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C/2017 K2 (PANSTARRS) is an Oort cloud comet previously observed to be activeat heliocentric distances r>20 au on what is likely its first passage throughthe inner solar system. We observed the comet on 2021 March 19-20 at r=6.82 aupre-perihelion and 8.35 deg phase angle with the Hubble Space Telescope (HST),and obtained high spatial resolution photometry and polarimetry mapping theproperties of dust over the coma prior to the onset of water ice sublimationactivity on the nucleus. We found clear radial gradients in the color andpolarization of the coma: the F475W-F775W (g'-i') reflectance slope increasedfrom ~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 fromabout -2% to -3.5% over the same range. The radial intensity profiles moreoverstrongly deviate from profiles simulated for stable dust grains. Near-infraredimagery obtained with the Palomar Hale Telescope on 2021 May 18 at r=6.34 aurevealed a continued absence of micron-sized grains in the tail, but showed noclear spatial gradient in JHKs colors. The observed patterns collectivelyappear consistent with the inner coma being optically dominated by sublimating,micron-sized water ice grains, unlike the tail of more stable, millimeter-sizedgrains. Finally, we evaluated these results alongside other Oort cloud comets,and found in a reanalysis of HST observations of C/2012 S1 (ISON) that thenear-nucleus polarimetric halo reported for that comet is likely anobservational artifact.

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