Large excess of heavy nitrogen in both hydrogen cyanide and cyanogen from comet 17P/Holmes

TL;DR

This study measures nitrogen isotopic ratios in cometary molecules, revealing a significant excess of heavy nitrogen in HCN and CN, which suggests early Solar System isotopic fractionation and that HCN likely is the main source of CN in comets.

Contribution

It provides the first measurements of 14N/15N ratios in HCN and CN in comet 17P/Holmes, showing they share the same non-terrestrial isotopic composition, indicating a common origin.

Findings

HCN and CN have similar non-terrestrial 14N/15N ratios.

The 15N excess indicates early Solar System nitrogen fractionation.

HCN likely the primary parent of CN in cometary atmospheres.

Abstract

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of October 24, 2007, we derive 14 N/15N = 139 +/- 26 in HCN, and 14N/15N = 165 +/- 40 in CN, establishing that HCN has the same non-terrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The 15N excess relative to the Earth atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of Solar System formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort-cloud and Kuiper-belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.