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Astron. Astrophys. 321, L13-L16 (1997)

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Letter to the Editor

First results of ISO-SWS observations of Saturn: detection of CO2, CH3 C2 H, C4 H2
and tropospheric H2 O *

Th. de Graauw 1, 2, H. Feuchtgruber 3, 4, B. Bézard 5, P. Drossart 5, Th. Encrenaz 5, D. A. Beintema 1, 4, M. Griffin 6, A. Heras 4, M. Kessler 4, K. Leech 4, E. Lellouch 5, P. Morris 4, 7, P. R. Roelfsema 1, 4, M. Roos-Serote 5, A. Salama 4, B. Vandenbussche 4, 8, E. A. Valentijn 1, 4, G. R. Davis 9 and D. A. Naylor 10

1 SRON, PO box 800, 9700 AV Groningen, The Netherlands
2 Kapteyn Astronomical Institute, PO box 800, 9700 AV Groningen, The Netherlands
3 Max-Planck Institut für Extraterrestrische Physik, Giessenbach Str.1, 85748 Garching, Germany
4 ISO Science Operation Center, Astrophysics Division, ESA, PO box 50727, 28080 Villafranca, Madrid, Spain
5 DESPA, Observatoire de Paris, 92195 Meudon, France
6 Queen Mary and Westfield College, Mile Ends Road, London E1 4NS, United Kingdom
7 SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
8 Instituut voor Sterrenkunde, University of Leuven, Celestÿnenlaan 200, 3001 Heverlee, Belgium
9 Institute of Space and Atmospheric Studies, University of Saskatchewan, 116 Science Place, Saaskatoon, Sask. S7N 5E2, Canada
10 Dept. of Physics, University of Lethbridge, 4401 University Drive, Lethbridge, Alta. T1K 3M4, Canada

Received 13 February 1997 / Accepted 6 March 1997


The spectrum of Saturn has been recorded between 4.5 and 16.0 µm with the grating mode of the Short-Wavelength Spectrometer (SWS) of ISO. The resolving power is 1500. The main results of this observation are (1) the detection of CO2, CH3 C2 H and C4 H2 in the stratosphere and (2) the detection of H2 O in the troposphere. In the 4.5-5.5 µm range, information is retrieved on the tropospheric composition (NH3, PH3, AsH3, GeH4, CH3 D and H2 O) down to pressure levels of several bars. Above 7 µm, the Saturn spectrum probes the upper troposphere and the lower stratosphere, at pressure levels ranging from 0.5 bar to 0.4 mbar. The CH4 emission band at 7.7 µm and the H2 -He continuum longward of 11 µm are used to retrieve the thermal profile, which is then used to derive the vertical distributions of minor species: NH3, PH3, CH3 D in the troposphere, and C2 H2 and C2 H6 in the stratosphere. Estimates of the CO2, CH3 C2 H and C4 H2 mean mixing ratios (above the 10-mbar level) are 3 10-10, 6 10-10 and 9 10-11 respectively. The retrieved disk-averaged thermal profile is found to be colder in the stratosphere than the Voyager 1 ingress radio-occultation profile by about 7 K at P = 0.5 mbar, and slightly warmer in the troposphere (about 5 K at 400 mbar and 3 K at 150 mb).

Key words: Planets - Saturn - infrared: solar system

* ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

Send offprint requests to: Th. de Graauw


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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998