Abstract
The diffuse component of radar echoes from geophysical surfaces is thought to result primarily from single and multiple scattering from rocks, fractures, ripples, and other wavelength-scale surface and subsurface structure. Analysis of the diffuse echo therefore provides an estimate of small-scale surface roughness. Useful theoretical scattering models are difficult to obtain due to the complexity of electromagnetic interactions at these scales, however, handicapping efforts to interpret radar backscatter from many geologically interesting targets. Interpretation of radar data from distant geophysical surfaces--e.g., planets, satellites, and asteroids--presents additional challenges because of the limited availability of ground truth for these bodies.

Numerical models provide an alternate means of studying diffuse surface scattering mechanisms. In this talk we describe a three-dimensional finite-difference time-domain (FDTD) model for calculating scattering from wavelength-scale objects of arbitrary shape and composition in the presence of a planetary regolith. We illustrate the technique with a visualization of the scattering process and demonstrate the validity of the FDTD approach by comparison with independent methods. We have applied our model to several current problems in planetary radar astronomy. The first example we present is a single-scattering model for diffuse radar backscatter from the Viking Lander 1 and 2 sites on Mars, in which we combine tabulated rock population data for these sites with FDTD radar cross section calculations for surface and subsurface objects of various shapes, ranging from a sphere to an accurate digitized model of a terrestrial rock. The second example is an FDTD analysis of scattering from wavelength-scale buried craters and retrorefractive lenses, mechanisms which have been proposed to explain "bizarre" radar echoes from the icy Galilean satellites of Jupiter.