Cyanogen, cyanoacetylene, and acetonitrile in comet 67P and their relationship to the cyano radical

TL;DR

This study analyzes the presence and relationships of cyanogen, cyanoacetylene, and acetonitrile in comet 67P, revealing their relative abundances and concluding they are unlikely primary sources of the CN radical.

Contribution

First measurements of cyanogen in a comet and analysis of their abundances relative to HCN, providing insights into the chemical complexity of cometary comae.

Findings

Cyanogen was often below detection limit but measurable during certain periods.

Nitriles studied are not sufficiently abundant to be primary CN parents.

Abundance ratios of NCCN to HCN tracked from inbound to outbound trajectory.

Abstract

The cyano radical (CN) is one of the most frequently remotely observed species in space, also in comets. Data from the high-resolution Double Focusing Mass Spectrometer (DFMS) on board the Rosetta orbiter, collected in the inner coma of comet 67P/Churyumov-Gerasimenko, revealed an unexpected chemical complexity, and, recently, also more CN than expected from photodissociation of its most likely parent hydrogen cyanide (HCN). This work is dedicated to the derivation of abundances relative to HCN of three cometary nitriles (including structural isomers) from DFMS data. Mass spectrometry of complex mixtures does not always allow distinction of structural isomers. We assumed the most stable and most abundant (in similar environments) structure in our analysis, that is HCN for CHN, CH3CN for C2H3N, HC3N for C3HN, and NCCN for C2N2. For cyanoacetylene (HC3N) and acetonitrile (CH3CN) the complete mission timeline was evaluated, while cyanogen (NCCN) often was below detection limit. By carefully selecting periods where cyanogen was above detection limit, we were able to follow the abundance ratio between NCCN and HCN from 3.16 au inbound to 3.42 au outbound. These are the first measurements of NCCN in a comet.We find that neither NCCN, nor any of the other two nitriles, is sufficiently abundant to be a relevant alternative parent to CN.