Aeronomy of the major planets: Photochemistry of ammonia and hydrocarbons

Abstract

The photochemistry of hydrocarbons and ammonia is reviewed for the cold, H2‐dominated atmospheres of the major planets and their satellites. Detailed discussions are given of primary dissociation paths, chemical kinetics, aeronomical models, the formation of complex hydrocarbons, and the role that these constituents and their photolysis products play in the energy balance of planetary atmospheres. It is concluded that the formation rate of complex hydrocarbons by gas phase kinetics is at most a few percent of the CH4 dissociation rate. On Jupiter, large concentrations of C2H6 and C2H2 are predicted, which are in reasonable agreement with observational estimates. A consideration of the Lyman α albedo, the UV albedo between 2000 and 2300 Å, and the NH3 photochemistry suggests that Jupiter's eddy diffusion coefficient is >108 cm² s−1 near the turbopause but only ∼104 cm² s−1 in the lower stratosphere. Large amounts of C2H6 and C2H2 may be present on Titan.