Titan’s atmosphere: An optimal gas mixture for aerosol production?

Abstract

Here 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. Analogs of Titan’s aerosols have been produced in different N 2–CH 4 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 ∼5% methane concentration is necessary to ensure a ∼1.5% 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 CH 4 concentration in agreement with Titan’s atmospheric CH 4 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 modeled 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 ∼5%. 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.