Discrete dipole model of radiative transfer in dense granular layers

We have developed a numerical Regolith Radiative Transfer (RRT) model based on the Discrete-Dipole Approximation (DDA), which iteratively solves the exact wave equations for a granular target in response to an incident plane wave and mutual interactions between all component grains. We extend the traditional DDA approach in two ways: the DDA target is laterally replicated using periodic boundary conditions, to represent a small piece of a nominally flat granular particle surface, and the emergent intensity is sampled in the near field of the target to avoid diffraction-like artifacts arising from its finite size. The approach is applicable in regimes for which the assumptions of current RRT models are invalid, such as wavelength-size particles which are closely packed. In this first paper we describe the technique and present tests and illustrative results for simple cases, using layers of varying filling fraction between 0.01 and 0.7, containing nominally spherical monomers of uniform composition (SiO2). We motivate the results with a simple “toy model” of scattering by a target with multiple interfaces. The model is easily extended to layers of particles which are heterogeneous in composition and arbitrarily shaped. c © 2012 Optical Society of America