Modélisation des climats de Titan et Vénus – EXOCLIMATS

Today, as space missions exploring planetary atmospheres are highly successful, with scientific teams now collecting a large amount of data, the study of the detailed aspects of all the mechanisms controling these complex atmospheric systems becomes possible. However, the interpretation of Cassini-Huygens (Titan) and Venus Express datasets will require the development of complex modeling tools. In particular, three-dimensional general circulation models (GCM), coupled to microphysics and photochemical models, already allowed significant progress in the comprehension of the atmosphere of Mars, its history, and its last and present interactions with the surface. Our teams from the Institute Pierre-Simon Laplace (LMD and SA), joined together on this project, carried out recently in this field several works which are internationally recognized, especially in the case of Mars, for which planet our GCM is now highly elaborated. Until now, our efforts could not be done at the same level for our Titan GCM, or for Venus, which GCM project has only recently started. However, rapid progress are now expected in the detailed study of these atmospheres, and therefore, the objective of this project is to allow for decisive developments of our models of Titan and Venus within the next three years, to go beyond their current limits. In the last three years, a collaboration between the LMD team and the team of Richard Fournier, now at LAPLACE in Toulouse, has made it possible to develop the radiative transfer computations that were essential to set up the general circulation model of Venus. This interdisciplinary work allowed for methodological developments, that have been applied successfully to the difficult conditions of this atmosphere. Our teams are strongly involved in the analysis of spacecraft datasets: SPICAV and VIRTIS on Venus Express, VIMS and CIRS (in collaboration with the LESIA) on Cassini. Synergy between fully operational numerical simulations and the current flow of spacebased observational data will be fundamental in the progress to be made in the years to come in our knowledge of planetary atmospheres mechanisms. These progress, applied in a comparative planetology approach, is essential for the comprehension of the climatic evolutions of the solar system planets, and a better apprehension of the required conditions for the emergence of life. Our general circulation model of Titan already includes couplings between atmospheric dynamics, radiative transfer, and the stratospheric aerosols layer. It also takes into account photochemistry, allowing the monitoring of atmospheric composition. The objective of this project is to extend the model beyond its current strong limitations: (1) To upgrade the radiative transfer computations, in order to raise the top level of the model, so that it can include the mesosphere. The significant role of processes taking place in this intermediate region between stratosphere and thermosphere seems to be shown by recent observations. (2) To extend the model to three dimensions, in order to represent the existence of non-axisymmetric processes in the troposphere, that must have fundamental implications on the methane cycle (e.g. precipitations), and its interactions with the surface. For Venus, our model does not take into account for the moment the existing couplings between the atmospheric dynamics, the planet-wide clouds, the atmospheric composition and the radiative transfer. This project therefore requires additional means for the rapid development of the microphysics and photochemical models, to complement this already three-dimensional dynamic model. We also want to extend this model vertically, as in the case of Titan, to include the mesosphere. This very dynamic area, much more variable than the denser atmosphere, is currently observed by the instrument SPICAV/Vénus Express (PI at Service d'Aéronomie). To reach these objectives, we request the recruitment of three post-doctoral positions: a specialist in radiative transfer (to work at the LAPLACE), to continue the work started on Venus, and to upgrade the model of Titan, in collaboration with the IAA of Granada, Spain; a specialist in atmospheric dynamics (to work at the LMD) to complete the vertical and three-dimensional extensions of the Titan GCM, and to thus study in an extensive way Titan's mesosphere, in strong connexion with the interpretation of space data. Finally a specialist in photochemistry (to work at SA) to develop the photochemical model which will be integrated into the Venus GCM, and to thus complete the couplings, the microphysics model being developed by a PhD student. These models improvements will be done in complementarity with the interpretation of available observational data, especially for the study of the interactions between atmosphere, surface and interior, work that started recently with the LPGN team, associated with this project.