ALMA Observations of HCN and its Isotopologues on Titan

TL;DR

This study uses ALMA sub-millimeter spectra to detect and analyze HCN isotopologues on Titan, providing new isotopic ratio measurements that inform models of Titan's atmospheric chemistry and fractionation processes.

Contribution

First detection of H$^{13}$C$^{15}$N on Titan and precise isotopic ratios for HCN, enhancing understanding of Titan's atmospheric composition and fractionation.

Findings

Detected H$^{13}$C$^{15}$N isotopologue on Titan.

Measured D/H ratio in HCN, indicating deuterium enrichment.

Confirmed elevated $^{14}$N/$^{15}$N ratio in HCN compared to N$_2$.

Abstract

We present sub-millimeter spectra of HCN isotopologues on Titan, derived from publicly available ALMA flux calibration observations of Titan taken in early 2014. We report the detection of a new HCN isotopologue on Titan, H$^{13}$C$^{15}$, and confirm an earlier report of detection of DCN. We model high signal-to-noise observations of HCN, H$^{13}$CN, HC$^{15}$N, DCN, and H$^{13}$C$^{15}$ to derive abundances and infer the following isotopic ratios: $^{12}$C/$^{13}$C = 89.8 $\pm$ 2.8, $^{14}$N/$^{15}$N = 72.3 $\pm$ 2.2, D/H = (2.5 $\pm$ 0.2)$\times$10$^{-4}$, and HCN/H$^{13}$C$^{15}$ = 5800 $\pm$ 270 (1$\sigma$ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of $\sim$2.3 elevation in $^{14}$N/$^{15}$N in HCN compared to N$_2$ and a lack of fractionation in $^{12}$C/$^{13}$C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of $\sim$2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.