36th DPS Meeting, 8-12 November 2004
Session 20 Titan
Poster I, Tuesday, November 9, 2004, 4:00-7:00pm, Exhibition Hall 1A

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[20.01] Methane abundance on Titan, obtained from the monodeuterated methane abundance

P. F. Penteado, C. A. Griffith (Lunar and Planetary Laboratory, Department of Planetary Sciences - University of Arizona), T. K. Greathouse (Lunar and Planetary Institute), M. J. Richter (University of Texas at Austin), J. H. Lacy (University of California at Davis), J. T. Rayner (Institute for Astronomy, University of Hawaii)

The second most abundant species on Titan's atmosphere, methane (CH4) has a central role in Titan's atmospheric dynamics and radiative balance. Its tropospheric abundance is likely to be controlled by condensation, cycling between surface deposits, vapor phase and tropospheric clouds. So far, different attempts to measure the methane content of Titan's atmosphere have led to conflicting results: Voyager's infrared spectra and the thermal profile from radio occultation data indicate a very high methane content, with up to 150% supersaturation in the troposphere. In contrast, the difference in visible and near infrared spectra between the dark and bright sides of Titan indicate that the total column abundance of methane is low, with no supersaturation occurring.

Measurement of the methane column abundance from its absorption bands is difficult due to the very high abundance present on Titan, that saturates all the bands that are strong enough to be well studied in laboratory. As the monodeuterated methane (CH3D) is less abundant than CH4 by a factor of 104, its strong and well-studied absorption bands are optically thin. So we measured the total column abundance of CH3D and the corresponding CH3D/CH4 ratio to infer the total methane column abundance. These were obtained using high resolution near infrared spectra obtained at NASA Infrared Telescope Facility, at 1.5\mum, with CSHELL - Cryogenic Near-IR Facility Spectrograph - and 8.6\mum with TEXES - Texas Echelon Cross-dispersed Echelle Spectrograph. In the methane window at 1.5\mum we observed the 3\nu2 absorption band of CH3D to measure the total column abundance of CH3D, where its bands were free from contamination of CH4 bands. At the 8.6\mum we measured the stratospheric emission lines of both CH4 and CH3D to obtain a CH3D/CH4 ratio without being significantly dependent of the temperature uncertainties.

Paulo Penteado is sponsored by the Brazilian Government through CAPES and NASA Planetary Astronomy Program.

The author(s) of this abstract have provided an email address for comments about the abstract: penteado@lpl.arizona.edu

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