DETECTION OF METHYL RADICALS (CH3) ON SATURN

We report the first detection of methyl radicals (CH3) in the upper at mosphere of Saturn. Emission from the nu(2) Q-branch of CH3 at 16.50 m u m was detected from observations with the Short-Wavelength Spectrome ter of the ISO satellite. A CH3 column density in the range 1.5-7.5 x 10(13) molec cm(-2) is derived, taking into account uncertainties in t he data calibration, CH3 band strength, and atmospheric model. Results are compared with predictions of photochemical models. The CH3 abunda nce is mostly sensitive to the poorly known methyl recombination rates and to the eddy mixing profile. The ISO observations imply either CH3 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.