Kinetics of hydrocarbons at low temperature: Suggestions based on photochemical models in the outer solar system

Abstract

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 model of hydrocarbon chemistry in the outer solar system, and the low-temperature hydrocarbon kinetics. 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. These observations imply a kinetically sensitive test to the measured and estimated hydrocarbon rate constants at low temperatures. We use the updated Caltech/JPL 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 (Jupiter, Saturn, Uranus, Neptune) and Titan.