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G. R. Gladstone, R. Link, S. A. Stern (SwRI), M. Festou (Observatoire Midi-Pyrenees), J. H. Waite (University of Michigan)
The origin of Titan's thick nitrogen and methane atmosphere is a compelling enigma. One key and still missing observable concerns the abundances of noble gases in general, and argon in particular. Detection of sufficient argon could indicate that the N2 and CO now found in the atmosphere came in with ice during Titan's accretion. Alternatively, if there is very little argon, then we have to turn to models starting with frozen ammonia, methane and water ice, indicating a more important role for the Saturn sub-nebula, and requiring subsequent modification by photochemistry. Current estimates on the fraction of argon in Titan's atmosphere are crude, and based only on indirect evidence, and range up to 25%. On Sept.~21, 2000, using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, we performed an observation of Titan to search for argon and to make a survey of Titan's dayglow in the 90--115~nm FUSE bandpass. No emissions were found in the 18~ks exposure, although only 7.4~ks were obtained when FUSE was in Earth's shadow where terrestrial airglow contamination is minimal. While no Ar, N, or N2 emissions were detected, 2-\sigma upper limits of 4~R (for Ar 104.8~nm) and 20~R (for N 113.4~nm) are found using the best of the FUSE data. There is a bump on the terrestrial geocorona H Ly\beta emission at 102.5~nm which may be due to Titan and a Titan Torus. The signal in the bump is about 400~R. Model estimates suggest that the Ly\beta brightness of Titan should be about 20~R and the Titan Torus in the 30--700~R range. For an assumed argon abundance of 5% the 104.8~nm emission is predicted to be 7~R, so the argon estimate is constraining already. The nitrogen estimate is very close to the model expectation of 15~R. An accurate determination of the abundance of argon on Titan would be useful in preparing for the arrival of the Cassini orbiter and Huygens probe at the Saturn system, so further FUSE observations of Titan are planned.
We gratefully acknowledge support from NASA through FUSE grant NAG5-9972.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.