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

Abstract

We report observations of deuterated acetylene (C2HD) at 19.3μm (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 8.0μm (745 cm−1) and 13.4μm (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 (1.22−0.21+0.27)× 10−4, consistent with that in methane obtained in previous studies. Possible sources of fractionation at different steps of the acetylene photochemistry are investigated.