Detection of the Methyl Radical on Neptune

We report the first detection of the methyl radical (CH 3 ) in the upper atmosphere of Neptune. Observations with the Short-Wavelength Spectrometer of the Infrared Space Observatory ( ISO ) satellite at a resolving power of 2200 revealed several emission features from the ν 2 Q -branch of CH 3 around 16.50 μm. The column density of methyl radicals above the 0.2 mbar level is 1.6 +1.2 −0.9 × 10 13 molecules cm -2 . Results are compared with predictions of photochemical models. The CH 3 abundance is mostly sensitive to the eddy mixing profile and to the poorly known methyl recombination rate at low pressure. Using Slagle et al.'s expression for this rate, the present observations imply an eddy mixing coefficient at the methane homopause in the range of 2-9 × 10 6 cm 2 s -1 . Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients.