Laboratory study of the bidirectional reflectance of powdered surfaces: On the asymmetry parameter of asteroid photometric data

We performed laboratory experiments on the bidirectional reflectance of powdered surfaces of dunite, graphite, and Allende (CV3) and Gao (H5) meteorites. The particle size of each sample varied from tens to hundreds of micrometers. The absolute bidirectional reflectance was determined for two ranges of phase angle: from 2° to 80° at 0° angle of incidence and from 2° to 155° at 75° angle of incidence. The phase angle was incremented every 1° in between 2° and 5° and every 10° between 10° and 150°. We determined the Hapke parameters and found that the values of the asymmetry parameter retrieved for most of the samples were positive when the coverage of the phase angle was wide, 2° to 155°, although the values derived from remote sensing instruments on asteroid flyby missions have been negative or nearly zero. Among our sample surfaces, only those of graphite, Allende, and Gao with particle sizes of hundreds micrometers show negative or nearly zero values. The single-scattering albedos determined for the Gao samples are comparable to the values of S-class asteroids, while those for the Allende samples are much larger than those of C-class asteroids.