Methane photochemistry in the outer planets

The methane photochemistry of the outer planets is discussed in the light of existing data and models. Preliminarily, a historical summary is given of the theoretical and observational work on hydrocarbons on the outer planets and Titan. An extensive compilation of the photoprocesses involving C(1)-C(4) hydrocarbons is then presented, describing the absorption features vs wavelengths, the states responsible for absorption, primary and secondary decompositions by photoabsorption, and quantum yields for these processes. The modeling of chemical reactions that are likely to occur in the upper atmospheres of the outer planets is discussed. Present problems are mathematically formulated and the physical parameters which appear in relevant systems of equations such as temperature, eddy diffusion coefficient, and mixing ratio of methane, are discussed. Finally, results for several atmospheric and chemical models for the outer planets are considered.