FIRST RESULTS OF ISO-SWS OBSERVATIONS OF SATURN - DETECTION OF CO2, CH3C2H, C4H2 AND TROPOSPHERIC H2O

The spectrum of Saturn has been recorded between 4.5 and 16.0 mu m wit h the grating mode of the Short-Wavelength Spectrometer (SWS) of ISO. The resolving power is 1500. The main results of this observation are (1) the detection of CO2 CH3C2H and C4H2 in the stratosphere and (2) t he detection of H2O in the troposphere. In the 4.5-5.5 mu m range, inf ormation is retrieved on the tropospheric composition (NH3, PH3, AsH3, GeH4, CH3D and H2O) down to pressure levels of several bars. Above 7 mu m, the Saturn spectrum probes the upper troposphere and the lower s tratosphere, at pressure levels ranging from 0.5 bar to 0.4 mbar. The CH4 emission band at 7.7 mu m and the H-2-He continuum longward of 11 mu m are used to retrieve the thermal profile, which is then used to d erive the vertical distributions of minor species: NH3, PH3, CH3D in t he troposphere, and C2H2 and C2H6 in the stratosphere. Estimates of th e CO2, CH3C2H and C4H2 mean mixing ratios (above the 10-mbar level) ar e 3 10(-10), 6 10(-10) and 9 10(-11) respectively. The retrieved disk- averaged thermal profile is found to be colder in the stratosphere tha n the Voyager 1 ingress radio-occultation profile by about 7 K at P = 0.5 mbar, and slightly warmer in the troposphere (about 5 K at 400 mba r and 3 K at 150 mb).