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

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4. Radiative transfer treatment

The outgoing infrared flux is calculated with a line-by-line radiative transfer code which was primarily developed for the analysis of Galileo/NIMS data of Jupiter (see for example Roos-Serote et al. 1998) and adapted to Saturn. We only consider the opacities of phosphine, ethane and acetylene, which are the only compounds having significant opacities in the wavelength range we study. The molecular spectroscopic parameters for PH3, C2H2 and C2H6 are taken from the GEISA databank (Jacquinet-Husson et al. 1998). This code also includes the Collision Induced Absorption (CIA) due to H2-H2 and H2-He collision (Borysow et al. 1985).

Parameters

We present here the characteristics of the thermal profile, phosphine, ethane and acetylene abundances, and cloud structure we used in our models. The determination of these parameters have generally been done only for a precise location and temporal variations certainly affect them. We then just consider the previously observed values as a starting point, the aim of our work being to give information on the variations of these parameters.

  • Thermal profile
    We use the profile derived by Lindal et al. (1985) from the radio occultation of Voyager by Saturn's atmosphere at a latitude of 36.5o N. This profile extends from 1300 mbars to 0.2 mbar and shows a tropopause temperature of 82K. Hubbard et al. (1997) determined the thermal structure of Saturn's mesosphere, above the profile derived by Lindal et al. (1985), from the occultation of [FORMULA]Sco. This observation was made at near-infrared wavelengths and probed equatorial latitudes. They showed that the mesosphere of Saturn is nearly isothermal (between 140 and 150K). We construct our standard temperature profile, quoted [FORMULA] in the following, from these two observations: the lower part consists in the profile of Lindal et al. (1985) and the upper part (above 1 mbar) is isothermal (141K).

  • Abundances
    From the analysis of the Voyager IRIS spectra acquired in August 1981, Courtin et al. (1984) derived mixing ratios of phosphine, ethane and acetylene equal to 1.4[FORMULA]0.8[FORMULA]10-6, 3[FORMULA]1.1[FORMULA]10-6 and 2.1[FORMULA]1.4[FORMULA]10-7 respectively, with the following vertical distributions: C2H6 and C2H2 have constant mixing ratios above 20-50mbar, and PH3 has a constant mixing ratio below 3-6 mbar. These results were derived from spectra probing northern latitudes (20o-40o N), around the location of the Voyager 2 ingress temperature profile (36.5o N). Phosphine is present only in the lower part of the atmosphere of Saturn (Courtin et al. 1984), below levels around 3-6 mbar, therefore, its mixing ratio at higher altitudes is set to zero. Ethane and acetylene, for their part, are produced in the stratosphere and then transported to upper and lower levels. Photochemical models calculate the vertical distribution of such compounds and show that their mixing ratios are not constant with altitudes (Strobel 1983, Gladstone et al. 1996, Moses et al. 1999, Ollivier et al. 2000a). Therefore the vertical distribution of ethane and acetylene is an additional parameter. In order to obtain the vertical distribution of C2H2 and C2H6, we use an improved version of the model of Ollivier et al. (2000a). Their results show a discrepancy between the calculated ethane mole fraction and the values derived from the observations of Voyager/IRIS or ISO/SWS. Some corrections of the chemical scheme have been done and with an adequate eddy diffusion coefficient profile, a good agreement between the observed and calculated ethane and acetylene mole fraction is obtained (a detailed explanation of the corrections will be available in Ollivier et al. 2000b). Fig. 4 presents the resulting ethane and acetylene mole fraction vertical profile, which we use in our radiative transfer calculations. The vertical distribution of the molecules is then not anymore a parameter.

  • Cloud structure
    The cloud structure is the less constrained parameter. Many studies of Saturn's clouds have been made from visible images, but their optical properties in the infrared are not well known. In order to fit the ISO spectra, de Graauw et al. (1997) had to include some clouds in their radiative transfer model. They considered two cloud layers: the first cloud is located at a pressure of 1.55 bar and has a transmission of 0.2, and the second cloud, at 0.55 bar, has a transmission 0.9. In our analysis, we considered the same structure without any spatial variations and the clouds are supposed to be grey absorbers.

[FIGURE] Fig. 4. Vertical distribution of ethane and acetylene mole fraction. These distributions are the same as in Ollivier et al. (2000b), where one can find the values of the different parameters used to calculate these profiles. The results of different observations are presented. The diamond and the square correspond respectively to the ethane and acetylene mole fraction derived by Courtin et al. (1984) from Voyager/IRIS spectra. The triangle is the value of ethane mole fraction retrieved from ISO spectra (de Graauw et al. 1997) and the two crosses represent two values of acetylene mole fraction derived by de Graauw et al. (1997) for two different altitudes.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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