ICARUS 38, 411--419 (1979) Far-Infrared Photometry of Planets: Saturn and Venus R. COURTIN Laboratoire de Physique SteUaire et Plandtaire, 91370 Verri~res le Buisson, France P. L~NA, M. DE MUIZON, AND D. ROUAN Universit~ Paris V I I et Observatoire-Equipe de Recherche A ssoci~e au CN RS, 92190 Meudon, France C. NICOLLIER Space Science Department, ESTEC, The Netherlands AND J. WIJNBERGEN Space Science Group, Groningen, The Netherlands Received July 17, 1978; revised December 21, 1978 We present far-infrared observations of Saturn and Venus made within four spectral bands (31 to 38, 47 to 67, 71 to 94, and 114 to 196 ~tm) using a 32-cm airborne telescope during May 1977. The set of brightness temperatures obtained from Saturn is analyzed on the basis of thermal models of the atmosphere of this planet. The best agreement is obtained with an effective temperature of about 95°K for the planet itself and a ring contribution cor- responding to brightness temperatures ranging from 55 to 70°K. These values of the tem- perature of the ring system are smaller than the ones measured at shorter wavelengths and could be indicative of a decreasing emissivity of the rings in the far infrared. I. INTRODUCTION Far-infrared data on planetary fluxes are scarce and difficult to obtain in a reli- able fashion. However, they play two important roles : (i) The planets, up to Saturn, are con- venient calibration sources for infrared observations carried out above the tropo- pause and beyond 25 tLm in wavelength. (ii) The fluxes, or brightness tempera- tures at a given wavelength, provide in- formation on the atmospheric structure, especially for Venus, Jupiter, Saturn, Uranus, and Neptune, because of the strong wavelength dependence of the opacity. Moreover, the total infrared flux, compared to the absorbed solar radiation, provides a test of the radiative budget of the planet. The data described in this paper deal with Saturn and Venus. The data on Saturn are analyzed in Section III and the results are discussed in Section IV. A subsequent paper will interpret the Venus data. 411 0019-1035/79/060411-09502.00/0 Copyright O 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.