Abstract
We report the first detection of the methyl radical (CH3) 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 ν2 Q-branch of CH3 around 16.50 μm. The column density of methyl radicals above the 0.2 mbar level is 1.6+1.2−0.9 × 1013 molecules cm-2. Results are compared with predictions of photochemical models. The CH3 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 × 106 cm2 s-1. Recombination rates higher than used in current photochemical models would lead to larger eddy mixing coefficients.
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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.