Note on the HST Image of Comet 2I/Borisov Taken on 2019 October 12

Abstract: The image of comet 2I/Borisov taken by the Hubble Space Telescope on October 12 shows a fan-like feature in the inner coma, which extends over a few arcseconds from the nuclear condensation, its axis pointing a little west of the north. It is proposed that this protrusion is a halo of centimeter-sized and larger dust grains or pebbles released at low velocities from the nucleus at large heliocentric distances (on the order of 10 AU or more) before perihelion and subjected to very low radiation pressure accelerations. Unlike potential microscopic dust from ongoing activity, this population of large grains should show no silicate feature near 10 microns in the infrared and its presence in 2I/Borisov can be tested observationally. The existence of material of a similar size range in Oort Cloud comets with perihelia near or beyond the snow line has long been documented by the characteristic orientation of their tails.