Fluorescence in the 3 micron bands of methane on Jupiter and Saturn from ISO/SWS observations

P. Drossart ¹, Th. Fouchet ¹, J. Crovisier ¹, E. Lellouch ¹, Th. Encrenaz ¹, H. Feuchtgruber ², J.P. Champion ³,

¹ Observatoire de Paris/Meudon

² Max Planck Institute, Garching, Germany

³ Université de Bourgogne, France

Observations of Jupiter and Saturn with ISO/SWS (Encrenaz et al., Astron. Astrophys., 315, L397, 1996; de Graauw et al., Astron. Astrophys., 321, L13, 1997) show evidences for methane emission features in the n3 band range between 3.2 and 3.4 , at the flux level of a few Jansky. Possible interpretations of these emissions are thermal emission in a hot stratospheric layer, heated by precipitating particles, or fluorescent scattering of solar light in vibrational bands of CH₄. Synthetic calculations favors the second interpretation, and a complete model of fluorescence will be presented for Jupiter and Saturn. On Jupiter, the thermal structure of the atmosphere and the composition are adapted from Gladstone et al. (Icarus, 119, 1; 1996), the thermal profile being retrieved from the n4 band of methane observed in the same ISO spectra. Line parameters for the CH₄ vibrational bands are calculated from the STDS spectroscopic databank (Tyuterev et al., J. Quant. Spectrosc. Rad. Transfer, 52, 459; 1994). In particular, evidences are obtained for fluorescence within the n₃ band complex of methane, as well as between overtone vibration bands cascading in the 3.3  range. The solar absorption occurs within the 3.3  range (Pentad vibration band system), the 2.3  range (Octad band system) and the 1.7  range (Tetradecad band system), the absorption occuring predominantly in the 10 mbar, 60 mbar and 160 mbar atmospheric pressure range respectively. A similar model will be obtained for Saturn methane emissions. Constraints on the vertical distribution of methane will be obtained from the modelling, giving a determination of the vertical distribution of CH₄ in the upper stratosphere. Such a determination will be useful to constrain the photochemical models of Jupiter and Saturn, which depends on the eddy diffusion coefficient, a parameter still poorly known in the upper atmospheres of giant planets. These observations will also help improving the models of thermal balance in the upper stratospheres of Jupiter and Saturn, where most of the solar absorption occurs.