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

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 <sup>2

1</sup> 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

Abstract

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 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 , is compatible with the data. The associated water mole fraction is about at an altitude of 400 km (column density of 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) 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)

This article contains no SIMBAD objects.

Contents

• 1. Introduction
• 2. Observations
  • 2.1. Strategy for data acquisition
  • 2.2. Reduction of the data
• 3. Modelling and results
• 4. Discussion
• References

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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