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

Abstract

Abstract. 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 highresolution (3-5 km) 70-cm radar data for the nearside with earlier calibrated full-disk observations at •e same wavelength to provide a reasonable estimate of the lunar backscatter coefficient. These ci'm 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 Ti02 and FeO abundances inferred from Earth-based telescopic and Clementinc multispectral .•bservations. These data imply that (1) the TiO 2 (ilmenite) content of the regolith controls variations in 70-cm depo!arized echo strength among mare units, with higher titanium abundance leading to lower echoes; (2) changes in the average 70-cm return for a given TiO 2 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.

Volcanic and impact deposits of the Moon's Aristarchus Plateau: A new view from Earth-based radar images

“…The maximum depth of probing is constrained by the microwave loss properties of the target material, but is typically 10-20 wavelengths. The loss properties are determined by the composition of the lunar material, with greater amounts of ilmenite, in particular, leading to greater attenuation of the incident signal (Carrier et al, 1991;Campbell et al, 1997). Radar studies show that lunar pyroclastics have very low backscatter due to their low volume rock abundance, and perhaps to greater microwave losses (Zisk et al, 1977;Gaddis et al, 1985).…”

Stratigraphy and composition of lava flows in Mare Nubium and Mare Cognitum

“…These can be estimated by obtaining the bulk oxide content in the regolith, dominated by SiO 2 , FeO, TiO 2 , Al 2 O 3 , MgO, and CaO (Papike et al 1991). The abundances of mafic minerals vary across the maria due to lateral differences in magma source regions and fractionation histories (Campbell et al 1997). Most of the information on composition and maturity of the regolith comes from spectrophotometric observations and orbital geochemistry data that have been calibrated according to measured values for the Apollo and Luna landing sites (Campbell et al 1997).…”

The titanium contents of lunar mare basalts

“…The recent Galileo and Clementine missions allowed us to take a global look at the composition of the Moon. Previous comprehensive mapping efforts have estimated Ti02 (Charette et al, 1974;Head, 1976;Johnson et al, 1977;Head et al, 1978;Pieters, 1978;Davis, 1980;Jaumann, 1991;Johnson et al, 1991;Greeley et al, 1993;Pieters et al, 1993;Melendrez et al, 1994;Campbell et al, 1997), and it has been suggested that relative FeO content can be estimated for a few cases from the depth of the 1 .O p m absorption band (Pieters, 1978). Recently, analytical techniques for the accurate determination of FeO (Lucey et al, 1995) and Ti02 (Lucey et al, 1996(Lucey et al, , 1998Blewett et al, 1997) abundances of lunar surface units from Clementine and Galileo spectral reflectance data have been developed.…”