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Photochemical modeling of CH_3 abundances in the outer solar system

Lee, Anthony Y. T. and Yung, Yuk L. and Moses, Julianne (2000) Photochemical modeling of CH_3 abundances in the outer solar system. Journal of Geophysical Research E, 105 (E8). pp. 20207-20225. ISSN 0148-0227.

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Recent measurements of methyl radicals (CH_3) 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 CH_3 on Saturn above 10 mbar and Neptune above the 0.2 mbar pressure level are (2.5–6.0) × 10^(13) cm^(−2) and (0.7–2.8) × 10^(13) 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 CH_3 abundances in the upper atmosphere of the giant planets and Titan. We identify the key reactions that control the concentrations of CH_3 in the model, such as the three-body recombination reaction, CH_3 + CH_3 + M → C_2H_6 + 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 + CH_3 + M → CH_4 + M, H + C_2H_3 → C_2H_2 + H_2, ^1CH_2 + H_2 → CH_3 + H, and H + C_2H_5 → 2 CH_3, the branching ratios of CH_4 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 CH_3 abundances for both Saturn (5.1×10^(13) cm^(−2)) and Neptune (2.2×10^(13) 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 CH_3 column concentrations (for p≤0.2 mbar) in the atmospheres of Jupiter (3.3×10^(13) cm^(−2)), Uranus (2.5×10^(12) cm^(−2)), and Titan (1.9×10^(15) cm^(−2)) may be checked by future observations.

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Yung, Yuk L.0000-0002-4263-2562
Moses, Julianne0000-0002-8837-0035
Alternate Title:Photochemical modeling of CH3 abundances in the outer solar system
Additional Information:© 2000 by the American Geophysical Union. Received September 20, 1999; revised April 6, 2000; accepted April 11, 2000. We thank W. DeMore and K. Bayes for valuable theoretical chemical rate constant estimates. We also thank M. Allen, M. Gerstell, and two anonymous referees for helpful comments. This research is supported by NASA grant NAG5-6263 to the California Institute of Technology.
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Issue or Number:E8
Record Number:CaltechAUTHORS:20140820-144239502
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Official Citation:Lee, A. Y. T., Y. L. Yung, and J. Moses (2000), Photochemical modeling of CH3 abundances in the outer solar system, J. Geophys. Res., 105(E8), 20207–20225, doi:10.1029/1999JE001186.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48743
Deposited By: Ruth Sustaita
Deposited On:20 Aug 2014 22:39
Last Modified:06 Nov 2019 18:19

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