Astron. Astrophys. 356, 347-356 (2000)

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

J.L. Ollivier ¹, F. Billebaud ¹, P. Drossart ², M. Dobrijévic ¹, M. Roos-Serote ³, T. August-Bernex ¹ and I. Vauglin <sup>*  4

1</sup> Observatoire de Bordeaux, BP 89, 32270 Floirac, France
² DESPA, Observatoire de Paris, 92195 Meudon, France
³ Observatorio Astronomico de Lisboa, 1349-018 Lisboa, Portugal
⁴ Observatoire de Lyon, 69561 Saint Genis Laval, France

Received 1 October 1999 / Accepted 27 January 2000

Abstract

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^o of solar longitude). These observations were made using C10µ, a 6464 pixel camera, at 6 different wavelengths (10.91, 11.69, 12.47, 13.09, 13.29 and 13.48 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 m), (ii) a very bright north-polar emission (especially at 11.69 and 12.47 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^o N and 40^o N, and greatly increases (about +10K) beyond 60^o 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^o N). But, from comparison with previous works (Tokunaga et al. 1978), we consider that this latter explanation is not likely.

Key words: planets and satellites: individual: Saturn infrared: solar system

* Visiting Astronomer, Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii

Send offprint requests to: J.L. Ollivier

Correspondence to: ollivier@observ.u-bordeaux.fr

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000

helpdesk.link@springer.de