Reanalysis of Clementine bistatic radar data from the lunar South Pole

On April 9, 1994, the Clementine spacecraft high-gain antenna was aimed toward the Moon's surface, and the resulting 13-cm wavelength radio echoes were received on Earth. Using these data, we have found that the lunar surface generally follows a Lambertian bistatic scattering function sigma(0) = K-D cos theta(i) cos theta(s) with K(D)similar to 0.003 for the opposite (expected) sense of circular polarization and K(D)similar to 0.001 for the same (unexpected) sense. But there are important deviations (of up to 50% in some parts of the echo spectrum) from this simple form, Based on an earlier analysis of these same data, Nozette et al. [1996] claimed detection of an enhancement in echoes with the same sense circular polarization from regions near the South Pole in a near-backscatter geometry. Such behavior would be consistent with presence of perhaps large quantities of water ice near the pole. We have been unable to reproduce that result. Although we find weak suggestions of enhanced echoes at the time of South Pole backscatter, similar features are present at earlier and later times, adjacent frequencies, and in the opposite circular polarization. If enhanced backscatter is present, it is not unique to the South Pole; if not unique to the pole, then ice appears less likely as an explanation for the enhancement.