Polarimetry as a tool to assess the surface properties of asteroids and the Moon

Abstract

The polarization of sunlight scattered by atmosphereless Solar System bodies depends on the surface properties of the scattering medium. The surface properties of the scattering medium can be inferred from the study of polarization of the scattered sunlight. The polarization is generally measured as a function of phase angle. This paper reviews the exploration of phase-polarization curves to characterize the surfaces of asteroids and the Moon. The effects of particle size and composition on the phase-polarization curve are also discussed. Methods for estimating characteristic grain sizes from polarization measurements, and empirical relationships between polarimetric parameters and the geometric albedo are also discussed.  However, these empirical relationships need proper calibration. The albedo values of asteroids, obtained from different polarimetric parameters, show a remarkable consistency, except for Barbarian asteroids. The locations of most of the asteroids in the P_min-α_inv plane were found in a region between those occupied by bare rocks (α_inv < 18⁰)  and by lunar fines (18⁰ ≤ α_inv ≤ 25⁰), indicating that asteroid surfaces are covered by a regolith that is coarser than lunar surface, but finer than bare rocks. Lunar surface particle sizes were estimated to be about 50–100 mm.