Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen's Hydrogen Cyanide

TL;DR

This study reports a surprising enrichment of nitrogen-15 isotopes in comet 46P/Wirtanen's hydrogen cyanide, suggesting greater isotopic diversity in Jupiter-family comets than previously recognized, with implications for Solar System formation.

Contribution

First detection of significant $^{15}$N enrichment in HCN of a Jupiter-family comet, expanding understanding of isotopic variations in cometary materials.

Findings

$^{14}$N/$^{15}$N ratio of 68±27 in 46P's HCN

No significant $^{13}$C fractionation observed

Implication of larger isotopic enrichments in Jupiter-family comets

Abstract

46P/Wirtanen is a Jupiter-family comet, probably originating from the Solar System's Kuiper belt, that now resides on a 5.4 year elliptical orbit. During its 2018 apparition, comet 46P passed unusually close to the Earth (within 0.08 au), presenting an outstanding opportunity for close-up observations of its inner coma. Here we present observations of HCN, H$^{13}$CN and HC$^{15}$N emission from 46P using the Atacama Compact Array (ACA). The data were analyzed using the SUBLIME non-LTE radiative transfer code to derive $^{12}$C/$^{13}$C and $^{14}$N/$^{15}$N ratios. The HCN/H$^{13}$CN ratio is found to be consistent with a lack of significant $^{13}$C fractionation, whereas the HCN/HC$^{15}$N ratio of $68\pm27$ (using our most conservative $1\sigma$ uncertainties), indicates a strong enhancement in $^{15}$N compared with the solar and terrestrial values. The observed $^{14}$N/$^{15}$N ratio is also significantly lower than the values of $\sim140$ found in previous comets, implying a strong $^{15}$N enrichment in 46P's HCN. This indicates that the nitrogen in Jupiter-family comets could reach larger isotopic enrichments than previously thought, with implications for the diversity of $^{14}$N/$^{15}$N ratios imprinted into icy bodies at the birth of the Solar System.