Titan's atmosphere: an optimal gas mixture for aerosol production?

International audienceHere we present the first quantitative study of the gas to solid particle conversion in a Radio Frequency dusty plasma experiment simulating the complex atmospheric reactivity on Titan. Analogues of Titan's aerosols have been produced in different N2-CH4 gas mixtures. Using in situ mass spectrometry, it has been found that, by varying the initial methane concentration, aerosols could be produced in methane steady state concentrations similar to Titan's atmospheric conditions. In our experiment, an initial not, vert, similar5% methane concentration is necessary to ensure a not, vert, similar1.5% methane steady state concentration in the plasma. The tholin mass production rate has been quantified as a function of the initial methane concentration. A maximum was found for a steady state CH4 concentration in agreement with Titan's atmospheric CH4 concentrations. At this maximum, the tholin C/N ratio is about 1.45 and the carbon gas to solid conversion yield is about 35%. We have modelled the mass production rate by a parabolic function, highlighting two competitive chemical regimes controlling the tholin production efficiency: an efficient growth process which is proportional to the methane consumption, and an inhibiting process which opposes the growth process and dominates it for initial methane concentrations higher than not, vert, similar5%. To explain these two opposite effects, we propose two mechanisms: one involving HCN patterns in the tholins for the growth process, and one involving the increasing amount of atomic hydrogen in the plasma as well as the increase in aliphatic contributions in the tholins for the inhibiting process. This study highlights new routes for understanding the chemical growth of the organic aerosols in Titan's atmosphere