The shadowing effect in regolith-type media: Numerical modeling

We present in this paper the results of computer simulation of the shadowin g effect in a medium composed of nontransparent particles under the assumpt ion of Lambertian light scattering from the particle surfaces. Within the g eometric-optics approximation, the performed simulation allows one to obtai n, with an accuracy better than 1%, the photometric characteristics of such multilayer media. For optically dense statistically homogeneous media, the packing density is the only parameter influencing backscattering: the lowe r the packing density of the medium, the more pronounced the opposition amp lification of brightness is. This is also true for media composed of partic les different in size. Phase dependences for mono- and polydispersed media with equal packing densities were found to be identical. Phase curves for a monolayer of hemispherical particles randomly arranged on a smooth Lambert ian substrate are always convex upward. In the phase-angle range from 0 deg rees to 40 degrees, the phase dependence of brightness for the monolayer of spherical particles may be almost linear. In a two-layer model, an optical ly thin layer of small particles lying on a layer of large particles can co nsiderably enhance the opposition effect. However, the enhancement depends strongly on the way the layers adjoin each other. A medium consisting of sp arsely arranged large particles on a smooth substrate made up of small part icles also exhibits a steeper phase curve near opposition.