Publications

Bright and dark patterns seen in ultraviolet images of Venus are mainly due to scattering clouds and two major absorbers: one is sulfur dioxide, and the other absorbs sunlight, whose identity is still uncertain. Sulfur dioxide is the chemical precursor of clouds that control the planetary albedo, and the unidentified absorber contributes to absorbing solar energy. Understanding how sulfur dioxide is distributed and contributes to cloud formation is crucial for studying the Venusian climate. The Ultraviolet Imager (UVI) onboard JAXA's Venus orbiter Akatsuki has been taking ultraviolet images at two wavelengths: 283 nm, the center of the absorption band of sulfur dioxide, and 365 nm, that of the unidentified absorber. However, since the absorption bands of sulfur dioxide and the unidentified absorber overlap, we developed a method to distinguish their effects and derive their individual distributions. We calculated the local time and latitudinal distribution from the obtained data set and found that they can be explained by the thermal tide, which is a planetary-scale wave excited by solar heating. We also investigated the long-term variations of sulfur dioxide and unidentified absorbers to examine their relationship to atmospheric motion.

Strong absorption is observable in Venus' atmosphere in the near-ultraviolet wavelength region. A combination of isomers of the SO dimer (OSSO) have previously been proposed as the cause of this absorption. Using 3D photochemical and dynamical atmospheric modeling, including state-of-the-art kinetics, we predict the concentration of OSSO in the Venusian atmosphere. Using radiative transfer modeling, we predict the effect the modeled concentration of OSSO would have on the observed Venusian reflectance and compare to published observations taken by the MASCS instrument on board the MESSENGER spacecraft in 2007. We find that the predicted OSSO concentration is too low to explain the observed absorption by a factor of 1000 and conclude that OSSO cannot be a major contributor to the unknown UV absorber on Venus.

 Remotely measuring the composition of the Venusian atmosphere below the clouds is challenging, yet yields invaluable insights about the atmospheric chemistry, circulation and interaction with the surface and interior of the planet. The VIRTIS-H instrument on board ESA’s Venus Express orbiter (2006–2014) provides a rich data set in this regard, thanks to its ability to observe and analyze, on the night side of the planet, the infrared radiation emitted by the deep atmospheric layers. The results of our analyses confirm the previously observed trends for the variations of two trace gases (carbon monoxide and carbonyl sulfide) with latitude, explained by the combined effects of chemical reactions and transport by the atmospheric circulation. Variations of carbon monoxide may also be linked to the variations of ground elevation, confirming the link between surface topography and atmospheric circulation. However, we were unable to separate the signature of heavy water vapor from ordinary water vapor or to detect any variations in sulfur dioxide, both of which require more powerful infrared instruments such as those planned on future Venus orbiters such as ESA’s EnVision.

CO is an extremely interesting trace species in the Martian atmosphere. It has been used for both dynamical and photochemical studies of the atmosphere. But its vertical distribution has not been systematically measured until the arrival of the Exomars Trace Gas Orbiter (TGO). We use observations of the NOMAD (Nadir and Occultation for Mars Discovery) spectrometer onboard TGO to retrieve full profiles of mixing ratios of CO up to 100 km with a good vertical resolution. The retrievals cover two Martian seasons during which a global dust storm event occurred. We have found the behavior of CO during this event to be governed by local chemistry as well as by the long range transport. During the dust storm, CO mixing ratios are depleted all over the globe while over the southern high latitudes, we discover an increase in CO due to transport from low latitudes during the end of the southern winter. The dynamical effect of global transport is found in the vertical distribution of CO during the southern summer. Another important result, where the local chemistry might be at play is the increase of CO in the low altitudes over low and midlatitudes during the decay phase of the GDS.

The characterization of water vapor in the atmosphere is important for understanding the cycle of water on Mars and it is crucial in most of the atmospheric processes taking place in its current climate. The observation technique of the Nadir and Occultation for Mars Discovery onboard ExoMars Trace Gas Orbiter using solar occultations allows a high resolution vertical sampling of the atmosphere, permitting characterization of the H2O vertical distribution. In this work, we analyze the H2O distribution in the Martian atmosphere during the southern spring in Martian years 34 and 35. A Global Dust Storm event during the first one allowed us to study the atmospheric H2O and its response in the same season under intense and regular dusty conditions. We found that during intense dust storms, water vapor is present at higher altitudes rather than in regular atmospheric dust activity. This shows high concentrations of about 150 ppmv up to 80 km. As a consequence of the dust intensification, we observed an increase in the altitude of the 50 ppmv water vapor layer. Here, we report observations of atmospheric layers where H2O abundances exceed the theoretical needed saturation limit even when small particles are present

The detailed variation of temperature and density with altitude is of paramount importance to characterize the atmospheric state and to constrain the chemistry and dynamics as a whole. The Nadir and Occultation for Mars Discovery (NOMAD) instrument on board the Trace Gas Orbiter (TGO) has among its key targets the characterization of the thermal state with unprecedented vertical resolution. This is the target of this work, where we analyzed transmittance spectra obtained from the NOMAD solar occultation channel, with a state-of-the-art retrieval scheme, adapted from Earth to Mars conditions and geometry. We applied it to the first year of TGO observations, which covered the last two Mars seasons of Mars Year 34. The results permit to study the temperature structure up to 90 km and its seasonal and latitudinal variations, revealing the impact of the MY34 Global Dust Storm, a warm layer at mesospheric altitudes not present in climate models, more frequent cold pockets than in current global climate models, and generally, colder temperature at those altitudes, all of which can be of importance for the validation of these climate models.

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.