Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)

TL;DR

This study sets a stringent upper limit on the HNC/HCN ratio in comet 73P/Schwassmann-Wachmann, suggesting low HNC production from polymers and highlighting thermal degradation of macromolecules as a plausible source.

Contribution

First measurement of a low HNC/HCN ratio in this comet, providing insights into volatile composition and chemical processes in cometary environments.

Findings

HNC/HCN ratio is less than 1.1%, much lower than in other comets

Comet 73P/Schwassmann-Wachmann is depleted in most volatiles except HCN

Thermal degradation of macromolecules may explain HNC variations in comets

Abstract

We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11, 2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of ~7 lower than the values measured previously in moderately active comets at 1 AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile species, except of HCN. The low HNC/HCN ratio thus argues against HNC production from polymers produced from HCN. However, thermal degradation of macromolecules, or polymers, produced from ammonia and carbon compounds, such as acetylene, methane, or ethane appears a plausible explanation for the observed variations of the HNC/HCN ratio in moderately active comets, including the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar polymers have been invoked previously to explain anomalous 14N/15N ratios measured in cometary CN.