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Astron. Astrophys. 350, L13-L17 (1999)

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Letter to the Editor

Detection of C2H4 in Neptune from ISO/PHT-S observations

B. Schulz 1, Th. Encrenaz 2, B. Bézard 2, P.N. Romani 3, E. Lellouch 2 and S.K. Atreya 4

1 ISO Data Centre, ESA, P.O. Box 50727, E-28080 Madrid, Spain
2 DESPA, Observatoire de Paris, F-92195 Meudon, France
3 NASA Goddard Space Flight Center, Code 693.2, Greenbelt, MD 20771, USA
4 The University of Michigan, Ann Arbor, MI 48109-1243, USA

Received 27 July 1999 / Accepted 6 September 1999


The 6-12 µm spectrum of Neptune has been recorded with the PHT-S instrument of the Infrared Space Observatory (ISO) at a resolution of 0.095 µm. In addition to the emissions of CH4, CH3D and C2H6 previously identified, the spectrum shows the first firm identification of ethylene C2H4. The inferred column density above the 0.2-mbar level is in the range (1.1-3)[FORMULA]1014 molecules cm-2. To produce this low amount, previous photochemical models invoked rapid mixing between the source and sink regions of C2H4. We show that this requirement can be relaxed if recent laboratory measurements of CH4 photolysis branching ratios at Lyman [FORMULA] are used.

Key words: planets and satellites: general – planets and satellites: individual: Neptune – infrared: solar system

Send offprint requests to: B. Schulz (bschulz@iso.vilspa.esa.es)

© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999