First results of ISO-SWS observations of Saturn: detection of CO_2_, CH_3_C_2_H, C_4_H_2_ and tropospheric H_2_O.
Beintema, D. A.; Lellouch, E.; Encrenaz, T.; de Graauw, T.; Morris, P.; Feuchtgruber, H.; Drossart, P.; Griffin, M.; Salama, A.; Roelfsema, P. R.; Vandenbussche, B.; Heras, A.; Naylor, D. A.; Roos-Serote, M.; Leech, K.; Valentijn, E. A.; Bezard, B.; Kessler, M.; Davis, G. R.
Abstract
The spectrum of Saturn has been recorded between 4.5 and 16.0μm with 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 CO_2_, CH_3_C_2_H and C_4_H_2_ in the stratosphere and (2) the detection of H_2_O in the troposphere. In the 4.5-5.5μm range, information is retrieved on the tropospheric composition (NH_3_, PH_3_, AsH_3_, GeH_4_, CH_3_D and H_2_O) down to pressure levels of several bars. Above 7μm, the Saturn spectrum probes the upper troposphere and the lower stratosphere, at pressure levels ranging from 0.5bar to 0.4mbar. The CH_4_ emission band at 7.7μm and the H_2_-He continuum longward of 11μm are used to retrieve the thermal profile, which is then used to derive the vertical distributions of minor species: NH_3_, PH_3_, CH_3_D in the troposphere, and C_2_H_2_ and C_2_H_6_ in the stratosphere. Estimates of the CO_2_, CH_3_C_2_H and C_4_H_2_ mean mixing ratios (above the 10-mbar level) are 3x10^-10^, 6x10^-10^ and 9x10^-11^ respectively. The retrieved disk-averaged thermal profile is found to be colder in the stratosphere than the Voyager 1 ingress radio-occultation profile by about 7K at P=0.5mbar, and slightly warmer in the troposphere (about 5K at 400mbar and 3K at 150mb).