Spectrogoniometer for measuring planetary surface materials at small phase angles.

The surfaces of airless bodies exhibit an anomalous increase in brightness as their faces become fully illuminated to the observer. This opposition effect is generally explained as the disappearance of mutual shadowing among the particles of the optically active portion of the regolith. Models suggest that the regolith's porosity and albedo are the primary factors which determine the effect's amplitude and angular dependence. By using collimated laser light and a pellicle beam splitter, the JPL spectrogoniometer has obtained measurements down to 0 degrees of samples of controlled porosity and albedo. The results of our first measurements show that dark porous surfaces are not the only ones to exhibit large opposition surges. Fits of our measurements to a computer program based on a shadowing model are in good agreement for porous surfaces. In the case of compacted surfaces, the model underpredicts the size of the increase below 3 degrees .