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.