Regolith surface reflectance: A new attempt to model

The reflectance coefficient of the regolith layer of celestial bodies has b een studied in relation to the physical properties of regolith particles (s ize, refractive index, and packing density) on the basis of an accurate num erical radiative-transfer algorithm for a semi-infinite flat layer. Using t he geometric-optics approximation, we have found that a shape mixture of ra ndomly oriented spheroids can successfully model the single-scattering phas e function of independent soil grains. In order to take into account the ef fect of packing density in a regolith layer, the concept of the so-called s tatic structure factor was used. The main effect of increasing packing dens ity is to suppress the forward-scattering peak of the phase function and to increase the albedo of the reflecting surface. We also investigated the in fluence of fine dust on the reflected light. An addition of small particles not only increases the surface albedo, but also changes the brightness pro file and enhances the backscattering. Although the problem of unique soluti on, which is inherent in the retrieval of the properties of a medium from t he measurements of the intensity of light scattered by this media, cannot b e removed in the proposed model, the procedure used here, in contrast to wi dely used approximations, allows us to fit observational data with a set of real characteristics of the regolith. Semiempirical approaches are able to fit the measurements well with a small number of free parameters, but they do not explicitly contain crucial physical characteristics of the regolith such as grain sizes or the refractive index. We compared the numerical sol ution of the radiative-transfer equation with the Hapke approximation, whic h is most often used by investigators. The errors introduced by the Hapke m odel are small only for near-isotropic scattering by isolated particles. Ho wever, independent regolith grains are known to scatter light mainly in the forward direction.