Soutenance

The Earth and Titan, largest satellite of Saturn are often compared by their atmospheric similarities: both have a dense atmosphere mainly made of nitrogen, with a pressure of the order of magnitude of a bar at the surface. However their composition significantly differs because of the second mostly abundant constituent: oxygen on Earth and methane on Titan. Terrestrial atmosphere is therefore oxidazing whereas Titanian atmosphere is reducing. This major difference leads to radically different chemistry patterns for organic species: oxidation lyses organic skeletons whereas photochemistry in reducing conditions leads to an efficient growth of the carbon chains. A reducing atmospheric chemistry therefore produces complex structures and a wide-range of possible chemical functions, providing abiotic organic materials of interest for astrobiology. Titan’s atmosphere is therefore considered as a presently observable prebiotic model for Early Earth. During my career I studied both oxidazing and reducing atmospheric mechanisms and I am now specifically interested in the contribution of ionic species to chemical growth. Those efficient ionic processes are actually involved in the production of solid organic haze surrounding Titan.

I use and develop global approaches: both modeling and experimental simulations of reactive atmospheres.

The modeling activity is focused on the building of a fully coupled chemistry model for Titan’s upper atmosphere, involving photo-dissociation, neutral-neutral reactions, ionization, ion-neutral reactions, and dissociative ion-electron recombination. One of the difficulties for Titan atmospheric reactivity is the lack of knowledge on numerous processes: either because the chemistry of N2-CH4 systems is simply mostly unknown, or because the low temperature [100-200K] in Titan’s atmosphere raises extrapolation issues for the kinetics parameters of the known processes which have often been studied at room temperature. In collaboration with LCP and LAB I am developing an original probabilistic modeling of Titan’s atmospheric reactivity in order to faithfully describe the degree of knowledge on each process composing the model.

The experimental simulation is made by a global approach: an energy source comparable with solar irradiation initiates the chemistry in an initial gas mixture similar to the main atmospheric composition. A strong interest of this method in the case of Titan is the direct comparison of several observables with in-situ measurements of the Cassini-Huygens space mission. A radio-frequency plasma experiment named PAMPRE offers the possibility to study in the laboratory the whole reactive chain leading to the production of solid organic aerosols from the gas phase. The validity of the setup and its interest to study Titan’s atmospheric chemistry is now well established. With this experimental platform I study the transition from N2-CH4 initial mixtures towards,1) semi-volatile products responsible for nucleation and 2) aerosol production and growth and their temporal correlations with reactive gas-phase species in the medium. This part of the project aims to complete our present limited knowledge on the chemistry involving large nitrogen rich species.

With the methodology developed, I moreover plan to extend my research to the case of prebiotic chemistry on Early Earth.