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[27.02] Detection of Methyl Radical in Neptune's Atmosphere from ISO-SWS Observations

P. N. Romani (NASA - Goddard Space Flight Center), B. B\'ezard, T. Encrenaz (Observatoire de Paris, Section de Meudon, France), H. Feuchtgruber (ISO Science Operations Center, ESA, Madrid, Spain)

We report the first detection of the methyl radical (CH₃) in the atmosphere of Neptune. Spectra were recorded on 12 November 1997 using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) in the grating mode. The spectral range 16.44-16.56 microns was observed during 50 minutes of integration at a resolving power of approximately 1500. The final spectrum shows prominent emission from the v₂ band of CH₃ centered at 16.497 microns with a signal-to-noise ratio of about 30. This emission feature can be reproduced with a CH₃ column density of about 2 x 10¹³ molecules cm^-2 in the upper atmosphere. We then used the observations to constrain a one-dimensional hydrocarbon photochemical model. Model output, CH₃ mixing ratio vs. altitude, was used to generate synthetic spectra that were then compared to the observed spectrum. We will report on the model sensitivity to the assumed methane mixing ratio in the lower stratosphere, eddy mixing profiles, H flux from the ionosphere, and uncertainties in the photochemical reaction scheme - particularly the reactions CH₃ + CH₃ and CH₃ + H.