Microphysics of dust in a distant comet C/2017 K2 (PanSTARRS) retrieved by means of polarimetry

We conducted the earliest polarimetric survey of Comet C/2017 K2 (PanSTARRS) over 10 epochs during February-March 2021, while the comet's heliocentric distance was decreasing from 7.135 au to 6.801 au. Within an aperture radius rho = 10,0 0 0 km, we found slightly varying negative polarization with median value PQ approximate to -2.1% at phase angle alpha approximate to 8 degrees This appears in quantitative agreement with measurements of other distant comets. The negative polarization reveals submicron and micron-sized dust particles within the coma constrain the imaginary part of the refractive index in at least one type of particles to Im(m) <= 0.04. Models earlier developed in Kochergin et al. [1] for the negative polarization in distant Comet 29P/Schwassmann-Wachmann are similar to the negative polarization of Comet C/2017 K2 (PanSTARRS), suggesting its coma could be populated by Mg-rich silicate particles and/or water-ice particles, which are mixed with organic particles and/or amorphous-carbon particles. Modeling dust motion suggests that the inner coma is populated by particles whose age is up to 15 days since their emanation from the nucleus.(c) 2022 Elsevier Ltd. All rights reserved.