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

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 $68\pm 27$ (using our most conservative $1\ddot{I}ƒ$ 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 $\sim 140$ 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.

Accepted for publication in PSJ, September 2024