Detection of C_2H_4 in Neptune from ISO/PHT-S observations
Lellouch, E.; Atreya, S. K.; Schulz, B.; Bézard, B.; Romani, P. N.; Encrenaz, Th.
Spain, France, United States
Abstract
The 6-12 mu m spectrum of Neptune has been recorded with the PHT-S instrument of the Infrared Space Observatory (ISO) at a 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. The inferred column density above the 0.2-mbar level is in the range (1.1-3)x10(14) molecules cm(-2) . To produce this low amount, previous photochemical models invoked rapid mixing between the source and sink regions of C_2H_4. We show that this requirement can be relaxed if recent laboratory measurements of CH_4 photolysis branching ratios at Lyman alpha are used.