Detection of the Methyl Radical on Neptune

Abstract

We report the Ðrst detection of the methyl radical in the upper atmosphere of Neptune. Obser-(CH 3) vations with the Short-Wavelength Spectrometer of the Infrared Space Observatory (ISO) satellite at a resolving power of 2200 revealed several emission features from the Q-branch of around 16.50 l 2 CH 3 km. The column density of methyl radicals above the 0.2 mbar level is molecules cm~2. 1.6 ~0.9 `1.2 ] 1013 Results are compared with predictions of photochemical models. The abundance is mostly sensitive CH 3 to the eddy mixing proÐle and to the poorly known methyl recombination rate at low pressure. Using Slagle et al.Ïs expression for this rate, the present observations imply an eddy mixing coefficient at the methane homopause in the range of 2È9 ] 106 cm2 s~1. Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients. Subject headings : infrared : solar system È planets and satellites : individual (Neptune)