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Astron. Astrophys. 315, L393-L396 (1996)

ISO LWS measurement of the far-infrared spectrum of Saturn*

G.R. Davis1, M.J. Griffin2, D.A. Naylor3, P.G. Oldham2, B.M. Swinyard4, P.A.R. Ade2, S.B. Calcutt5, Th. Encrenaz6, T. De Graauw7, D. Gautier6, P.G.J. Irwin5, E. Lellouch6, G.S. Orton8, C. Armand9, M. Burgdorf9, A. Di Giorgio9, D. Ewart9, C. Gry9, K.J. King4, T. Lim9, S. Molinari9, M. Price9, S. Sidher9, A. Smith9, D. Texier9, N. Trams9, and S.J. Unger4,

1 Institute of Space and Atmospheric Studies, University of Saskatchewan, 116 Science Place, Saskatoon, Sask., S7N 5E2, Canada
2 Dept. of Physics, Queen Mary and Westfield College, Mile End Road, London, E1 4NS, UK
3 Dept. of Physics, University of Lethbridge, 4401 University Drive, Lethbridge, Alta., T1K 3M4, Canada
4 Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX, UK
5 Atmospheric, Oceanic and Planetary Physics, Dept. of Physics, Oxford University, Parks Road, Oxford, OX1 3DW, UK
6 Observatoire de Paris-Meudon, 5 Place Janssen, F-92195 Meudon Cedex, Paris, France
7 SRON, PO Box 800, Groningen, The Netherlands
8 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
9 LWS Instrument Dedicated Team, VILSPA, Villafranca, Spain

Received 4 July 1996 / Accepted 2 August 1996

Abstract. The spectrum of Saturn from 43 to 197 img10.gifm was measured with the ISO Long Wavelength Spectrometer (LWS) during the performance verification phase of the mission. The measurements were made using the LWS in grating mode, with spectral resolutions of 0.29 img10.gifm from 43 to 90 img10.gifm and 0.6 img10.gifm from 90 to 197 img10.gifm. The spectrum was compared with an atmospheric radiative-transfer model and four results were obtained: first, the slope of the measured continuum within each detector passband is in good agreement with the model; second, absorption features due to ammonia and phosphine were unambiguously detected, and all detected features were attributed to these two molecules; third, the ammonia absorption features agree reasonably well with the nominal model (based on Voyager IRIS measurements); and fourth, the phosphine absorption features disagree with the nominal model. Superior agreement with the measured spectrum was obtained with a modified PH3 profile in which the tropospheric mixing ratio was increased to 7x10-6 and the cutoff due to photodissociation was lowered to 300 mbar. These results are based on trial observations during performance verification of the LWS and provide an indication of the results we expect to obtain when the spectrum of Saturn is measured comprehensively later in the mission.

Key words: planets and satellites: individual: Saturn - 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) and with the participation of ISAS and NASA

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