The extraordinary composition of the "blue comet" C/2016 R2 (PanSTARRS)

TL;DR

This study provides a detailed multi-wavelength analysis of comet C/2016 R2, revealing an unusual composition rich in N2 and CO, distinct from other comets, suggesting a unique formation history or origin.

Contribution

It is the first comprehensive multi-wavelength characterization of comet C/2016 R2, highlighting its unique volatile composition and potential origin beyond the N2 ice line.

Findings

Comet C/2016 R2 is rich in N2 and CO, unlike typical comets.

The comet is dust-poor compared to other observed comets.

Its composition suggests formation beyond the N2 ice line or as a fragment of a differentiated transneptunian object.

Abstract

We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). This comet was observed on 23-24 January 2018 with the IRAM 30m telescope, and in January to March 2018 with the Nan\c{c}ay radio telescope. Visible spectroscopy was performed in December 2017 and February 2018 with small amateur telescopes. We report on measurements of CO, CH3OH, H2CO and HCN production rates, and on the determination of the N2/CO abundance ratio. Several other species, especially OH, were searched for but not detected. The inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of about 2.8 AU. The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N2 and CO and dust poor. This suggests that this comet might belong to a very rare group of comets formed beyond the N2 ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers.