Polarization of disintegrating Comet C/2019 Y4 (ATLAS)

ABSTRACT We observe Comet C/2019 Y4 (ATLAS) before and after its disintegration while making polarimetric measurements over a wide range of phase angles. The disintegration event was marked with a dramatic growth of the positive polarization branch that is consistent with a large relative abundance of absorbing material of up to (96.5 ± 3.4) per cent. This polarization spike relaxed as the carbonaceous particles are preferentially swept from the coma due to solar-radiation pressure. The observations suggest that the primordial material stored within comets is extremely rich in carbonaceous material. The pristine cometary material is processed by subsequent solar interactions, forming a refractory crust on the nucleus surface. Polarimetry provides a means of measuring the volume ratio of carbonaceous material, and hence the weathering that has occurred on the comet due to these interactions. The polarimetric response of Comet C/2019 Y4 (ATLAS) appears similar to that of Comet C/1995 O1 (Hale-Bopp), except on few epochs that are similar to that of Comet C/1996 B2 (Hyakutake).