Detection of C_2H_4 in Neptune from ISO/PHT-S Observations

Details Detection of C_2H_4 in Neptune from ISO/PHT-S Observations Detection of C_2H_4 in Neptune from ISO/PHT-S Observations

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.6904e&link_type=abstract

American Astronomical Society, DPS meeting #31, #69.04

Astronomy and Astrophysics

Astrophysics

Scientific paper

The 6 -- 12 mu m 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 mu m. In addition to the emissions of CH_4, CH_3D and C_2H_6 previously identified, the spectrum shows the first firm identification of ethylene C_2H_4 through the Q-branch of its nu _7 band at 10.5 mu m. Radiative transfer calculations were carried out in the same way as in Bezard et al. (Astrophys. J. 515, 868, 1999). The inferred column density is in the range 1.1--3 10(14) molecules cm(-2) . To produce the low value of C_2H_4, 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 C_2H_4. However, this requirement can be relaxed if recent laboratory measurements of CH_4 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 CH_4 branching ratios at Lyman alpha , the ultimate source of C_2H_4, and the rate coefficient of H + C_2H_4, the dominant sink of C_2H_4, at low temperature and pressure.

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