Properties of dust in inner cometary comae from polarimetric observations

Evidence for changes in the properties of dust particles in cometary comae is provided by observations of the linear polarization of the solar light they scatter, which only depends upon the observational conditions and the dust properties. Local polarimetric changes in a coma under fixed observational conditions point out changes in the properties of the dust (e.g. size distribution, morphology, complex refractive indices and thus albedo). After evidence was obtained from local polarimetric observations of Halley's comet with Giotto spacecraft, techniques of remote polarimetric imaging have been developed, providing polarimetric images of the comae of some comets [e.g. 1]. Up to three different regions may be noticed: a background coma, jet-like features with a higher polarization, and a polarimetric halo extending on less than 2000 km around the photometric center. The possible presence of dust particles with different properties in the innermost coma is of major importance for the Rosetta mission, expected to rendezvous with comet 67P/Churyumov-Gerasimenko in 2014 and to approach its nucleus, while releasing the Philae lander from an altitude smaller than 10 km. Polarimetric observations, as interpreted through both experimental and numerical simulations [e.g., 2, 3], together with recent Stardust and Deep Impact in situ missions, provide clues to the following properties: the dust particles are quite small (0.1 micrometer to a few millimeters); they are very porous and irregular, possibly consisting of fluffy aggregates; they mostly consist of silicates and carbonaceous compounds, although the presence of icy grains cannot be ruled out in the innermost coma. We will discuss the origin of changes between the properties of dust particles present in the innermost coma and those of found further away from the nucleus, as mostly deduced from recent laboratory simulations of dust polarimetric properties. Support from CNES is acknowledged. [1] Hadamcik et al. JQSRT 79, 661, 2003. [2] Hadamcik et al. Icarus 190, 660, 2007. [3] Lasue et al. Icarus 199, 129, 2009