Photometry of particulate surfaces at extremely small phase angles

We present results of goniometric photometry of different samples at extremely small phase angles using a new laboratory setup with a laser light source (λ=0.63 μm). No opposition effect of carbon soot is found in the range 0.008-1.5°; whereas, MgO deposits and very disperse porous SiO2 powder demonstrate a narrow brightness spike of significant amplitude. We find a strong dependence of phase curve slope in the range on albedo when studying mixtures of white chalk and carbon soot. These measurements as applied to interpretation of opposition effect observed for Kuiper belt objects suggest that the albedo of these objects cannot be small. Size particle separates of olivine reveal prominent opposition spikes independent of the particle size. Spherical particles with a diameter near 50 μm show significant differences in their scattering behavior depending on whether they form a monolayer or thick particulate surfaces. The latter show a sharp opposition surge unlike the monolayer. Except for the monolayer of spheres, no particulate surface in our measurements yet exhibits phase curve rounding as the phase angle approaches zero.