Astronomy and Astrophysics Letters dedicated to ISO

ISO LWS measurement of the far-infrared spectrum of Saturn

G.R. Davis, M.J. Griffin, D.A. Naylor, P.G. Oldham, B.M. Swinyard, P.A.R. Ade, S.B. Calcutt, Th. Encrenaz, T. De Graauw, D. Gautier, P.G.J. Irwin, E. Lellouch, G.S. Orton, C. Armand, M. Burgdorf, A. Di Giorgio, D. Ewart, C. Gry, K.J. King, T. Lim, S. Molinari, M. Price, S. Sidher, A. Smith, D. Texier, N. Trams and S.J. Unger


The spectrum of Saturn from 43 to 197 mu m 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 mu m from 43 to 90 mu m and 0.6 mu m from 90 to 197 mu m. 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 PH sub three profile in which the tropospheric mixing ratio was increased to 710 minus six and the cutoff due to photodissociation was lowered to 300mbar. 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.

Planets and satellites: Individual: Saturn - Infrared: solar system

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