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

TL;DR

This study measures the D/H ratio in Titan's acetylene using high-resolution infrared spectroscopy, providing insights into atmospheric chemistry and isotopic fractionation processes.

Contribution

First detection of deuterated acetylene (C₂HD) in Titan's atmosphere using high-resolution spectral observations and comprehensive analysis of isotopic ratios.

Findings

D/H ratio in acetylene is (1.22^{+0.27}_{-0.21}) × 10^{-4}

D/H ratio in acetylene is consistent with that in methane from previous studies

Spectral lines of C₂HD detected with high signal-to-noise ratio

Abstract

We report observations of deuterated acetylene (C$_2$HD) at 19.3 $\mu$m (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 $\nu_4$ band were clearly detected with a S/N ratio up to 10. Spectral intervals around 8.0 $\mu$m (745 cm$^{-1}$) and 13.4 $\mu$m (1247 cm$^{-1}$) containing acetylene (C$_2$H$_2$) and methane (CH$_4$) lines respectively, were observed during the same run to constrain the disk-averaged C$_2$H$_2$ 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 C$_2$HD/C$_2$H$_2$ abundance ratio, is (1.22$^{+0.27}_{-0.21})$ $\times$ 10$^{-4}$, consistent with that in methane obtained in previous studies. Possible sources of fractionation at different steps of the acetylene photochemistry are investigated.