The D/H ratio in Titan's acetylene from high spectral resolution IRTF/Texes observations

<p>In July 2017, we used the Texes high-resolution spectrometer at the NASA Infrared Telescope Facility to observe Titan in the 19.3-&#181;m (519 cm^-1) region where the &#957;₄ band of deuterated acetylene (C₂HD) is located. Six individual lines were clearly detected with a S/N ratio up to 10. Spectral intervals around 8.0 (745 cm^-1) and 13.4 &#181;m (1247 cm^-1) were observed during the same run to constrain the disk-averaged temperature profile and acetylene (C₂H₂) abundance profile respectively. Telluric correction and flux calibration were obtained from observations of asteroid Hygiea. Constraints from Cassini/CIRS observations in 2017 around the sub-Earth latitude were also used to constrain the atmospheric model and check the flux calibration. The D/H ratio is derived from the C₂HD/C₂H₂ abundance ratio. Preliminary results of this analysis indicate a D/H ratio in acetylene around 1.7&#215;10^-5 with an uncertainty of about 10%, in agreement with the previous, less precise, determination from Cassini/CIRS measurements. This ratio is slightly larger than that in methane (approximately 1.3&#215;10^-5), which suggests some fractionation at work in the photochemical production of acetylene from methane.</p>