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 yr 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. 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 ± 27 (using our most conservative 1 σ 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 ∼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.

Keywords

• Comets
• Neutral coma gases
• Comae
• Small Solar System bodies
• Astrochemistry
• Cosmochemistry