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