A multiple-scattering model of the diffuse component of lunar radar echoes

We propose that the average diffuse component of lunar radar echoes results from the cumulative effect of multiple scattering within the ejecta blanket of fresh young craters. Using a multiple-scattering polarization computer program, we compare our model with a variety of observations of the diffuse component and find a general agreement between the two. We show that multiple scattering makes a significant contribution to the partial depolarization of incident completely polarized radar signals, although single scattering may also be important. Mean indices of refraction of 1.3 and 1.6 are inferred for the ejecta blanket at wavelengths of 3.8 and 23 cm, respectively. These values are consistent with measurements made on an Apollo 11 fine sample and indicate an increasing density with depth below the surface. We hypothesize that the difference between the diffuse reflectivities of mare and highland areas is due chiefly to differences between the microwave absorption properties of rocks and dust in the two regions. If that is so, radar maps, properly prepared to display such differences, can serve as geological maps of the mineral or minerals (perhaps ilmenite) that are the principal microwave absorbers.