An analysis of Clementine data obtained from a UVVIS camera and simulating laboratory photometric and polarimetric measurements is presented with the use of a new photometric threeparameter function combining the shadow-hiding and coherent backscatter mechanisms. The fit of calculated curves to the average brightness phase function of the Moon derived from Clementine data indicates that the coherent backscatter component is nonzero. The average amplitude of the opposition surge of the Moon in the range of phase angles 0• -1• is approximately 10%. The Clementine data also show a flattening of phase-dependent brightness at angles less than 0.25• that is caused by the angular size of the solar disk. The lunar brightness phase curves at small phase angles are nearly the same in different wavelengths even though at larger phase angles (5• -50 • ) the lunar surface becomes distinctly redder with increasing phase angle. According to the model, the lack of wavelength-dependent brightness variations at small phase angles can be due to quasifractal properties of the lunar surface. Results of related laboratory measurements suggest that: (1) besides the narrow coherent backscatter opposition spike there is a broad component which can contribute to phase angles up to 10• and (2) a component of coherent backscatter can be important even for low albedo surfaces. The latter testifies the opposition effect of the lunar surface to be substantially formed by the coherent backscatter mechanism.

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[1] We mapped the photometric characteristics of the lunar surface for several small areas using Clementine UVVIS camera images. The maps of the phase function steepness showed several anomalous sites. Several small fresh impact craters have anomalous halos in these maps. The phase function within the halos is less steep than for the surrounding mare surface. We interpret these halos to be due to geologically recent impact-caused alteration of the equilibrium millimeter-scale regolith structure. This equilibrium structure is established through micrometeoritic bombardment at a geologically short timescale. An anomaly of the same signature was found at the Apollo 15 landing site. We interpret it as being a result of the regolith structure alteration with the lander jets. A unique photometric anomaly not correlated with albedo was found within the Reiner Gamma Formation. We suggest that this anomaly is genetically related to the formation, which indicates its young age. This favors the impact hypothesis for the nature of the Reiner Gamma Formation. Our study showed that mapping of photometric characteristics is a new powerful tool in studies of the surfaces of atmosphereless bodies. Future photometric studies of the Moon with existing and new data sets are promising for a search for traces of recent seismic events, studies of the recent population of meteoroids in the inner solar system, an advance in the understanding of swirls, etc.

Abstract. Photometric properties of the lunar surface in visual and near-infrared light were studied using raw images obtained with UVVIS camera during the Clementine mission. The investigation focused on several specific regions on the lunar surface, each of which was observed by Clementine at a variety of different illumination and viewing geometries. Through these observations, the dependence of the surface brightness on the observation/illumination geometry was studied. It was shown that the disk component of this dependence, that is, the variations of brightness at constant phase angle, is different for different mare areas. The color of the lunar surface also changes with changing of the observation/illumination geometry, even if under constant phase angle. The Reiner Gamma formation displays unusual photometric properties. They are consistent with the surface being smoother than the typical mare regolith surface. The UVVIS images taken at the smallest phase angles were used to study the opposition spike, that is, the sharp increase of the surface brightness near the opposition. Steepness of the phase dependence of brightness varies over a wide range for different sites.