Dust Evolution in the Coma of Distant, Inbound Comet C/2017 K2 (PANSTARRS)

TL;DR

This study uses high-resolution Hubble Space Telescope observations to analyze dust properties in the coma of distant comet C/2017 K2, revealing sublimating micron-sized water ice grains and radial gradients in color and polarization.

Contribution

It provides the first detailed characterization of dust evolution in C/2017 K2's coma before water ice sublimation, highlighting the presence of sublimating ice grains and challenging previous interpretations of polarimetric halos.

Findings

Radial increase in reflectance slope from 4.5% to 7% per 100 nm

Strengthening of negative polarization from -2% to -3.5%

Absence of micron-sized grains in the tail, indicating sublimating water ice grains

Abstract

C/2017 K2 (PANSTARRS) is an Oort cloud comet previously observed to be active at heliocentric distances r>20 au on what is likely its first passage through the inner solar system. We observed the comet on 2021 March 19-20 at r=6.82 au pre-perihelion and 8.35 deg phase angle with the Hubble Space Telescope (HST), and obtained high spatial resolution photometry and polarimetry mapping the properties of dust over the coma prior to the onset of water ice sublimation activity on the nucleus. We found clear radial gradients in the color and polarization of the coma: the F475W-F775W (g'-i') reflectance slope increased from ~4.5% per 100 nm within ~10,000 km of the nucleus up to ~7% per 100 nm by ~50,000 km, while the negative polarization in F775W (i') strengthened from about -2% to -3.5% over the same range. The radial intensity profiles moreover strongly deviate from profiles simulated for stable dust grains. Near-infrared imagery obtained with the Palomar Hale Telescope on 2021 May 18 at r=6.34 au revealed a continued absence of micron-sized grains in the tail, but showed no clear spatial gradient in JHKs colors. The observed patterns collectively appear consistent with the inner coma being optically dominated by sublimating, micron-sized water ice grains, unlike the tail of more stable, millimeter-sized grains. Finally, we evaluated these results alongside other Oort cloud comets, and found in a reanalysis of HST observations of C/2012 S1 (ISON) that the near-nucleus polarimetric halo reported for that comet is likely an observational artifact.