Measurements of CH₃D and CH₄ 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 CH ₃D 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_-0.9^+1.3, km amagat, close to the column abundance of an atmosphere with 100% saturation in the entire troposphere.