The CN isotopic ratios in comets

TL;DR

This study measures the isotopic ratios of carbon and nitrogen in CN radicals across 23 comets to understand their origins and the early solar system's chemical processes, finding consistent ratios regardless of comet origin.

Contribution

It provides the first comprehensive measurement of {$^{12}$C/$^{13}$C} and {$^{14}$N/$^{15}$N} ratios in CN for a diverse sample of comets, revealing uniform isotopic compositions.

Findings

Carbon isotopic ratio matches terrestrial value

Nitrogen isotopic ratio is lower than terrestrial, indicating fractionation

Ratios are consistent across different comet types and distances

Abstract

Our aim is to determine the isotopic ratios {$^{12}$C/$^{13}$C}{} and {$^{14}$N/$^{15}$N}{} in a variety of comets and link these measurements to the formation and evolution of the solar system. The {$^{12}$C/$^{13}$C}{} and {$^{14}$N/$^{15}$N}{} isotopic ratios are measured {for} the CN radical by means of high-resolution optical spectra of the R branch of the B-X (0,0) violet band. {23 comets from different dynamical classes} have been observed, sometimes at various heliocentric {and nucleocentric} distances, in order to estimate possible variations of the isotopic ratios in parent molecules. The {$^{12}$C/$^{13}$C}{} and {$^{14}$N/$^{15}$N}{} isotopic ratios in CN are remarkably constant (average values of, respectively, $91.0\pm3.6$ and $147.8\pm5.7$) within our measurement errors, for all comets whatever their origin or heliocentric distance. While the carbon isotopic ratio does agree with the terrestrial value (89), the nitrogen ratio is a factor of two lower than the terrestrial value (272), indicating a fractionation in the early solar system, or in the protosolar nebula, common to all the comets of our sample. This points towards a common origin of the comets independently of their birthplaces, and a relationship between HCN and CN.