Properties of dust particles in comets from photometric and polarimetric observations of 67P

International audienceRemote observations of the light scattered by dust particles in comet 67P/Churyumov–Gerasimenko (67P/C-G) (photometry and imaging polarimetry) allow us to compare the deduced physical properties of these particles with in situ measurements provided by the Rosetta mission. Onboard experiments on the dust confirm properties inferred from polarimetric observations and from laboratory experiments, namely the existence of large dark particles in the size range 10–500 μm, which could be fluffy and compact aggregates of smaller grains (submicron- to micron-sized) and possibly of compact particles. The 2008–2009 apparition of comet 67P/C-G allowed us to observe the comet with two telescopes under a French–Indian collaboration. From the photometric and polarimetric observations, we inferred the presence of slow-moving, large compact particles in the coma before perihelion and eventually after. The additional ejection of fluffy aggregates of submicron-sized grains was consistent with observations after perihelion. In 2014–2015, as part of a worldwide campaign, observations of 67P/C-G were carried out in order to follow the evolution of the dust coma during the Rosetta mission. The campaign included observations with the Hubble Space Telescope, and photometric observations with the Himalayan Chandra Telescope in India. The comet was very active for about three months after perihelion, showing collimated jets, the structure of which changed over time, and a long dust tail. Using new published polarization values for observations in 2010, we were able to compare pre- and post-perihelion data and also to build partial phase curves for 67P/C-G (limited to phase angles smaller than 40°)