Saturn Tropospheric Water Measured with ISO/SWS

Computer Science – Sound

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Observations of Saturn with ISO/SWS (de Graauw et al., Astron. Astrophys., 1997) have provided well calibrated disk-averaged spectra over a wide spectral range (2.3--45 mu m), 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 (>= 5 bars) are sounded in the 5 mu m range, where solar reflected sunlight and thermal emission combine to form the Saturn spectrum. Molecular absorption is dominated by PH_3 and NH_3 between 4.5 and 5.5 mu m, with deep abundances relative to H_2 of 3.5x 10(-6) and 1.0x 10(-4) respectively. H_2O 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 et al., 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 H_2O 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 (e.g. Orton et al., Science, 1996). On the contrary, the 5 mu m thermal emission of saturnian belts does not exhibit contrasts as large as the Jovian belts. The low saturnian H_2O 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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