Photochemical modeling of CH3 abundances in the outer solar system

Abstract

Recent measurements of methyl radicals (CH3) in the upper atmospheres of Saturn and Neptune by the Infrared Space Observatory (ISO) provide new constraints to photochemical models of hydrocarbon chemistry in the outer solar system. The derived column abundances of CH3 on Saturn above 10 mbar and Neptune above the 0.2 mbar pressure level are (2.5–6.0) × 1013 cm−2 and (0.7–2.8) × 1013 cm−2, respectively. We use the updated Caltech/Jet Propulsion Laboratory photochemical model, which incorporates hydrocarbon photochemistry, vertical molecular and bulk atmospheric eddy diffusion, and realistic radiative transfer modeling, to study the CH3 abundances in the upper atmosphere of the giant planets and Titan. We identify the key reactions that control the concentrations of CH3 in the model, such as the three‐body recombination reaction, CH3 + CH3 + M → C2H6 + M. We evaluate and extrapolate the three‐body rate constant of this reaction to the low‐temperature limit (1.8×10−16 T−3.75 e−300/T, T<300 K) and compare methyl radical abundances in five atmospheres: Jupiter, Saturn, Uranus, Neptune, and Titan. The sensitivity of our models to the rate coefficients for the reactions H + CH3 + M → CH4 + M, H + C2H3 → C2H2 + H2, 1CH2 + H2 → CH3 + H, and H + C2H5 → 2 CH3, the branching ratios of CH4 photolysis, vertical mixing in the five atmospheres, and Lyman α photon enhancement at the orbit of Neptune have all been tested. The results of our model CH3 abundances for both Saturn (5.1×1013 cm−2) and Neptune (2.2×1013 cm−2) show good agreement with ISO Short Wavelength Spectrometer measurements. Using the same chemical reaction set, our calculations also successfully generate vertical profiles of stable hydrocarbons consistent with Voyager and ground‐based measurements in these outer solar system atmospheres. Predictions of CH3 column concentrations (for p≤0.2 mbar) in the atmospheres of Jupiter (3.3×1013 cm−2), Uranus (2.5×1012 cm−2), and Titan (1.9×1015 cm−2) may be checked by future observations.