Detection of methyl radicals (CH_3) on Saturn

Abstract

We report the first detection of methyl radicals (CH_3) in the upper atmosphere of Saturn. Emission from the nu_2 Q-branch of CH_3 at 16.50 mu m was detected from observations with the Short-Wavelength Spectrometer of the ISO satellite. A CH_3 column density in the range 1.5-7.5x10(13) molec cm(-2) is derived, taking into account uncertainties in the data calibration, CH_3 band strength, and atmospheric model. Results are compared with predictions of photochemical models. The CH_3 abundance is mostly sensitive to the poorly known methyl recombination rates and to the eddy mixing profile. The ISO observations imply either CH_3 recombination rates larger than used in current photochemical models, or an eddy mixing coefficient near the methane homopause smaller than inferred from Voyager ultraviolet measurements. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of NASA and ISAS. The SWS instrument is a joint project of the SRON and the MPE