The D/H ratio in Titan’s acetylene from high spectral resolution IRTF/TEXES observations

We report observations of deuterated acetylene (C₂HD) at 19.3 μm (519 cm⁻¹) with the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility in July 2017. Six individual lines from the Q-branch of the ν₄ band were clearly detected with a S/N ratio up to 10. Spectral intervals around 8.0 μm (745 cm⁻¹) and 13.4 μm (1247 cm⁻¹) containing acetylene (C₂H₂) and methane (CH₄) lines respectively, were observed during the same run to constrain the disk-averaged C₂H₂ abundance profile and temperature profile. Cassini observations with the Composite Infrared Spectrometer (CIRS) were used to improve the flux calibration and help to constrain the atmospheric model. The measured D/H ratio in acetylene, derived from the C₂HD/C₂H₂ abundance ratio, is (1.22+0.27-0.21) × 10⁻⁴, consistent with that in methane obtained in previous studies. Possible sources of fractionation at different steps of the acetylene photochemistry are investigated.