  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.
