Estimation of Mars radar backscatter from measured surface rock populations

Reanalysis of rock population data at the Mars Viking Lander sites has yiel ded updated values of rock fractional surface coverage (about 0.16 at both sites, including outcrops) and new estimates of rock burial depths and axia l ratios. These data, are combined with a finite difference time domain (FD TD) numerical scattering model to estimate diffuse backscatter due to rocks at both the Lander 1 (VL1) and Lander 2 (VL2) sites. We consider single sc attering from both surface and subsurface objects of various shapes, rangin g from an ideal sphere to an accurate digitized model of a terrestrial rock . The FDTD cross-section calculations explicitly account for the size, shap e, composition, orientation, and burial state of the scattering object, the incident wave angle and polarization, and the composition of the surface. We calculate depolarized specific cross sections at 12.6 cm wavelength due to lossless rock-like scatterers of about 0.014 at VL1 and 0.023 at VL2, wh ich are comparable to the measured ranges of 0.019-0.032 and 0.012-0.018, r espectively. We also discuss the variation of the diffuse cross section as the local angle of incidence, theta(i), changes. Numerical calculations for a limited set of rock shapes indicate a marked difference between the angu lar backscattering behavior of wavelength-scale surface and subsurface rock s: while subsurface rocks scatter approximately as a cosine power law, surf ace rocks display a complex variation, often with peak backscattering at hi gh incidence angles (theta(i) = 70 degrees-75 degrees).