Quantification of porosity and surface roughness in laboratory measurements of the bidirectional reflectance of asteroid surface analogues

Abstract

Abstract We studied the effects of the surface roughness and porosity of a sample layer on its light-scattering properties in laboratory experiments using fly ash, iron, graphite, and olivine powders. Three types of surface structure were prepared: compacted, knocked, and fluffy surfaces. The surface roughness is represented by the mean slope angle of small facets on the surface. We found a positive correlation between the surface roughness and the porosity of the layer. The bidirectional reflectance of the surface at the wavelength of a He-Ne laser (633 nm) was measured to illustrate the influence of surface structure on scattering properties, with the incidence angle fixed at 0°, while varying the phase angle from 2 to 80°. The reflectance of a relatively rough surface was lower than that of a relatively smooth one for all of the materials measured. The reflectance measured at 30° in phase angle decreased by between ∼25 and ∼60%. This effect may explain the discrepancy between the absolute reflectance in previous laboratory results and the observed results for C class asteroids (Kamei and Nakamura, 2002; Nakamura et al., 2002).