Titan's atmosphere: an optimized mixture for gas-particle conversion
Résumé
The complex organic chemistry that takes place in Titan's dense atmosphere, composed mainly of N2 and CH4, and that leads to the production of aerosols has been puzzling scientists for many years. Until recently, these aerosols were thought to be formed in the lower stratosphere, at altitudes about a few hundred kilometres, where the atmosphere is dense and neutral. The recent observations of the INMS and the CAPS instruments on Cassini showed that these organic aerosols are actually formed at higher altitudes, in the ionosphere, where an active chemistry coupling ion and neutral species occurs. To try and understand this chemistry, we use a dusty plasma experiment that reproduces in a laboratory setting, the atmospheric reactivity on Titan. This experiment uses a Capacitively Coupled Plasma discharge, produced in a continuous gas flow, to induce the chemistry between N2 and CH4. More complex molecules are then formed in the reactive medium and lead to the production of solid particles, tholins, analogues of Titan's aerosols. In the study presented here, different nitrogen-methane mixtures (from 1% to 10% CH4) have been used to study the influence of methane on the chemistry and on the subsequent production of aerosols. Using in situ mass spectrometry measurements, we have been able to infer the real CH4 concentration in the plasma in steady state, i.e. when the tholins are produced. We have thus determined which CH4 concentration needs to be used to simulate Titan's atmospheric composition. The mass of tholins produced has also been carefully measured. The correlation between the resulting tholins production rates and the real methane concentration in the reactive medium has shown that the stoechiometry responsible for a maximum production of solid particles seems to be corresponding to Titan's conditions.