Photochemistry of the Reducing Atmospheres of Jupiter, Saturn and Titan

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Abstract
The photochemistry of atmospheric constituents in the cold atmospheres of Jupiter, Saturn and Titan is reviewed: H2, CH4, NH3, PH3, H20, N2, etc. A detailed discussion of major dissociation paths and essential chemical reactions is given with the emphasis on a first-order description of the concentrations of simple molecules in the photochemical regions of these atmospheres. The composition as measured by Voyager spacecraft instruments, IUE and ground-based telescopes is compared with the predictions of aeronomical models and generally acceptable agreement is found for Jupiter, provided the C2H6 mixing ratio is ∼ 1 × 10−6. More complex molecules do not exceed ppb mixing ratios as a consequence of large H-atom concentrations and the efficiency of H-atom cracking reactions. Even on Titan with lower H-atom concentrations these reactions are apparently effective in suppressing the concentrations of C4H2, C3H4, HC3N, and C2N2.