[Previous] | [Session 27] | [Next]

[27.01] Saturn Tropospheric Water Measured with ISO/SWS

P. Drossart (Paris Obs.)

Observations of Saturn with ISO/SWS (de Graauw {\it et al.}, {\em Astron. Astrophys.}, 1997) have provided well calibrated disk-averaged spectra over a wide spectral range (2.3--45~\mum), at a resolving power of 1500. These spectra give access to the atmospheric composition at many different levels, depending on the wavelength. As ground-based observations have demonstrated, the deepest levels (\geq~5~bars) are sounded in the 5~\mum range, where solar reflected sunlight and thermal emission combine to form the Saturn spectrum. Molecular absorption is dominated by PH₃ and NH₃ between 4.5 and 5.5~\mum, with deep abundances relative to H₂ of 3.5\times 10^-6 and 1.0\times 10^-4 respectively. H₂O is detected in the troposphere of Saturn from several absorption lines. A full interpretation of the Saturn spectrum is made with a model similar to the one used for Galileo/NIMS interpretation (Roos-Serote {\it et al.}, {\em J. Geophys. Res.}, 1998). In addition to the solar reflected component by a cloud layer at about 520~mbar, a thermal emission from a deeper cloud (1.5 bar) is needed to fit the continuum. A spectral contribution from the rings is also present. The abundance of H₂O is measured at about 1.5% only of a saturation profile. Such a low saturation value is reminiscent of the water measurements in the hot spots of Jupiter, as measured by Voyager/IRIS, Galileo Probe and NIMS, and could be representative of dry area on Saturn. On Jupiter, the thermal emission from the hot spots, despite their small relative area, dominates the 5 micron spectrum of the average disk, because of the high contrasts in brightness temperatures ({\it e.g.} Orton {\it et al.}, {\em Science}, 1996). On the contrary, the 5~\mum thermal emission of saturnian belts does not exhibit contrasts as large as the Jovian belts. The low saturnian H₂O abundance measured by ISO, if not relevant to the whole planet, would imply that large inhomogeneities must be present at smaller spatial scales, than observed in ground-based images. Cassini observations, in particular with VIMS observations in the 5 micron range, should help understanding the origin of Saturn apparent dryness.