Description: Publication date: June 2018 Source: Icarus, Volume 307 Author(s): Peng Hong, Yasuhito Sekine, Tsutoni Sasamori, Seiji Sugita Formation of organic aerosols driven by photochemical reactions has been observed and suggested in CH 4 -containing atmospheres, including Titan and early Earth. However, the detailed production and growth mechanisms of organic aerosols driven by solar far ultraviolet (FUV) light remain poorly constrained. We conducted laboratory experiments simulating photochemical reactions in a CH 4 CO 2 atmosphere driven by the FUV radiations dominated by the Lyman-α line. In the experiments, we analyzed time variations in thickness and infrared spectra of solid organic film formed on an optical window in a reaction cell. Gas species formed by FUV irradiation were also analyzed and compared with photochemical model calculations. Our experimental results show that the growth rate of the organic film decreases as the CH 4 /CO 2 ratio of reactant gas mixture decreases, and that the decrease becomes very steep for CH 4 /CO 2  〈 1. Comparison with photochemical model calculations suggests that polymerizations of gas-phase hydrocarbons, such as polyynes and aromatics, cannot account for the growth rate of the organic film but that the addition reaction of CH 3 radicals onto the organic film with the reaction probability around 10 −2 can explain the growth rate. At CH 4 /CO 2  〈 1, etching by O atom formed by CO 2 photolysis would reduce or inhibit the growth of the organic film. Our results suggest that organic aerosols would grow through CH 3 addition onto the surface during the precipitation of aerosol particles in the middle atmosphere of Titan and early Earth. On Titan, effective CH 3 addition would reduce C 2 H 6 production in the atmosphere. On early Earth, growth of aerosol particles would be less efficient than those on Titan, possibly resulting in small-sized monomers and influencing UV shielding.