Multiple Scattering of Light by the Surfaces of Asteroids (Invited)

Polarimetric and photometric observations of asteroids and other atmosphereless Solar System objects show two ubiquitous effects at small solar phase angles: linear polarization parallel to the scattering plane (negative polarization) and a nonlinear increase of brightness toward the backscattering direction (opposition effect). The observations are modeled using a radiative-transfer coherent-backscattering method (RT-CB) applied to a realistic particulate model for an asteroid's surface. The method makes use of so-called phenomenological fundamental single scatterers (Muinonen & Videen, JQSRT 113, 2385, 2012; for the validity of RT-CB, see Muinonen et al., ApJ 760, 118, 2012). The new modeling allows us to constrain the single-scattering albedo, phase function, and polarization characteristics as well as the mean free path length between successive scatterings. With the help of the Discrete-Dipole Approximation, the T-matrix method, and experimental measurements, the modeling allows us to put constraints on the size, shape, and refractive index of the fundamental scatterers. We illustrate the application of RT-CB by interpreting the polarimetric observations of, e.g., the C, M, S, and E-class asteroids.