Dust Coma of Comet C/2017 K2 (PANSTARRS) Pre-Perihelion at 6–7 au

C/2017 K2 (PANSTARRS) is a bright Oort cloud comet observed to be unusually active at r > 20 au that is likely making its first passage through the inner solar system. We observed the comet on 2021 March 19 at r = 6.8 au pre-perihelion and 8.3 degrees phase angle with ACS/WFC on the Hubble Space Telescope, and collected high resolution optical imaging photometry and polarimetry to map the properties of dust throughout the coma, prior to the onset of water ice sublimation activity on the nucleus. No radial asymmetry could be distinguished in color or polarization maps of the inner coma, although a bright, southward-pointing jet-like fan is evident in intensity maps. However, the F475W-F775W (g’-i’) reflectance color slope increases from a low ~4%/100 nm near the nucleus up to a more typical ~5.5%/100 nm beyond ~40,000 km from the nucleus, while the negative polarization in F775W (i’) strengthens from -2% to -3% over the same range. These radial trends in color and polarization are broadly consistent with the initial presence and gradual sublimation of sub-micron to micron-scale water ice inclusions embedded in the dark, millimeter-sized grains that dominate the comet’s optical coma and tail. We also collected broadband near-infrared imaging using WIRC on the Palomar Hale Telescope on 2021 May 18 at r = 6.3 au and found JHK reflectance to be slightly bluer than average. The J-H and J-K colors were radially uniform out to ~60,000 km to <0.03 and <0.05 magnitudes, respectively, suggesting that the measured colors are likely not sufficiently sensitive to discern the 1.5 and 2.0 μm water ice absorption bands and, instead, reflect the absorbance of the refractory grain components.