7-km-thick convective layer at the cloud top (70-km altitude) caused by the solar absorption of the unknown ultraviolet absorber.

T. Navarro, G. Schubert, and S. Lebonnois. Atmospheric mountain wave generation on Venus and its influence on the solid planet's rotation rate. Nature Geoscience, 11:487-491, 2018. [ bib | DOI | PDF version | ADS link ]

The Akatsuki spacecraft observed a 10,000-km-long meridional structure at the top of the cloud deck of Venus that appeared stationary with respect to the surface and was interpreted as a gravity wave. Additionally, over four Venus solar days of observations, other such waves were observed to appear in the afternoon over equatorial highland regions. This indicates a direct influence of the solid planet on the whole Venusian atmosphere despite dissimilar rotation rates of 243 and 4 days, respectively. How such gravity waves might be generated on Venus is not understood. Here, we use general circulation model simulations of the Venusian atmosphere to show that the observations are consistent with stationary gravity waves over topographic highsor mountain wavesthat are generated in the afternoon in equatorial regions by the diurnal cycle of near-surface atmospheric stability. We find that these mountain waves substantially contribute to the total atmospheric torque that acts on the planet's surface. We estimate that mountain waves, along with the thermal tide and baroclinic waves, can produce a change in the rotation rate of the solid body of about 2 minutes per solar day. This interplay between the solid planet and atmosphere may explain some of the difference in rotation rates (equivalent to a change in the length of day of about 7 minutes) measured by spacecraft over the past 40 years.

P. L. Read and S. Lebonnois. Superrotation on Venus, on Titan, and Elsewhere. Annual Review of Earth and Planetary Sciences, 46:175-202, 2018. [ bib | DOI | PDF version | ADS link ]

The superrotation of the atmospheres of Venus and Titan has puzzled dynamicists for many years and seems to put these planets in a very different dynamical regime from most other planets. In this review, we consider how to define superrotation objectively and explore the constraints that determine its occurrence. Atmospheric superrotation also occurs elsewhere in the Solar System and beyond, and we compare Venus and Titan with Earth and other planets for which wind estimates are available. The extreme superrotation on Venus and Titan poses some difficult challenges for numerical models of atmospheric circulation, much more difficult than for more rapidly rotating planets such as Earth or Mars. We consider mechanisms for generating and maintaining a superrotating state, all of which involve a global meridional overturning circulation. The role of nonaxisymmetric eddies is crucial, however, but the detailed mechanisms may differ between Venus, Titan, and other planets.

A. Sánchez-Lavega, S. Lebonnois, T. Imamura, P. Read, and D. Luz. The Atmospheric Dynamics of Venus. Space Science Reviews, 212:1541-1616, 2017. [ bib | DOI | PDF version | ADS link ]

We review our current knowledge of the atmospheric dynamics of Venus prior to the Akatsuki mission, in the altitude range from the surface to approximately the cloud tops located at about 100 km altitude. The three-dimensional structure of the wind field in this region has been determined with a variety of techniques over a broad range of spatial and temporal scales (from the mesoscale to planetary, from days to years, in daytime and nighttime), spanning a period of about 50 years (from the 1960s to the present). The global panorama is that the mean atmospheric motions are essentially zonal, dominated by the so-called super-rotation (an atmospheric rotation that is 60 to 80 times faster than that of the planetary body). The zonal winds blow westward (in the same direction as the planet rotation) with a nearly constant speed of 100 m s^{-1} at the cloud tops (65-70 km altitude) from latitude 50degN to 50degS, then decreasing their speeds monotonically from these latitudes toward the poles. Vertically, the zonal winds decrease with decreasing altitude towards velocities 1-3 m s^{-1} in a layer of thickness 10 km close to the surface. Meridional motions with peak speeds of 15 m s^{-1} occur within the upper cloud at 65 km altitude and are related to a Hadley cell circulation and to the solar thermal tide. Vertical motions with speeds 1-3 m s^{-1} occur in the statically unstable layer between altitudes of 50 - 55 km. All these motions are permanent with speed variations of the order of 10%. Various types of wave, from mesoscale gravity waves to Rossby-Kelvin planetary scale waves, have been detected at and above cloud heights, and are considered to be candidates as agents for carrying momentum that drives the super-rotation, although numerical models do not fully reproduce all the observed features. Momentum transport by atmospheric waves and the solar tide is thought to be an indispensable component of the general circulation of the Venus atmosphere. Another conspicuous feature of the atmospheric circulation is the presence of polar vortices. These are present in both hemispheres and are regions of warmer and lower clouds, seen prominently at infrared wavelengths, showing a highly variable morphology and motions. The vortices spin with a period of 2-3 days. The South polar vortex rotates around a geographical point which is itself displaced from the true pole of rotation by 3 degrees. The polar vortex is surrounded and constrained by the cold collar, an infrared-dark region of lower temperatures. We still lack detailed models of the mechanisms underlying the dynamics of these features and how they couple (or not) to the super-rotation. The nature of the super-rotation relates to the angular momentum stored in the atmosphere and how it is transported between the tropics and higher latitudes, and between the deep atmosphere and upper levels. The role of eddy processes is crucial, but likely involves the complex interaction of a variety of different types of eddy, either forced directly by radiative heating and mechanical interactions with the surface or through various forms of instability. Numerical models have achieved some significant recent success in capturing some aspects of the observed super-rotation, consistent with the scenario discussed by Gierasch (J. Atmos. Sci. 32:1038-1044, 1975) and Rossow and Williams (J. Atmos. Sci. 36:377-389, 1979), but many uncertainties remain, especially in the deep atmosphere. The theoretical framework developed to explain the circulation in Venus's atmosphere is reviewed, as well as the numerical models that have been built to elucidate the super-rotation mechanism. These tools are used to analyze the respective roles of the different waves in the processes driving the observed motions. Their limitations and suggested directions for improvements are discussed.

S. S. Limaye, S. Lebonnois, A. Mahieux, M. Pätzold, S. Bougher, S. Bruinsma, S. Chamberlain, R. T. Clancy, J.-C. Gérard, G. Gilli, D. Grassi, R. Haus, M. Herrmann, T. Imamura, E. Kohler, P. Krause, A. Migliorini, F. Montmessin, C. Pere, M. Persson, A. Piccialli, M. Rengel, A. Rodin, B. Sandor, M. Sornig, H. Svedhem, S. Tellmann, P. Tanga, A. C. Vandaele, T. Widemann, C. F. Wilson, I. Müller-Wodarg, and L. Zasova. The thermal structure of the Venus atmosphere: Intercomparison of Venus Express and ground based observations of vertical temperature and density profiles^✰. Icarus, 294:124-155, 2017. [ bib | DOI | PDF version | ADS link ]

The Venus International Reference Atmosphere (VIRA) model contains tabulated values of temperature and number densities obtained by the experiments on the Venera entry probes, Pioneer Venus Orbiter and multi-probe missions in the 1980s. The instruments on the recent Venus Express orbiter mission generated a significant amount of new observational data on the vertical and horizontal structure of the Venus atmosphere from 40 km to about 180 km altitude from April 2006 to November 2014. Many ground based experiments have provided data on the upper atmosphere (90-130 km) temperature structure since the publication of VIRA in 1985. The “Thermal Structure of the Venus Atmosphere” Team was supported by the International Space Studies Institute (ISSI), Bern, Switzerland, from 2013 to 2015 in order to combine and compare the ground-based observations and the VEx observations of the thermal structure as a first step towards generating an updated VIRA model. Results of this comparison are presented in five latitude bins and three local time bins by assuming hemispheric symmetry. The intercomparison of the ground-based and VEx results provides for the first time a consistent picture of the temperature and density structure in the 40 km-180 km altitude range. The Venus Express observations have considerably increased our knowledge of the Venus atmospheric thermal structure above ~40 km and provided new information above 100 km. There are, however, still observational gaps in latitude and local time above certain regions. Considerable variability in the temperatures and densities is seen above 100 km but certain features appear to be systematically present, such as a succession of warm and cool layers. Preliminary modeling studies support the existence of such layers in agreement with a global scale circulation. The intercomparison focuses on average profiles but some VEx experiments provide sufficient global coverage to identify solar thermal tidal components.

The differences between the VEx temperature profiles and the VIRA below 0.1 mbar/95 km are small. There is, however, a clear discrepancy at high latitudes in the 10-30 mbar (70-80 km) range. The VEx observations will also allow the improvement of the empirical models (VTS3 by Hedin et al., 1983 and VIRA by Keating et al., 1985) above 0.03 mbar/100 km, in particular the 100-150 km region where a sufficient observational coverage was previously missing. The next steps in order to define the updated VIRA temperature structure up to 150 km altitude are (1) define the grid on which this database may be provided, (2) fill what is possible with the results of the data intercomparison, and (3) fill the observational gaps. An interpolation between the datasets may be performed by using available General Circulation Models as guidelines.

An improved spatial coverage of observations is still necessary at all altitudes, in latitude-longitude and at all local solar times for a complete description of the atmospheric thermal structure, in particular on the dayside above 100 km. New in-situ observations in the atmosphere below 40 km are missing, an altitude region that cannot be accessed by occultation experiments. All these questions need to be addressed by future missions.

S. Lebonnois and G. Schubert. The deep atmosphere of Venus and the possible role of density-driven separation of CO₂ and N₂. Nature Geoscience, 10:473-477, 2017. [ bib | DOI | PDF version | ADS link ]

With temperatures around 700 K and pressures of around 75 bar, the deepest 12 km of the atmosphere of Venus are so hot and dense that the atmosphere behaves like a supercritical fluid. The Soviet VeGa-2 probe descended through the atmosphere in 1985 and obtained the only reliable temperature profile for the deep Venusian atmosphere thus far. In this temperature profile, the atmosphere appears to be highly unstable at altitudes below 7 km, contrary to expectations. We argue that the VeGa-2 temperature profile could be explained by a change in the atmospheric gas composition, and thus molecular mass, with depth. We propose that the deep atmosphere consists of a non-homogeneous layer in which the abundance of N₂-the second most abundant constituent of the Venusian atmosphere after CO₂-gradually decreases to near-zero at the surface. It is difficult to explain a decline in N₂ towards the surface with known nitrogen sources and sinks for Venus. Instead we suggest, partly based on experiments on supercritical fluids, that density-driven separation of N₂ from CO₂ can occur under the high pressures of Venus's deep atmosphere, possibly by molecular diffusion, or by natural density-driven convection. If so, the amount of nitrogen in the atmosphere of Venus is 15% lower than commonly assumed. We suggest that similar density-driven separation could occur in other massive planetary atmospheres.

M. Lefèvre, A. Spiga, and S. Lebonnois. Three-dimensional turbulence-resolving modeling of the Venusian cloud layer and induced gravity waves. Journal of Geophysical Research (Planets), 122:134-149, 2017. [ bib | DOI | PDF version | ADS link ]

The impact of the cloud convective layer of the atmosphere of Venus on the global circulation remains unclear. The recent observations of gravity waves at the top of the cloud by the Venus Express mission provided some answers. These waves are not resolved at the scale of global circulation models (GCM); therefore, we developed an unprecedented 3-D turbulence-resolving large-eddy simulations (LES) Venusian model using the Weather Research and Forecast terrestrial model. The forcing consists of three different heating rates: two radiative ones for solar and infrared and one associated with the adiabatic cooling/warming of the global circulation. The rates are extracted from the Laboratoire de Météorlogie Dynamique Venus GCM using two different cloud models. Thus, we are able to characterize the convection and associated gravity waves in function of latitude and local time. To assess the impact of the global circulation on the convective layer, we used rates from a 1-D radiative-convective model. The resolved layer, taking place between 1.0 × 10⁵ and 3.8 × 10⁴ Pa (48-53 km), is organized as polygonal closed cells of about 10 km wide with vertical wind of several meters per second. The convection emits gravity waves both above and below the convective layer leading to temperature perturbations of several tenths of kelvin with vertical wavelength between 1 and 3 km and horizontal wavelength from 1 to 10 km. The thickness of the convective layer and the amplitudes of waves are consistent with observations, though slightly underestimated. The global dynamics heating greatly modify the convective layer.

G. Gilli, S. Lebonnois, F. González-Galindo, M. A. López-Valverde, A. Stolzenbach, F. Lefèvre, J. Y. Chaufray, and F. Lott. Thermal structure of the upper atmosphere of Venus simulated by a ground-to-thermosphere GCM. Icarus, 281:55-72, 2017. [ bib | DOI | PDF version | ADS link ]

We present here the thermal structure of the upper atmosphere of Venus predicted by a full self-consistent Venus General Circulation Model (VGCM) developed at Laboratoire de Météorologie Dynamique (LMD) and extended up to the thermosphere of the planet. Physical and photochemical processes relevant at those altitudes, plus a non-orographic GW parameterisation, have been added. All those improvements make the LMD-VGCM the only existing ground-to-thermosphere 3D model for Venus: a unique tool to investigate the atmosphere of Venus and to support the exploration of the planet by remote sounding. The aim of this paper is to present the model reference results, to describe the role of radiative, photochemical and dynamical effects in the observed thermal structure in the upper mesosphere/lower thermosphere of the planet. The predicted thermal structure shows a succession of warm and cold layers, as recently observed. A cooling trend with increasing latitudes is found during daytime at all altitudes, while at nighttime the trend is inverse above about 110 km, with an atmosphere up to 15 K warmer towards the pole. The latitudinal variation is even smaller at the terminator, in agreement with observations. Below about 110 km, a nighttime warm layer whose intensity decreases with increasing latitudes is predicted by our GCM. A comparison of model results with a selection of recent measurements shows an overall good agreement in terms of trends and order of magnitude. Significant data-model discrepancies may be also discerned. Among them, thermospheric temperatures are about 40-50 K colder and up to 30 K warmer than measured at terminator and at nighttime, respectively. The altitude layer of the predicted mesospheric local maximum (between 100 and 120 km) is also higher than observed. Possible interpretations are discussed and several sensitivity tests performed to understand the data-model discrepancies and to propose future model improvements.

S. Lebonnois, N. Sugimoto, and G. Gilli. Wave analysis in the atmosphere of Venus below 100-km altitude, simulated by the LMD Venus GCM. Icarus, 278:38-51, 2016. [ bib | DOI | PDF version | ADS link ]

A new simulation of Venus atmospheric circulation obtained with the LMD Venus GCM is described and the simulated wave activity is analyzed. Agreement with observed features of the temperature structure, static stability and zonal wind field is good, such as the presence of a cold polar collar, diurnal and semi-diurnal tides. At the resolution used (96 longitudes × 96 latitudes), a fully developed superrotation is obtained both when the simulation is initialized from rest and from an atmosphere already in superrotation, though winds are still weak below the clouds (roughly half the observed values). The atmospheric waves play a crucial role in the angular momentum budget of the Venus's atmospheric circulation. In the upper cloud, the vertical angular momentum is transported by the diurnal and semi-diurnal tides. Above the cloud base (approximately 1 bar), equatorward transport of angular momentum is done by polar barotropic and mid- to high-latitude baroclinic waves present in the cloud region, with frequencies between 5 and 20 cycles per Venus day (periods between 6 and 23 Earth days). In the middle cloud, just above the convective layer, a Kelvin type wave (period around 7.3 Ed) is present at the equator, as well as a low-latitude Rossby-gravity type wave (period around 16 Ed). Below the clouds, large-scale mid- to high-latitude gravity waves develop and play a significant role in the angular momentum balance.

J.-L. Bertaux, I. V. Khatuntsev, A. Hauchecorne, W. J. Markiewicz, E. Marcq, S. Lebonnois, M. Patsaeva, A. Turin, and A. Fedorova. Influence of Venus topography on the zonal wind and UV albedo at cloud top level: The role of stationary gravity waves. Journal of Geophysical Research (Planets), 121:1087-1101, 2016. [ bib | DOI | PDF version | ADS link ]

Based on the analysis of UV images (at 365 nm) of Venus cloud top (altitude 67 2 km) collected with Venus Monitoring Camera on board Venus Express (VEX), it is found that the zonal wind speed south of the equator (from 5degS to 15degS) shows a conspicuous variation (from -101 to -83 m/s) with geographic longitude of Venus, correlated with the underlying relief of Aphrodite Terra. We interpret this pattern as the result of stationary gravity waves produced at ground level by the uplift of air when the horizontal wind encounters a mountain slope. These waves can propagate up to the cloud top level, break there, and transfer their momentum to the zonal flow. Such upward propagation of gravity waves and influence on the wind speed vertical profile was shown to play an important role in the middle atmosphere of the Earth by Lindzen (1981) but is not reproduced in the current GCM of Venus atmosphere from LMD. (Laboratoire de Météorologie Dynamique) In the equatorial regions, the UV albedo at 365 nm varies also with longitude. We argue that this variation may be simply explained by the divergence of the horizontal wind field. In the longitude region (from 60deg to -10deg) where the horizontal wind speed is increasing in magnitude (stretch), it triggers air upwelling which brings the UV absorber at cloud top level and decreases the albedo and vice versa when the wind is decreasing in magnitude (compression). This picture is fully consistent with the classical view of Venus meridional circulation, with upwelling at equator revealed by horizontal air motions away from equator: the longitude effect is only an additional but important modulation of this effect. This interpretation is comforted by a recent map of cloud top H₂O, showing that near the equator the lower UV albedo longitude region is correlated with increased H₂O. We argue that H₂O enhancement is the sign of upwelling, suggesting that the UV absorber is also brought to cloud top by upwelling.

S. Lebonnois, V. Eymet, C. Lee, and J. Vatant d'Ollone. Analysis of the radiative budget of the Venusian atmosphere based on infrared Net Exchange Rate formalism. Journal of Geophysical Research (Planets), 120:1186-1200, 2015. [ bib | DOI | PDF version | ADS link ]

A detailed one-dimensional analysis of the energy balance in Venus atmosphere is proposed in this work, based on the Net Exchange Rate formalism that allows the identification in each altitude region of the dominant energy exchanges controlling the temperature. Well-known parameters that control the temperature profile are the solar flux deposition and the cloud particle distribution. Balance between solar heating and infrared energy exchanges is analyzed for each region: upper atmosphere (from cloud top to 100 km), upper cloud, middle cloud, cloud base, and deep atmosphere (cloud base to surface). The energy accumulated below the clouds is transferred to the cloud base through infrared windows, mostly at 3-4 μm and 5-7 μm. The continuum opacity in these spectral regions is not well known for the hot temperatures and large pressures of Venus's deep atmosphere but strongly affects the temperature profile from cloud base to surface. From cloud base, upward transport of energy goes through convection and short-range radiative exchanges up to the middle cloud where the atmosphere is thin enough in the 20-30 μm window to cool directly to space. Total opacity in this spectral window between the 15 μm CO₂ band and the CO₂ collision-induced absorption has a strong impact on the temperature in the cloud convective layer. Improving our knowledge of the gas opacities in these different windows through new laboratory measurements or ab initio computations, as well as improving the constraints on cloud opacities would help to separate gas and cloud contributions and secure a better understanding of Venus's atmosphere energy balance.

D. Grassi, R. Politi, N. I. Ignatiev, C. Plainaki, S. Lebonnois, P. Wolkenberg, L. Montabone, A. Migliorini, G. Piccioni, and P. Drossart. The Venus nighttime atmosphere as observed by the VIRTIS-M instrument. Average fields from the complete infrared data set. Journal of Geophysical Research (Planets), 119:837-849, 2014. [ bib | DOI | PDF version | ADS link ]

We present and discuss here the average fields of the Venus atmosphere derived from the nighttime observations in the 1960-2350 cm^-1 spectral range by the VIRTIS-M instrument on board the Venus Express satellite. These fields include: (a) the air temperatures in the 1-100 mbar pressure range (˜85-65 km above the surface), (b) the altitude of the clouds top, and (c) the average CO mixing ratio. A new retrieval code based on the Bayesian formalism has been developed and validated on simulated observations, to statistically assess the retrieval capabilities of the scheme once applied to the VIRTIS data. The same code has then been used to process the entire VIRTIS-M data set. Resulting individual retrievals have been binned on the basis of local time and latitude, to create average fields. Air temperature fields confirm the general trends previously reported in Grassi et al. (2010), using a simplified retrieval scheme and a more limited data set. At the lowest altitudes probed by VIRTIS (˜65 km), air temperatures are strongly asymmetric around midnight, with a pronounced minima at 3LT, 70degS. Moving to higher levels, the air temperatures first become more uniform in local time (˜75 km), then display a colder region on the evening side at the upper boundary of VIRTIS sensitivity range (˜80 km). As already shown by Ignatiev et al. (2008) for the dayside, the cloud effective altitude increases monotonically from the south pole to the equator. However, the variations observed in night data are consistent with an overall variation of just 1 km, much smaller than the 4 km reported for the dayside. The cloud altitudes appear slightly higher on the evening side. Both observations are consistent with a less vigorous meridional circulation on the nightside of the planet. Carbon monoxide is not strongly constrained by the VIRTIS-M data. However, average fields present a clear maximum of 80 ppm around 60degS, well above the retrieval uncertainty. Once the intrinsic low sensitivity of VIRTIS data in the region of cold collar is kept in mind, this datum is consistent with a [CO] enrichment toward the poles driven by meridional circulation.

E. Marcq and S. Lebonnois. Simulations of the latitudinal variability of CO-like and OCS-like passive tracers below the clouds of Venus using the Laboratoire de Météorologie Dynamique GCM. Journal of Geophysical Research (Planets), 118:1983-1990, 2013. [ bib | DOI | PDF version | ADS link ]

The lower atmosphere of Venus below the clouds is a transitional region between the relatively calm lowermost scale height and the superrotating atmosphere in the cloud region and above. Any observational constraint is then welcome to help in the development of general circulation models of Venus, a difficult task considering the thickness of its atmosphere. Starting from a state-of-the-art 3-D Venus General Circulation Model (GCM), we have included passive tracers in order to investigate the latitudinal variability of two minor gaseous species, carbonyl sulfide (OCS) and carbon monoxide (CO), whose vertical profiles and mixing ratios are known to vary with latitude between 30 and 40km. The relaxation to chemical equilibrium is crudely parametrized through a vertically uniform time scale τ. A satisfactory agreement with available observations is obtained with 10⁸sτ_CO510⁸ s and 10⁷sτ_OCS10⁸ s. These results, in addition to validating the general circulation below the clouds, are also helpful in characterizing the chemical kinetics of Venus' atmosphere. This complements the much more sophisticated chemical models which focus more on thermodynamical equilibrium.

S. Lebonnois, C. Covey, A. Grossman, H. Parish, G. Schubert, R. Walterscheid, P. Lauritzen, and C. Jablonowski. Angular momentum budget in General Circulation Models of superrotating atmospheres: A critical diagnostic. Journal of Geophysical Research (Planets), 117:12004, 2012. [ bib | DOI | PDF version | ADS link ]

To help understand the large disparity in the results of circulation modeling for the atmospheres of Titan and Venus, where the whole atmosphere rotates faster than the surface (superrotation), the atmospheric angular momentum budget is detailed for two General Circulation Models (GCMs). The LMD GCM is tested for both Venus (with simplified and with more realistic physical forcings) and Titan (realistic physical forcings). The Community Atmosphere Model is tested for both Earth and Venus with simplified physical forcings. These analyses demonstrate that errors related to atmospheric angular momentum conservation are significant, especially for Venus when the physical forcings are simplified. Unphysical residuals that have to be balanced by surface friction and mountain torques therefore affect the overall circulation. The presence of topography increases exchanges of angular momentum between surface and atmosphere, reducing the impact of these numerical errors. The behavior of GCM dynamical cores with regard to angular momentum conservation under Venus conditions provides an explanation of why recent GCMs predict dissimilar results despite identical thermal forcing. The present study illustrates the need for careful and detailed analysis of the angular momentum budget for any GCM used to simulate superrotating atmospheres.

C. F. Wilson, E. Chassefière, E. Hinglais, K. H. Baines, T. S. Balint, J.-J. Berthelier, J. Blamont, G. Durry, C. S. Ferencz, R. E. Grimm, T. Imamura, J.-L. Josset, F. Leblanc, S. Lebonnois, J. J. Leitner, S. S. Limaye, B. Marty, E. Palomba, S. V. Pogrebenko, S. C. R. Rafkin, D. L. Talboys, R. Wieler, L. V. Zasova, and C. Szopa. The 2010 European Venus Explorer (EVE) mission proposal. Experimental Astronomy, 33:305-335, 2012. [ bib | DOI | PDF version | ADS link ]

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an `M-class' mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (25degC and 0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.

A. Migliorini, D. Grassi, L. Montabone, S. Lebonnois, P. Drossart, and G. Piccioni. Investigation of air temperature on the nightside of Venus derived from VIRTIS-H on board Venus-Express. Icarus, 217:640-647, 2012. [ bib | DOI | PDF version | ADS link ]

We present the spatial distribution of air temperature on Venus' night side, as observed by the high spectral resolution channel of VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), or VIRTIS-H, on board the ESA mission Venus Express. The present work extends the investigation of the average thermal fields in the northern hemisphere of Venus, by including the VIRTIS-H data. We show results in the pressure range of 100-4 mbar, which corresponds to the altitude range of 65-80 km. With these new retrievals, we are able to compare the thermal structure of the Venus' mesosphere in both hemispheres. The major thermal features reported in previous investigations, i.e. the cold collar at about 65-70degS latitude, 100 mbar pressure level, and the asymmetry between the evening and morning sides, are confirmed here. By comparing the temperatures retrieved by the VIRTIS spectrometer in the North and South we find that similarities exist between the two hemispheres. Solar thermal tides are clearly visible in the average temperature fields. To interpret the thermal tide signals (otherwise impossible without day site observations), we apply model simulations using the Venus global circulation model Venus GCM (Lebonnois, S., Hourdin, F., Forget, F., Eymet, V., Fournier, R. [2010b]. International Venus Conference, Aussois, 20-26 June 2010) of the Laboratoire de Météorologie Dynamique (LMD). We suggest that the signal detected at about 60-70deg latitude and pressure of 100 mbar is a diurnal component, while those located at equatorial latitudes are semi-diurnal. Other tide-related features are clearly identified in the upper levels of the atmosphere.

L. Cottereau, N. Rambaux, S. Lebonnois, and J. Souchay. The various contributions in Venus rotation rate and LOD. Astronomy Astrophysics, 531:A45, 2011. [ bib | DOI | arXiv | PDF version | ADS link ]

Context. Thanks to the Venus Express Mission, new data on the properties of Venus could be obtained, in particular concerning its rotation. <BR /> Aims: In view of these upcoming results, the purpose of this paper is to determine and compare the major physical processes influencing the rotation of Venus and, more particularly, the angular rotation rate. <BR /> Methods: Applying models already used for Earth, the effect of the triaxiality of a rigid Venus on its period of rotation are computed. Then the variations of Venus rotation caused by the elasticity, the atmosphere, and the core of the planet are evaluated. <BR /> Results: Although the largest irregularities in the rotation rate of the Earth on short time scales are caused by its atmosphere and elastic deformations, we show that the irregularities for Venus are dominated by the tidal torque exerted by the Sun on its solid body. Indeed, as Venus has a slow rotation, these effects have a large amplitude of two minutes of time (mn). These variations in the rotation rate are greater than the one induced by atmospheric wind variations that can reach 25-50 s of time (s), depending on the simulation used. The variations due to the core effects that vary with its size between 3 and 20 s are smaller. Compared to these effects, the influence of the elastic deformation caused by the zonal tidal potential is negligible. <BR /> Conclusions: As the variations in the rotation of Venus reported here are close to 3 mn peak to peak, they should influence past, present, and future observations, thereby providing further constraints on the planet's internal structure and atmosphere.

H. F. Parish, G. Schubert, C. Covey, R. L. Walterscheid, A. Grossman, and S. Lebonnois. Decadal variations in a Venus general circulation model. Icarus, 212:42-65, 2011. [ bib | DOI | PDF version | ADS link ]

The Community Atmosphere Model (CAM), a 3-dimensional Earth-based climate model, has been modified to simulate the dynamics of the Venus atmosphere. The most current finite volume version of CAM is used with Earth-related processes removed, parameters appropriate for Venus introduced, and some basic physics approximations adopted. A simplified Newtonian cooling approximation has been used for the radiation scheme. We use a high resolution (1deg by 1deg in latitude and longitude) to take account of small-scale dynamical processes that might be important on Venus. A Rayleigh friction approach is used at the lower boundary to represent surface drag, and a similar approach is implemented in the uppermost few model levels providing a 'sponge layer' to prevent wave reflection from the upper boundary. The simulations generate superrotation with wind velocities comparable to those measured in the Venus atmosphere by probes and around 50-60% of those measured by cloud tracking. At cloud heights and above the atmosphere is always superrotating with mid-latitude zonal jets that wax and wane on an approximate 10 year cycle. However, below the clouds, the zonal winds vary periodically on a decadal timescale between superrotation and subrotation. Both subrotating and superrotating mid-latitude jets are found in the approximate 40-60 km altitude range. The growth and decay of the sub-cloud level jets also occur on the decadal timescale. Though subrotating zonal winds are found below the clouds, the total angular momentum of the atmosphere is always in the sense of superrotation. The global relative angular momentum of the atmosphere oscillates with an amplitude of about 5% on the approximate 10 year timescale. Symmetric instability in the near surface equatorial atmosphere might be the source of the decadal oscillation in the atmospheric state. Analyses of angular momentum transport show that all the jets are built up by poleward transport by a meridional circulation while angular momentum is redistributed to lower latitudes primarily by transient eddies. Possible changes in the structure of Venus' cloud level mid-latitude jets measured by Mariner 10, Pioneer Venus, and Venus Express suggest that a cyclic variation similar to that found in the model might occur in the real Venus atmosphere, although no subrotating winds below the cloud level have been observed to date. Venus' atmosphere must be observed over multi-year timescales and below the clouds if we are to understand its dynamics.

D. Grassi, A. Migliorini, L. Montabone, S. Lebonnois, A. Cardesìn-Moinelo, G. Piccioni, P. Drossart, and L. V. Zasova. Thermal structure of Venusian nighttime mesosphere as observed by VIRTIS-Venus Express. Journal of Geophysical Research (Planets), 115:9007, 2010. [ bib | DOI | PDF version | ADS link ]

The mapping IR channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) on board the Venus Express spacecraft observes the CO₂ band at 4.3 μm at a spectral resolution adequate to retrieve the atmospheric temperature profiles in the 65-96 km altitude range. Observations acquired in the period June 2006 to July 2008 were used to derive average temperature fields as a function of latitude, subsolar longitude (i.e., local time, LT), and pressure. Coverage presented here is limited to the nighttime because of the adverse effects of daytime non-LTE emission on the retrieval procedure and to southernmost latitudes because of the orientation of the Venus-Express orbit. Maps of air temperature variability are also presented as the standard deviation of the population included in each averaging bin. At the 100 mbar level (about 65 km above the reference surface), temperatures tend to decrease from the evening to the morning side despite a local maximum observed around 20-21LT. The cold collar is evident around 65S, with a minimum temperature at 3LT. Moving to higher altitudes, local time trends become less evident at 12.6 mbar (about 75 km) where the temperature monotonically increases from middle latitudes to the southern pole. Nonetheless, at this pressure level, two weaker local time temperature minima are observed at 23LT and 2LT equatorward of 60S. Local time trends in temperature reverse about 85 km, where the morning side is the warmer. The variability at the 100 mbar level is maximum around 80S and stronger toward the morning side. Moving to higher altitudes, the morning side always shows the stronger variability. Southward of 60S, standard deviation presents minimum values around 12.6 mbar for all the local times.

S. Lebonnois, F. Hourdin, V. Eymet, A. Crespin, R. Fournier, and F. Forget. Superrotation of Venus' atmosphere analyzed with a full general circulation model. Journal of Geophysical Research (Planets), 115:6006, 2010. [ bib | DOI | PDF version | ADS link ]

A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55-60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation.

V. Eymet, R. Fournier, J.-L. Dufresne, S. Lebonnois, F. Hourdin, and M. A. Bullock. Net exchange parameterization of thermal infrared radiative transfer in Venus' atmosphere. Journal of Geophysical Research (Planets), 114:11008, 2009. [ bib | DOI | PDF version | ADS link ]

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.

A. Sánchez-Lavega, R. Hueso, G. Piccioni, P. Drossart, J. Peralta, S. Pérez-Hoyos, C. F. Wilson, F. W. Taylor, K. H. Baines, D. Luz, S. Erard, and S. Lebonnois. Variable winds on Venus mapped in three dimensions. Geophysical Research Letters, 35:13204, 2008. [ bib | DOI | PDF version | ADS link ]

We present zonal and meridional wind measurements at three altitude levels within the cloud layers of Venus from cloud tracking using images taken with the VIRTIS instrument on board Venus Express. At low latitudes, zonal winds in the Southern hemisphere are nearly constant with latitude with westward velocities of 105 ms^-1 at cloud-tops (altitude ˜ 66 km) and 60-70 ms^-1 at the cloud-base (altitude ˜ 47 km). At high latitudes, zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear (values lower than 15 ms^-1), indicating the possibility of a vertically coherent vortex structure. Meridional winds at the cloud-tops are poleward with peak speed of 10 ms^-1 at 55deg S but below the cloud tops and averaged over the South hemisphere are found to be smaller than 5 ms^-1. We also report the detection at subpolar latitudes of wind variability due to the solar tide.

J.-L. Bertaux, A.-C. Vandaele, O. Korablev, E. Villard, A. Fedorova, D. Fussen, E. Quémerais, D. Belyaev, A. Mahieux, F. Montmessin, C. Muller, E. Neefs, D. Nevejans, V. Wilquet, J. P. Dubois, A. Hauchecorne, A. Stepanov, I. Vinogradov, A. Rodin, J.-L. Bertaux, D. Nevejans, O. Korablev, F. Montmessin, A.-C. Vandaele, A. Fedorova, M. Cabane, E. Chassefière, J. Y. Chaufray, E. Dimarellis, J. P. Dubois, A. Hauchecorne, F. Leblanc, F. Lefèvre, P. Rannou, E. Quémerais, E. Villard, D. Fussen, C. Muller, E. Neefs, E. van Ransbeeck, V. Wilquet, A. Rodin, A. Stepanov, I. Vinogradov, L. Zasova, F. Forget, S. Lebonnois, D. Titov, S. Rafkin, G. Durry, J. C. Gérard, and B. Sandel. A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H₂O and HDO. Nature, 450:646-649, 2007. [ bib | DOI | PDF version | ADS link ]

Venus has thick clouds of H₂SO₄ aerosol particles extending from altitudes of 40 to 60km. The 60-100km region (the mesosphere) is a transition region between the 4day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO₂, HCl, HF, H₂O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60km. Here we report the detection of an extensive layer of warm air at altitudes 90-120km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the `cryosphere' above 100km. We also measured the mesospheric distributions of HF, HCl, H₂O and HDO. HCl is less abundant than reported 40years ago. HDO/H₂O is enhanced by a factor of ˜2.5 with respect to the lower atmosphere, and there is a general depletion of H₂O around 80-90km for which we have no explanation.

P. Drossart, G. Piccioni, J. C. Gérard, M. A. Lopez-Valverde, A. Sanchez-Lavega, L. Zasova, R. Hueso, F. W. Taylor, B. Bézard, A. Adriani, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, J. P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. Coradini, A. di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. Irwin, Y. Langevin, S. Lebonnois, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, D. M. Stam, D. Titov, G. Visconti, M. Zambelli, C. Tsang, E. Ammannito, A. Barbis, R. Berlin, C. Bettanini, A. Boccaccini, G. Bonnello, M. Bouyé, F. Capaccioni, A. Cardesin, F. Carraro, G. Cherubini, M. Cosi, M. Dami, M. de Nino, D. Del Vento, M. di Giampietro, A. Donati, O. Dupuis, S. Espinasse, A. Fabbri, A. Fave, I. Ficai Veltroni, G. Filacchione, K. Garceran, Y. Ghomchi, M. Giustizi, B. Gondet, Y. Hello, F. Henry, S. Hofer, G. Huntzinger, J. Kachlicki, R. Knoll, D. Kouach, A. Mazzoni, R. Melchiorri, G. Mondello, F. Monti, C. Neumann, F. Nuccilli, J. Parisot, C. Pasqui, S. Perferi, G. Peter, A. Piacentino, C. Pompei, J.-M. Réess, J.-P. Rivet, A. Romano, N. Russ, M. Santoni, A. Scarpelli, A. Sémery, A. Soufflot, D. Stefanovitch, E. Suetta, F. Tarchi, N. Tonetti, F. Tosi, and B. Ulmer. A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express. Nature, 450:641-645, 2007. [ bib | DOI | PDF version | ADS link ]

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 (90-120km 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 4.3m, extending from 90 to 120km altitude, and of night-side O₂ emission extending from 95 to 100km. The CO₂ emission peak occurs at ˜115km and varies with solar zenith angle over a range of ˜10km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O₂ peak emission happens at 96km+/-1km, 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.

G. Piccioni, P. Drossart, A. Sanchez-Lavega, R. Hueso, F. W. Taylor, C. F. Wilson, D. Grassi, L. Zasova, M. Moriconi, A. Adriani, S. Lebonnois, A. Coradini, B. Bézard, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, J. P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. G. J. Irwin, Y. Langevin, M. A. Lopez-Valverde, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, D. M. Stam, D. Titov, G. Visconti, M. Zambelli, E. Ammannito, A. Barbis, R. Berlin, C. Bettanini, A. Boccaccini, G. Bonnello, M. Bouye, F. Capaccioni, A. Cardesin Moinelo, F. Carraro, G. Cherubini, M. Cosi, M. Dami, M. de Nino, D. Del Vento, M. di Giampietro, A. Donati, O. Dupuis, S. Espinasse, A. Fabbri, A. Fave, I. F. Veltroni, G. Filacchione, K. Garceran, Y. Ghomchi, M. Giustini, B. Gondet, Y. Hello, F. Henry, S. Hofer, G. Huntzinger, J. Kachlicki, R. Knoll, K. Driss, A. Mazzoni, R. Melchiorri, G. Mondello, F. Monti, C. Neumann, F. Nuccilli, J. Parisot, C. Pasqui, S. Perferi, G. Peter, A. Piacentino, C. Pompei, J.-M. Reess, J.-P. Rivet, A. Romano, N. Russ, M. Santoni, A. Scarpelli, A. Semery, A. Soufflot, D. Stefanovitch, E. Suetta, F. Tarchi, N. Tonetti, F. Tosi, and B. Ulmer. South-polar features on Venus similar to those near the north pole. Nature, 450:637-640, 2007. [ bib | DOI | PDF version | ADS link ]

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 50km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

P. Drossart, G. Piccioni, A. Adriani, F. Angrilli, G. Arnold, K. H. Baines, G. Bellucci, J. Benkhoff, B. Bézard, J.-P. Bibring, A. Blanco, M. I. Blecka, R. W. Carlson, A. Coradini, A. Di Lellis, T. Encrenaz, S. Erard, S. Fonti, V. Formisano, T. Fouchet, R. Garcia, R. Haus, J. Helbert, N. I. Ignatiev, P. G. J. Irwin, Y. Langevin, S. Lebonnois, M. A. Lopez-Valverde, D. Luz, L. Marinangeli, V. Orofino, A. V. Rodin, M. C. Roos-Serote, B. Saggin, A. Sanchez-Lavega, D. M. Stam, F. W. Taylor, D. Titov, G. Visconti, M. Zambelli, R. Hueso, C. C. C. Tsang, C. F. Wilson, and T. Z. Afanasenko. Scientific goals for the observation of Venus by VIRTIS on ESA/Venus express mission. Planetary and Space Science, 55:1653-1672, 2007. [ bib | DOI | PDF version | ADS link ]

The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the ESA/Venus Express mission has technical specifications well suited for many science objectives of Venus exploration. VIRTIS will both comprehensively explore a plethora of atmospheric properties and processes and map optical properties of the surface through its three channels, VIRTIS-M-vis (imaging spectrometer in the 0.3-1 μm range), VIRTIS-M-IR (imaging spectrometer in the 1-5 μm range) and VIRTIS-H (aperture high-resolution spectrometer in the 2-5 μm range). The atmospheric composition below the clouds will be repeatedly measured in the night side infrared windows over a wide range of latitudes and longitudes, thereby providing information on Venus's chemical cycles. In particular, CO, H ₂O, OCS and SO ₂ can be studied. The cloud structure will be repeatedly mapped from the brightness contrasts in the near-infrared night side windows, providing new insights into Venusian meteorology. The global circulation and local dynamics of Venus will be extensively studied from infrared and visible spectral images. The thermal structure above the clouds will be retrieved in the night side using the 4.3 μm fundamental band of CO ₂. The surface of Venus is detectable in the short-wave infrared windows on the night side at 1.01, 1.10 and 1.18 μm, providing constraints on surface properties and the extent of active volcanism. Many more tentative studies are also possible, such as lightning detection, the composition of volcanic emissions, and mesospheric wave propagation.

V. Formisano, F. Angrilli, G. Arnold, S. Atreya, K. H. Baines, G. Bellucci, B. Bezard, F. Billebaud, D. Biondi, M. I. Blecka, L. Colangeli, L. Comolli, D. Crisp, M. D'Amore, T. Encrenaz, A. Ekonomov, F. Esposito, C. Fiorenza, S. Fonti, M. Giuranna, D. Grassi, B. Grieger, A. Grigoriev, J. Helbert, H. Hirsch, N. Ignatiev, A. Jurewicz, I. Khatuntsev, S. Lebonnois, E. Lellouch, A. Mattana, A. Maturilli, E. Mencarelli, M. Michalska, J. Lopez Moreno, B. Moshkin, F. Nespoli, Y. Nikolsky, F. Nuccilli, P. Orleanski, E. Palomba, G. Piccioni, M. Rataj, G. Rinaldi, M. Rossi, B. Saggin, D. Stam, D. Titov, G. Visconti, and L. Zasova. The planetary fourier spectrometer (PFS) onboard the European Venus Express mission. Planetary and Space Science, 54:1298-1314, 2006. [ bib | DOI | PDF version | ADS link ]

The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm ^-1 (0.9-5.5 μm) and a long wavelength (LW) channel that covers 250-1700 cm ^-1 (5.5-45 μm). Both channels have a uniform spectral resolution of 1.3 cm ^-1. The instrument field of view FOV is about 1.6 deg (FWHM) for the short wavelength channel and 2.8 deg for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity). PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side. The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70-80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere. Measurement of the 15 μm CO ₂ band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study. PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.