Temporal variability of the physical properties of water-ice grains in the coma of comet C/2017 K2 (PANSTARRS)

The Oort cloud comet C/2017 K2 (PANSTARRS) discovered in May 2017 by the Pan-STARRS survey (Kaiser et al. 2002, SPIE, 4836, 154), has exhibited steady-state activity since at least R_h~23.7 au (Hui et al. 2018, AJ, 155, 25; Jewitt et al. 2021, AJ, 161, 188). Dynamical simulations by Krolikowska and Dybczynski (2018, A&A, 615, A170) indicate the comet passed its previous perihelion about 3 Myr ago, long enough to possibly consider C/2017 K2 as arriving from both Oort cloud distances and near-interstellar temperatures (Yang et al. 2021, ApJL, 914, L17). The long-distance activity, the dynamical properties, and the probable detection of solid-state water in the coma (Kareta et al. 2021, RNAAS, 5, 153) make C/2017 K2 a remarkable target to investigate the temporal evolution of the physical properties of water-ice grains (abundance, size, purity and crystallinity) as insolation and cometary activity vary with heliocentric distance. The ultimate goal is to determine if and how coma material evolves after leaving the nucleus and understand whether a unique set of ice properties can be consistent with all spectroscopic measurements, after accounting for observing circumstances (e.g., heliocentric distance).

We present Infrared Telescope Facility (IRTF)/SpeX observations of C/2017 K2 covering heliocentric distances from 5.7 au to 2.6 au inbound (perihelion is at 1.8 au on 2022 December 20). These data were obtained using the high-throughput low-resolution prism mode covering the wavelength range 0.7–2.52 μm. Measurements display evidence of the characteristic 1.5- and 2.0-μm water-ice absorption bands. However, variations in spectral slope and band strength on the order of 5% at 2.0 μm are observed. These spectral changes are indicative of variations in the ice-to-dust ratio and/or particle size of the water-ice grains possibly resulting from differences in solar insolation or changes at the nucleus surface, or a combination of the two effects. We interpret our spectroscopic results through the synergy of spectroscopic and sublimation lifetime models to provide a thorough understanding of the temporal evolution of the icy grain halos throughout the orbit. The results of C/2017 K2 are presented in the context of a systematic, low-resolution near-IR spectroscopic survey conducted in the last ten years to look for and characterize water-ice grain halos in cometary comae (Protopapa et al. 2018, ApJL, 862, L16; Protopapa et al. 2021, PSJ, 2, 176).

This work was funded by NASA through the Solar System Observations program via contract 80NSSC20K0673.