DPS Meeting, Madison, October 1998
Session 27. Solar System Astronomy with ISO and Prospects for SIRTF II
Invited Plenary Session, Wednesday, October 14, 1998, 10:35-11:40am, Madison Ballroom A and B

## [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.

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