Seasonal effects in the thermal structure of Saturn's stratosphere from infrared imaging at 10 microns

We present thermal infrared images of Saturn recorded with the Canada-France-Hawaii 3.6 meter Telescope (CFHT) in 1992, during Saturn's northern summer (145.5 degrees of solar longitude). These observations were made using C10 mu, a 64 x 64 pixel camera, at 6 different wavelengths (10.91, 11.69, 12.47, 13.09, 13.29 and 13.48 mu m), sensitive to phosphine (PH3), ethane (C2H6), and acetylene (C2H2). Many features are clearly visible, in particular (i) a bright north-equatorial belt (11.69, 12.47, 13.09, 13.29 and 13.48 mu m), (ii) a very bright north-polar emission (especially at 11.69 and 12.47 mu m), (iii) a darker zone corresponding to the ring occultation. We used an infrared radiative transfer code to model the emission of Saturn's atmosphere at the observed wavelengths. Three set of parameters are of importance: (i) the cloud distribution and properties, (ii) the thermal profile, and (iii) the compound abundances. From the present observations and from comparison with previous ones obtained by other authors (Tokunaga et al. 1978), we can conclude that the observed structures likely originate in the seasonal cycle of Saturn. Its effect concentrates essentially at levels located above the 200-300 mbar level. We also retrieve the latitudinal evolution of the thermal profile: we show that the tropospheric temperature decreases towards the pole by about 3K, whereas the stratospheric temperature has the same behavior as the tropospheric temperature between 15 degrees N and 40 degrees N, and greatly increases (about +10K) beyond 60 degrees N. We also explore the possibility of explaining these features with variable hydrocarbon abundances: an enhancement of a factor of 5.5 of the ethane mixing ratio and of a factor of 6 of the acetylene mixing ratio appears then necessary at high latitudes (60 degrees N). But, from comparison with previous works (Tokunaga et al. 1978), we consider that this latter explanation is not likely.