Asteroidal polarimetry, as compared to cometary polarimetry: the case of 21 Lutetia

Polarimetric measurements provide clues to the classification of solar system objects and to their local properties, through experimental and numerical simulations. Asteroidal polarimetry originates in solar light scattered by an irregular surface, as opposed to cometary polarimetry that comes from scattering by a low-density dust cloud. In both cases, polarimetric phase curves are somehow comparable, with key parameters such as the minimum in polarization, the inversion angle, the maximum in polarization, and their wavelength dependence. Differences between observations obtained at similar phase angles and wavelengths may be attributed to local changes, e.g. variagation of the surface revealed by the rotation for asteroidal polarimetry and existence of halos and jet features with different dust particles for cometary polarimetry. New observations of 21 Lutetia have been recently obtained in three colors and interpreted through laboratory simulations (1); they will be compared with other observations (2) and discussed in relation with the surface images obtained during the Rosetta flyby on 10 July 2010. Also, conclusions about the properties of dust in the coma of 67P/Churyumov-Gerasimenko, the rendezvous target of the Rosetta mission, as inferred from polarimetric observations, will be summarized (3,4).