Open AccessModeling of methane photolysis in the reducing atmospheres of the outer solar system
Author(s) -
Smith Nicholas S.,
Raulin François
Publication year - 1999
Publication title -
journal of geophysical research: planets
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/1998je900027
Subject(s) - photodissociation , methane , titan (rocket family) , atmosphere of titan , atmosphere (unit) , excited state , photochemistry , wavelength , materials science , chemistry , analytical chemistry (journal) , astrobiology , atomic physics , physics , environmental chemistry , thermodynamics , organic chemistry , optoelectronics
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.