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J. I. Moses (LPI), T. Fouchet (Oxford), B. Bézard, E. Lellouch (Obs. Paris-Meudon), G. R. Gladstone (SwRI), H. Feuchtgruber (MPI-EP), M. Allen (JPL/Caltech)
For both Jupiter and Saturn, the composition of the stratosphere is controlled in large part by disequilibrium chemistry initiated by methane photolysis. Because of the general similarities in stratospheric temperatures (~165 vs.~~140 K), methane abundances (~1.8 \times 10-3 vs.~\le\, 4.5 \times 10-3 mole fraction), and overall composition (major gases H2, He, and CH4) on Jupiter and Saturn, the same basic set of photochemical reaction schemes are expected to dominate in the upper atmospheres of the two planets. Indeed, hydrocarbon photochemical products such as CH3, C2H2, C2H4, C2H6, CH3C2H, C4H2, and C6H6 have now been observed in the stratospheres of both planets. Some interesting differences in the observed abundances of these molecules do exist, however, and theoretical photochemical models of both Jupiter and Saturn can shed light on the reasons for the compositional similarities and differences. Factors that can create compositional differences between the two planets include heliocentric distance (which can affect photolysis rates, H atom production rates, effective temperatures), transport effects (such as the strength of vertical mixing, stratospheric circulation patterns, transport from auroral regions), atmospheric temperatures (which can affect temperature-sensitive reaction rates and photolysis cross sections, aerosol formation), and auroral energy input. Through photochemical modeling, we will address the importance of each of these factors in explaining differences in the C2H6/C2H2 ratios, the C4H2 and C6H6 abundances, and similarities in the CH3C2H abundances on Jupiter and Saturn. Preliminary results suggest that temperature-dependent molecular diffusion coefficients rather than vastly different eddy diffusion coefficients may explain why the methane homopause is located at lower pressures (higher altitudes) on Saturn than on Jupiter. We also find that hydrocarbon abundances are extremely sensitive to the atomic hydrogen abundance, and Jupiter's low C4H2 abundance is consistent with the lower overall C2H2 abundance on Jupiter as compared with Saturn. However, several aspects of the observations are still not well understood.