Modeling Ice and Dust in Comet C/2017 K2 (Pan-STARRS)’s Coma

Abstract C/2017 K2 (Pan-STARRS) is a long-period comet that has been active since 24 au inbound and likely as far out as 35 au, far beyond where the sublimation of water ice can power cometary activity. Near-infrared spectroscopic observations were obtained with SpeX on the NASA Infrared Telescope Facility at a heliocentric distance of R H = 6.50 au, which show clear signs of water ice in K2’s coma. Subsequently, more near-infrared spectra of K2 were collected with the Near-Infrared High-Throughput Spectrograph instrument on the Lowell Discovery Telescope and FLAMINGOS-2 on the Gemini South Telescope in order to further characterize the initially detected water ice. The SpeX data show strong water ice absorption features with an overall red slope while the later observations show decreasing ice fractions. We ran several different spectral models utilizing a Markov chain Monte Carlo approach to characterize the dust and ice in the coma, resulting in a range of ice fractions from 5% to 12% in the SpeX data depending on the mixing method, ice crystallinity, and assumed scattering regime(s). Modeling the SpeX data showed two classes of solutions, one with redder dust and more ice and one with less red dust and less ice, with significant differences in the derived properties of interest like the typical size of the ice grains. We utilize the later dust-dominated observations to recontextualize our observations of K2’s icier coma further out and to facilitate a discussion of how its coma evolved as a function of heliocentric distance. We also discuss best practices in modeling reflected light from cometary comae.