Polarimetric observations and laboratory simulations of asteroidal surfaces: The case of 21 Lutetia

The Rosetta spacecraft flew by 21 Lutetia on July 2010. This event provides a unique opportunity to enhance our knowledge of solar system small bodies, by comparing the surface properties measured in situ and the properties deduced from the linear polarization of scattered light, and prepare future observations. The linear polarization is studied as a function of the phase angle at different wavelengths and compared to phase curves of M-type and C-type asteroids. In a second part of the work, 21 Lutetia's polarization phase curves are compared to phase curves measured in the laboratory for powdered carbonaceous chondrites suggested as eventual analogs by spectroscopic studies. The importance of the variation of the linear polarization as a function of the wavelength is emphasized. CV3-class meteorite is found to be the best polarimetric laboratory analog with an average size of regolith grain lower than 50 μm.

Research highlights► Updated values for polarimetric albedo of 21 Lutetia. ► 21 Lutetia unlikely to belong to C-type but possibly to W, a subset of M asteroids. ► Average size of regolith grains should be lower than 50 μm. ► Importance of polarimetric color effect for such studies. ► CV3 meteorite best polarimetric laboratory analog.