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

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1. Introduction

Many previous infrared observations of Saturn have led to the conclusion that a strong seasonal effect exists on Saturn in the upper atmosphere. Gillett and Orton (1975), using center-to-limb scans at 11.7 [FORMULA]m, showed that the south pole of Saturn was unexpectedly bright, and related this with the longer insolation of southern high latitudes, leading to a temperature enhancement. Rieke (1975) and Tokunaga et al. (1978) also observed such a feature at other wavelengths (respectively 12.35 [FORMULA]m and 17.8, 19.7, 22.7 [FORMULA]m) and derived the same conclusions. From the observations of the Voyager probes, Hanel et al. (1982) showed that the tropopause temperature is approximately 10K cooler in the north polar region than in the south polar region. Furthermore, from Voyager IRIS (Infrared Interferometer Spectrometer and Radiometer) spectra, Conrath and Pirraglia (1983) retrieved the latitudinal variations of the temperature in the troposphere at three pressure levels. Their results clearly showed a north/south asymmetry, as well as small scale thermal variations, which are assumed to be due to meridional circulation. This north/south asymmetry consists of an equator-to-pole temperature gradient which is lower towards the insolated pole than towards the pole which lies in the planetary shadow. This gradient is especially clear for the 150 mbar level (e.g. Fig. 1 of Conrath and Pirraglia, 1983), which corresponds to the upper troposphere.

Bézard et al. (1984) and Bézard & Gautier (1985) developed a seasonal climate model of Saturn's upper troposphere and stratosphere, which reproduces the strong thermal hemispheric asymmetry and its temporal evolution. They predicted a seasonal cycle with a temperature amplitude of about 30K at the 5 mbar levels of both poles, and they agreed with the observations of a warm south pole at the epoch of Voyager's observations. Moreover, they predicted a reverse situation for the following years, the north pole becoming hotter than the south pole since approximately 1985. Subsequent observations brought a confirmation: from infrared images of Saturn obtained in March 1989, Gezari et al. (1989) showed that the equator-to-north-pole temperature gradient is larger than the equator-to-south-pole gradient, confirming the presence of a seasonal effect. Conrath et al. (1990) used a radiative-dynamical code to calculate the temperatures of Saturn's atmosphere during the seasonal cycle, and Barnet et al. (1992) used an extension of the same code, but including the effects of the rings. These two models agree closely, and clearly showed the evolution of the atmosphere. In particular, the atmosphere at north summer solstice (90o of solar longitude) shows a structure which could explain quite well the observations of Gezari et al. (1989), which were obtained just after the north summer solstice, when the solar longitude of Saturn was 103.5o.

In this paper, we present observations of Saturn made with the ground-based mid-infrared camera "C10µ" at six different wavelengths (10.91, 11.69, 12.47, 13.09, 13.29, and 13.48 [FORMULA]m), in the thermal continuum and in the [FORMULA] ethane and [FORMULA] acetylene-band emissions. The emission of the atmosphere at these wavelengths is calculated with a line-by-line radiative transfer code and then compared with the observed fluxes. This will allow us to put constraints either on the temperature or on the hydrocarbon abundances. Ethane (C2H6) and acetylene (C2H2) are formed in the stratosphere of Saturn following methane (CH4) photodissociation by ultraviolet photons. Their abundances have been constrained by many observations, such as Voyager's IRIS (Courtin et al. 1984) and ISO (Infrared Space Observatory, de Graauw et al. 1997) spectra. However, these observations both present limitations. As the field of view of the ISO-SWS (Short-Wavelength Spectrometer) instrument was comparable to the size of Saturn, we only have a globally-averaged value of ethane and acetylene abundances. Regarding Voyager's abundance determinations, Courtin et al. (1984) only considered a limited latitudinal range. Accordingly, no information about the spatial variations of hydrocarbon abundances is available. It is yet likely that such variations occur in giant planets' atmospheres. Indeed, hydrocarbon abundances depend on many parameters, as shown by different photochemical models (see for example Atreya, 1986, Ollivier et al. 2000a): solar insolation, temperature profile, vertical transport... Moreover, a process like the energetic particle precipitation around the poles can produce an additional chemistry which can lead to a modification of compound abundances (Kostiuk et al. 1987). Beside these possible abundance effects, the large tilt of the spin axis should produce an evolution of the thermal structure of Saturn's atmosphere, as stated by previous works (Tokunaga et al. 1978, Gillett & Orton, 1975). Nevertheless, these previous works never consider the possible variation of hydrocarbon abundances. Consequently, we will consider two assumptions in the present analysis:

  • the first one considers the same ethane and acetylene abundances over all latitudes. The observed variations of the infrared flux are then only "thermal".

  • the second one includes the possibility of variations of the hydrocarbon abundances.

Section 2 presents the general characteristics of the observations. Section 3 is devoted to the data reduction and calibration procedures and presents the resulting images. In Sect. 4, we describe the radiative transfer code we used and the starting hypotheses. Then, Sect. 5 is devoted to the analysis of the images and Sect. 6 to the discussion of our results.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000