Titan Observations with ISO
A. Coustenis (DESPA, Observatoire de Paris), A. Salama (ISO Science Operations Center), Th. Encrenaz, E. Lellouch, D. Gautier (DESPA, Obs. de Paris), M. F. Kessler (ISO SOC, Madrid), Th. de Graauw (SRON), M. J. Griffin (QMW College), G. Orton (JPL), R. Wittemberg (DESPA, Obs. Paris)
The European satellite ISO (Infrared Space Observatory), launched in November 1995, is dedicated to the infrared observation of the sky by imaging, photometry and spectroscopy. Two spectrographs cover the 2.3-197 m range with a resolving power ranging from 200 to 2000 (grating mode) and the 11.4 - 180 m range with R = 10000-30000 (Fabry-Pérot mode). Observations of Titan were obtained in January 1997 with the SWS/Grating. The spectral resolution is about 5-10 times better than the IRIS-Voyager spectra, and the sensitivity is better than 1 Jy in the 7-16 micron range. Synthetic calculations to model the spectrum at the ISO resolution have been made by Coustenis et al. (Icarus 102, 240, 1993) and show a good agreement with the data.
In the thermal range (above 7 m) observed by ISO, emission features include all the hydrocarbons and nitriles previously detected by the Voyager/IRIS spectrometer. With the ISO/SWS resolution we have obtained new precise measurements of the abundances of these trace constituents on a disk average and have investigated the presence of other possible candidate molecules such as H O, C H (allene) and C H (benzene). These remain undetected as yet, but we have set upper limits to their abundances (e.g. for a constant-with-height water stratospheric abundance). A more accurate determination of the temperature profile will be obtained by analysing the methane band at 7.7 m.
New ISO/LWS/Grating and ISO/SWS/Fabry-Pérot observations of Titan are expected in June 1997.