Measurements of CH 3 D and CH 4 in Titan from Infrared Spectroscopy

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) × 10-5. Taken together, our measurements of the CH3D column abundance and the CH3D/CH4 ratio indicate a total CH4 column abundance of 4.2 km amagat, close to the column abundance of an atmosphere with 100% saturation in the entire troposphere.