Electrical properties of rocks and their significance for lunar radar observations

A wide variety of terrestrial rocks has been investigated to determine the range of variation of the high-frequency electrical properties. Both solid rocks and powders of various types have been measured at frequencies of 450 MHz and 35 GHz; some measurements have been made at elevated temperatures. Solid materials show wide variations in permittivity and absorption length, but apart from a small trend with silica content it is not clear that there is any pattern to the variations, and it is unlikely that measurements limited to these radar frequencies will be useful for identification. With powdered rocks, on the other hand, the variation from rock type to rock type is much smaller, but depends on the powder density in a quite predictable way. Consequently, radar information can be employed to make precise estimates of density and of density profiles even in the absence of information about composition. The long absorption lengths in powdered rocks indicate that radar reflections may occur at considerable depths in a regolith of the type found on the moon.