The search for Water Vapor on Titan

R. E. Samuelson, G. L. Bjoraker (NASA/GSFC), A. Coustenis, Th. Encrenaz (DESPA Obs. de Paris-Meudon), A. Salama (ISO Sci. Op. Center, Spain), Th. de Graauw (SRON, The Netherlands)

The discovery of carbon dioxide on Titan in 1981 opened a new vista in the chemistry of reducing atmospheres. Because the principal mechanism for the production of carbon dioxide is the reaction of CO with the hydroxyl radical OH, a search began for both CO and water vapor at this time. Water vapor should be injected into Titan's atmosphere from an influx of meteoritic debris abetted by sputtering and collisions associated with Saturn's icy satellites and rings. Photolysis of water vapor produces OH, and the reaction of OH with CO should then produce carbon dioxide. Impetus for this idea followed from the discovery of CO shortly thereafter. The recent acquistion of ISO Short Wavelength Spectrometer data (0.5 wavenumber resolution) between 220 and 340 wavenumbers has reduced the upper limit of globally averaged water vapor to about one part per billion above the 10 mbar level. Data to be acquired by the ISO Long Wavelength Spectrometer is expected either to reduce this upper limit substantially, or to yield the first detection of water vapor in Titan's atmosphere. Latest results will be reported.