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Astron. Astrophys. 336, L85-L89 (1998)

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Letter to the Editor

Evidence for water vapor in Titan's atmosphere from ISO/SWS data *

A. Coustenis 1, A. Salama 2, E. Lellouch 1, Th. Encrenaz 1, G.L. Bjoraker 3, R.E. Samuelson 3, Th. de , Graauw 4, H. Feuchtgruber 2 and M.F. Kessler 2

1 DESPA, Observatoire de Paris, F-92195 Meudon, France
2 ISO Science Operation Center, ESA, P.O. Box 50727, E-28080 Madrid, Spain
3 NASA/GSFC, Greenbelt, Maryland, USA
4 SRON, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 27 May 1998 / Accepted 2 July 1998


The infrared spectrum of Titan around 40 µm was recorded in the grating mode of the Short Wavelength Spectrometer (SWS) of ISO, with a resolving power of about 1900. Two emission features appear at 43.9 and 39.4 µm, where pure rotational water lines are expected. Line strengths are about 8 times the 1[FORMULA] statistical noise level. The H2O vertical profile for water suggested by the photochemical model of Lara et al. (1996), rescaled by a factor of about [FORMULA], is compatible with the data. The associated water mole fraction is about [FORMULA] at an altitude of 400 km (column density of [FORMULA] mol cm-2 above the surface). The inferred water influx at 700 km in Titan's atmosphere is in the range (0.8-2.8) [FORMULA] mol cm-2s-1.

Key words: planets and satellites: Titan – infrared: solar system

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of NASA and ISAS. The SWS instrument (P.I. Th. de Graauw) is a joint project of the SRON and the MPE. RES would like to thank D. Hamilton for illuminating discussions regarding dust transport in the Saturn System.

Send offprint requests to: A. Coustenis (e-mail: coustenis@obspm.fr)

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998