The backscattering ratio of comet 67P/Churyumov-Gerasimenko dust coma as seen by OSIRIS onboard Rosetta

Remote sensing observations of dust particles ejected from cometsprovide important hints on the intimate nature of the materials composing these primitive objects. The measurement of dust comabackscattering ratio, BSR, defined as the ratio of the reflectance at phase angle0° and 30°, helps tuning theoretical models aimed at solving the inverse scattering problemderiving information on the nature of the ejected particles. The Rosetta/ OSIRIScamera sampled the comaphase function of comet67P, with four series acquired at low phase anglesfrom 2015 January to 2016 May. We also added previously published data to our analysis to increase the temporal resolution of our findings. We measured a BSR in the range ∼ [1.7–3.6], broader than the range found in literature from ground-based observations of other comets. We found that during the post-perihelion phase, the BSR is systematically larger than the classical cometary dust values only for nucleocentric distances smaller than ∼100 km. We explain this trend in terms of a cloud of chunks orbiting the nucleus at distances <100 km ejected during perihelion and slowly collapsing on the nucleus over a few months because of the comagas drag. This also implies that the threshold particle size for the dust phase function to become similar to the nucleus phase function is between 2.5 mm and 0.1 m, taking into account previous Rosetta findings.