The spectrum of Saturn was measured with the ISO Long Wavelength Spectrometer (LWS) during both the performance verification and routine phase of the mission.
End to end grating scans were used to cover the wavelength range from 43 to
197 
m, and the spectrum obtained was compared with a multilayer,
radiative-transfer model of the saturnian atmosphere based on Voyager data.
It was found that the slope of the measured continuum within each detector
passband is in good agreement with the model and that most of the absorption
features are due to ammonia and phosphine. For the lines at long wavelengths,
however, there are discrepancies between model and observations. These could
be reduced in the case of Phosphine by lowering the model cutoff caused by
photodissociation to 300 mbar and increasing the assumed portion of this
molecule in the troposphere to 7 x 10-5. Also three methane lines were
detected which were deeper and broader than predicted by the model.
The LWS Fabry-Pérot was later used to detect for the first
time in the atmosphere of Saturn the R(1) rotational line of HD at 56.23
m. The measured spectrum, shown on the cover, has been compared with an
atmospheric model in which the D/H ratio was a free parameter.
The best model fit was obtained with a D/H ratio of 2.3 x 10-5; D/H values between 1.5 x 10-5 and 3.5 x 10-5 are however also compatible with the data. This result is intermediate between the saturnian value derived from ground-based observations of deuterated methane, and the jovian D/H ratio measured by the mass spectrometer in the Galileo probe.
These results were presented in the A & A Special ISO Issue (G. Davis et al. and M. Griffin et al.). Further LWS observations with a higher signal-to-noise ratio will be used to improve the accuracy of these tentative results.