2005ApJ...629L..53P

We measured the CH₄ column abundance in Titan's atmosphere through an analysis of Titan's monodeuterated methane (CH₃D) spectral features. CH₃D is several orders of magnitude less abundant in Titan's atmosphere than CH₄. Thus, unlike CH₄, the strong and well-studied CH₃D 3ν₂ lines are not saturated and provide a sensitive measure of its column abundance. We recorded the CH₃D 3ν₂ lines at 1.55 µm at NASA's Infrared Telescope Facility (IRTF) equipped with the Cryogenic Echelle Spectrograph. We derive a total integrated column abundance of 2.1±0.1 m amagat for CH₃D. We also measured stratospheric emission lines of both CH₃D and CH₄ at 8.6 µm at higher resolution than previously possible to better constrain the CH₃D/CH₄ ratio. These observations, recorded at the IRTF using the Texas Echelon Cross Echelle Spectrograph, were analyzed with radiative transfer calculations. We determine a CH₃D/CH₄ ratio of (50±10)x10^–5. Taken together, our measurements of the CH₃D column abundance and the CH₃D/CH₄ ratio indicate a total CH₄ column abundance of 4.2^+1.3_–0.9 km amagat, close to the column abundance of an atmosphere with 100% saturation in the entire troposphere.