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Th. Encrenaz, B. B\'ezard, E. Lellouch (DESPA, Obs. Paris), B. Schulz (ISO Data Center, ESA, Madrid), P. N. Romani (NASA/GSFC), S. K. Atreya (Un. of Michigan)
The 6 -- 12 \mum spectrum of Neptune has been recorded with the photometer of the Infrared Space Observatory (ISO) in its spectroscopic mode (PHT-S) with a spectral resolution of 0.095 \mum. In addition to the emissions of CH4, CH3D and C2H6 previously identified, the spectrum shows the first firm identification of ethylene C2H4 through the Q-branch of its \nu7 band at 10.5 \mum. Radiative transfer calculations were carried out in the same way as in Bézard et al. (Astrophys. J. 515, 868, 1999). The inferred column density is in the range 1.1--3 1014 molecules cm-2. To produce the low value of C2H4, previous photochemical models (Romani et al., Icarus 106, 442, 1993; Bishop et al., Plan. Space Sci. 46, 1, 1997) invoked rapid mixing between the source and sink regions of C2H4. However, this requirement can be relaxed if recent laboratory measurements of CH4 photolysis branching ratios at Lyman \alpha are used (Smith and Raulin, J. Geophys. Res. 104, 1873, 1999). There is still a need to accurately measure the CH4 branching ratios at Lyman \alpha, the ultimate source of C2H4, and the rate coefficient of H + C2H4, the dominant sink of C2H4, at low temperature and pressure.