Laboratory photometry of planetary regolith analogs - I. Effects of grain and packing properties on opposition effect

New laboratory phase curves are presented to act as ground truth for solar system regolith photometry. The effects of physical parameters such as grain size, surface roughness, and packing density on backscattering peak can be recognized, but it turns out that the same parameter may have different effects depending on the sample, and the parameter effects are mixed with each other. These are a strong limitations to modelling, since the scattering models usually do not distinguish between compositions or predict different effects of same parameters. Furthermore, attention should be paid not only to physical but chemical characteristics as well. This emphasizes the role of experiments in the study of the opposition effect and surface characterization. As the experimental approach to scattering study becomes more significant, the laboratory methods for backscattering must be constantly improved to suit better the remote sensing purposes and achieve a large database.