ALMA Observations of HCN and its Isotopologues on Titan

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.

Accepted for publication in Astrophysical Journal Letters. 14 pages, 3 figures, 2 tables

References in corpus (5)

• Ethyl cyanide on Titan: Spectroscopic detection and mapping using ALMA
• ALMA measurements of the HNC and HC${}_3$N distributions in Titan's atmosphere
• Isotopic Ratios of Carbon and Oxygen in Titan's CO using ALMA
• Herschel/PACS spectroscopy of trace gases of the stratosphere of Titan
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Cited by in corpus (28)

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• Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto
• Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini Comparisons from 2012 to 2015
• ALMA Spectral Imaging of Titan Contemporaneous with Cassini's Grand Finale
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• Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A
• Seasonal evolution of temperatures in Titan's lower stratosphere
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• The floatability of aerosols and waves damping on Titan's seas
• Escape and evolution of Titan's N${}_2$ atmosphere constrained by ${}^{14}$N/${}^{15}$N isotope ratios
• Nitrogen Atmospheres of the Icy Bodies in the Solar System
• A new spectroscopically-determined potential energy surface and \emph{ab initio} dipole moment surface for high accuracy HCN intensity calculations
• Measurement of CH${}_3$D on Titan at Submillimeter Wavelengths
• Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations
• Variability in Titan's Mesospheric HCN and Temperature Structure as Observed by ALMA
• N${}_2$ accretion, metamorphism of organic nitrogen, or both processes likely contributed to the origin of Pluto's N${}_2$
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• Unraveling the Interface Chemistry between HCN and Cosmic Silicates by the Interplay of Infrared Spectroscopy and Quantum Chemical Modeling
• Open Questions and Future Directions in Titan Science
• An intense thermospheric jet on Titan