In Search of Primitive Near-Earth Asteroids and of Some of their Surface Properties: Polarimetric Observations and Simulations as an Invaluable Tool

Within the solar system, primitive asteroids (of C, P or D taxonomic types) are low-albedo small bodies, rich in complex organic molecules. Together with cometary nuclei, they are assumed to represent the less degraded remnants of planetesimals. Analyses of their properties should thus provide clues to the origin and evolution of the early solar system. It is the reason why sample-return missions to primitive near-Earth asteroids (NEAs) are considered by major space agencies. Remote observations of the linear polarization of solar light scattered by NEAs can provide information of interest for the preparation of such missions. In the visible and near infrared domains, polarimetric phase curves of asteroids (which are covered by low-density dust layers forming a regolith) are smooth, with a shallow negative branch near opposition, an inversion region near 20° phase angle, and a wide positive branch. Such curves, typical of scattering by irregular dust particles, present different trends, corresponding to different taxonomic types. The positive branch is always higher for low-albedo primitive asteroids, than for medium or high-albedo asteroids, the differences getting larger with phase angles increasing from the inversion angle to about 90°. While measurements on the positive branch can hardly be obtained for main belt asteroids (because of the Sun-asteroids-Earth geometry), they are feasible for NEAs, making it possible to estimate their taxonomic type and even their geometric albedo. We will present an approach to optimize polarimetric observations with large telescopes, in order to point out, amongst all NEAs recently discovered but of yet unknown type, primitive objects of interest (possibly as back-up targets) for future space missions. We will also discuss recent developments in the interpretation of polarimetric observations of such objects (dependence of the linear polarization on the phase angle and the observational wavelength) in terms of dust physical and chemical properties, through simulations, with emphasis on experimental simulations under reduced gravity conditions.