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J.L. Ollivier, F. Billebaud (Observatoire de Bordeaux), P. Drossart (Observatoire de Paris-Meudon), M. Dobrij\'evic (Observatoire de Bordeaux), M. Roos-Serote (Observatorio Astronomico de Lisboa), T. August-Berneix (Observatoire de Bordeaux), I. Vauglin (Observatoire de Lyon)
We report here the analysis of thermal infrared images of Saturn. These images, obtained at the CFHT in December 1992, were acquired at six different wavelengths (10.91, 11.69, 12.47, 13.09, 13.29 and 13.48 \mum) located in the phosphine, ethane and acetylene emission bands, probing levels in the upper troposphere and lower stratosphere. Many features are visible on these images: (i) a darkening due to the rings, located south of the equator, (ii) a north equatorial bright belt extending from 20\rm{o}N to 40\rm{o}N, and (iii) a strong polar bright structure. These observations confirm the presence of a seasonal cycle on Saturn: during Voyager approach, the southern hemisphere was hotter than the northern hemisphere, and our images show that in December 1992, the hotter hemisphere is the northern one . The seasonal climate model of Saturn's stratosphere of Bézard and Gautier (1985) is also in agreement with this result. Using a radiative line-by-line transfer code, we obtained from these images the latitudinal variations of the temperature. The upper troposphere is quite isothermal over the northern hemisphere (its temperature shows a small decrease of about 3K towards the north pole), whereas the stratospheric temperature exhibits a large amplitude: from 15\rm{o}N to 40\rm{o}N, it is quite constant and beyond 50\rm{o}N the increase of the stratospheric temperature is of the order of 10K.
The author(s) of this abstract have provided an email address for comments about the abstract: ollivier@observ.u-bordeaux.fr