Modeling of methane photolysis in the reducing atmospheres of the outer solar system

It is important that methane photolysis is accurately treated in models of planetary atmospheres of the outer solar system and of the interstellar medium. Recent measurements of the H atom and H 2 yields in the photolysis of methane have been considered and appear to be consistent with quantum yields, at Lyman α (121.6 nm), of (R1) CH 3 + H = 0.41, (R2) 1 CH 2 + H2 = 0.53, (R3) 1 CH 2 + H + H = 0.0, (R4) 3 CH 2 + H + H = 0.0, and (R5) CH + H 2 + H = 0.06. At shorter wavelengths it is likely that the production of CH will increase due to the decomposition of excited CH 3 molecules, although further data are required to be able to accurately model the wavelength dependence of methane photolysis. At the low temperatures of Titan's atmosphere, for example, the possible temperature dependence of CH 4 absorption cross sections may need to be considered. By reference to other hydrocarbons, the absorption of methane may increase by 30% or more at 200 K with respect to room temperature values. These dependences, not previously considered in photochemical models of Titan, could significantly change the predicted composition of Titan's atmosphere and the chemical pathways implied.