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P. Drossart, Th. Fouchet, J. Crovisier, E. Lellouch, Th. Encrenaz (Paris Obs.), H. Feuchtgruber (Max Planck Inst.), J.-P. Champion (Univ. Bourgogne)
The observations of ISO/SWS between 3.0 and 3.4 micron exhibit faint emission features, at the level of few Jansky, both in Jupiter equatorial spectra and Saturn disk-averaged spectra, corresponding to the spectral location of methane \nu3 band group (Encrenaz {\it et al.}, {\em Astron. Astrophys.}, 1996; de Graauw {\it et al}, {\em Astron. Astrophys.}, 1997). A preliminary attempt to interpret the Jovian emission concluded to thermal emission in the upper stratosphere through the \nu3 band of methane, but this interpretation has now been ruled out by further investigations, including a joint fit of both fundamentals, \nu4 band at 7.7~\mum and \nu3 at 3.3~\mum. The best interpretation of the observed emissions is now obtained from a model of fluorescence of solar light through the rovibrational structure of the CH4 molecules. The model includes the absorption of solar photons in the \nu3 band (3.3~\mum), the 2.3~\mum and the 1.7~\mum band groups, with line parameters obtained from recent analysis of methane vibration bands (Tyuterev {\it et al}, {\em JQSRT}, 1994). The maximum of absorption peaks at atmospheric levels of 10~\mubar, 60~\mubar and 160~\mubar respectively, with the photochemical model of Gladstone {\it et al.} ({\em Icarus}, 1996). The re-emission of photons corresponds to transitions between various levels of the complex vibrational structure of methane; a non-monochromatic, non-LTE model of radiative transfer has been developped, following the scheme of vibrational relaxation of CH4 detailed in Appleby ({\em Icarus}, 1990). Fits to the observed ISO spectra give some constraints on the methane abundance vertical profile in the upper stratosphere of Jupiter and Saturn, which will be compared to current photochemical models of methane in these planets.