Unusual polarimetric response of interstellar Comet 3I/ATLAS: clues to the microphysics of particles populating its inner coma

ABSTRACT Using highly irregular agglomerated debris particles, we analyse the polarimetric observations of the second interstellar Comet 3I/ATLAS beyond the snow line. Unlike the vast majority of Solar system comets at shorter heliocentric distance, modelling of the polarization of the inner coma in Comet 3I/ATLAS necessarily requires water-ice particles (30–40 per cent by volume). Our modelling suggests the other two species to be Mg-rich silicate and organics/amorphous carbon, which are common for Solar system comets. The relative abundance of the Mg-rich silicates is lower than what was needed in modelling the polarization of Solar system comets. The modelling suggests that if Comet 3I/ATLAS were to keep its activity at heliocentric distances smaller than that of the snow line, it would be an extremely high Pmax comet with Pmax ≈ 36–40 per cent in red light.