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Earth-based Radar Study of the Lunar Regolith
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The composition and structure of the lunar regolith are of fundamental importance to the geologic study of the moon. Except lunar orbiters, earth-based radar is the most detailed measurement to provide insight into the upper 5-100m of the entire nearside hemisphere’s regolith and crust. Past works shown that radar scattering from the regolith can come from several sources, whose importance varies with the wavelength. Surface rocks and regolith sculpture will be the primary scatters at X-band wavelength (3cm); while larger blocks and those buried in the soil become progressively more important at S (12cm) and P (70cm) band; rock layers beneath the regolith contribute the primary echoes of the 7.5m wavelength. It is generally recognized that 70-cm radar wavelength, for its tens wavelength penetration depth and high sensitivities to the solid inclusion in the soil, can be used to inverse the composition, vertical structure and mechanical properties of the lunar regolith. The backscatter of 3.8cm wavelength is influenced both by topography and randomly distributed rocks on the surface of lunar regolith. For the mare, the 7.5m radar wavelength penetrates a considerable distance into the sub-regolith basalt and reflects the scattering and absorption properties of the basalt layers below the soil. 70-cm radar echoes are influenced to varying degrees by (1) the surface and subsurface rock population. (2) the depth and roughness of a substrate layer. (3) the dielectric properties of the rock and fine soil. (4) the possible influence on loss tangent by components excludes TiO2 and Fe. To examine the effects of these parameters, radar returns are divided into three sources: quasi-secular echoes from the upper surface of the regolith, echoes from a buried substrate and Mie scattering from surface and buried rocks.
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