A Large Outburst, Coma Asymmetries, and the Color of Comet 243P/NEAT

TL;DR

This study analyzes a significant outburst of comet 243P/NEAT, examining its activity, coma asymmetries, and water ice presence, and compares it with other comets to understand outburst mechanisms and ice detection challenges.

Contribution

It provides the first detailed analysis of comet 243P/NEAT's outburst and investigates the relationship between coma color, water ice, and outburst energetics in comets.

Findings

Large outburst likely ejected water ice grains but no ice signatures detected in photometry or spectra.

Coma asymmetries dominate the quiescent lightcurve variations, not activity level changes.

Outburst kinetic energies show grouping among different comets, suggesting common mechanisms.

Abstract

Water ice is a fundamental building material of comets and other bodies in the outer solar system. Yet, the properties of cometary water ice are challenging to study, due to its volatility and the typical distances at which comets are observed. Cometary outbursts, impulsive mass-loss events that can liberate large amounts of material, offer opportunities to directly observe and characterize cometary water ice. We present a study of comet 243P/NEAT, instigated by a $-3$ mag outburst that occurred in December 2018. Optical images and a 251-day lightcurve were examined to characterize the outburst and the comet's quiescent activity. Variations in the quiescent lightcurve appear to be dominated by coma asymmetries, rather than changing activity levels as the comet approached and receded from the Sun. Furthermore, the lightcurve shows evidence for 1 to 2 additional small outbursts ($-0.3$ mag) occurring in September 2018. The large December 2018 outburst likely ejected water ice grains, yet no signatures of ice were found in color photometry, a color map, nor a near-infrared spectrum. We discuss possible dynamical and thermal reasons for this non-detection. In this context, we examined the comae of comets 103P/Hartley 2 and C/2013 US$_{10}$ (Catalina), and show that a one-to-one mapping between continuum color and the presence of water ice cannot be supported. We also discuss possible causes for the large outburst, and find that there is an apparent grouping in the kinetic energy per mass estimates for the outbursts of 5 comets.