Packing properties of fine powders and the depth of the lunar dust layer

Previous experimental and theoretical studies by Hapke and Van Horn [1963] and Hapke [1963] of surfaces which reflect light like the moon support the hypothesis that the lunar surface is covered with a layer of fine rock dust. The material in this layer appears to be in an extremely unconsolidated state, having a bulk density of the order of one-eighth that of solid rock. Such photometric studies can give information concerning only the upper few millimeters of the lunar soil. However, analyses of radiofrequency observations indicate that extensive areas of the moon may be covered to depths of the order of a meter or greater by a substance which is also much less dense than solid rock. From the radar reflectivity of the lunar surface at meter wavelengths Evans [1962] infers a dielectric constant of K = 2.7; Troitsky [1962] estimates K = 1.6 on the basis of radio-thermal observations at centimeter wavelengths. Since the RF dielectric constants of stony meteorites and of terrestrial rocks range from about 4 to 45 [Fensler et al., 1962] it is clear that the density of the lunar soil is unusually low at least to depths to which RF radiation can penetrate; Troitsky estimates this depth to be of the order of 20 wavelengths or greater in the 3-cm-wavelength range.