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 regol ith and crust. We combine high-resolution (3-5 km) 70-cm radar data fo r the nearside with earlier calibrated full-disk observations at the s ame wavelength to provide a reasonable estimate of the lunar backscatt er coefficient. These data are tested against models for echoes from a buried substrate and Mie scattering from surface and buried rocks. Th ese mechanisms are expected to dominate the 70-cm radar echo, with the ir relative importance determined by the rock population, regolith dep th, substrate roughness, and the loss tangent of the soil. Results ind icate that the 70-cm radar echo for the maria comes largely from Mie s cattering 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 eclips e temperature maps, crater-population age estimates for the maria, and to TiO2 and FeO abundances inferred from Earth based telescopic and C lementine multispectral observations. These data imply that (1) the Ti O2 (ilmenite) content of the regolith controls variations in 70-cm dep olarized echo strength among mare units, with higher titanium abundanc e 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 di stinguish between variations in rock population with age and calibrati on effects; (3) the 7.5-m radar echoes are controlled by the age of th e mare basalt flows, with older deposits having a greater degree of fr acturing and higher backscatter. Future mapping at 12.6-cm and 70-cm w avelengths will help to resolve some of the issues raised here.