Regolith composition and structure in the lunar maria: Results of long-wavelength radar studies

Radar measurements at 70-cm and 7.5-m wavelengths provide insight into the structure and composition of the upper 5-100 m of the lunar regolith and crust. We combine high-resolution (3-5 km) 70-cm radar data for the nearside with earlier calibrated full-disk observations at the same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These data are tested against models for echoes from a buried substrate and Mie scattering from surface and buried rocks. These mechanisms are expected to dominate the 70-cm radar echo, with their relative importance determined by the rock population, regolith depth, substrate roughness, and the loss tangent of the soil. Results indicate that the 70-cm radar echo for the maria comes largely from Mie scattering by rocks buried within the fine soil. Radar scattering from a buried substrate is not likely to greatly affect the observed return. We also compared the 70-cm and 7.5-m radar images to infrared eclipse temperature maps, crater-population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth based telescopic and Clementine multispectral observations. These data imply that (1) the TiO2 (ilmenite) content of the regolith controls variations in 70-cm depolarized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70-cm return for a given TiO2 abundance between maria of different ages do occur, but uncertainties in the current radar data do not allow us to uniquely distinguish between variations in rock population with age and calibration effects; (3) the 7.5-m radar echoes are controlled by the age of the mare basalt flows, with older deposits having a greater degree of fracturing and higher backscatter. Future mapping at 12.6-cm and 70-cm wavelengths will help to resolve some of the issues raised here.