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J. Hillier, B. J. Buratti (JPL), K. Hill (CalTech)
The mechanism responsible for the lunar opposition effect is an area of current debate. Recent studies of the lunar opposition effect have yielded contradictory results. Hapke et al. (1993), studying polarized light from lunar samples, concluded the surge is predominantly due to coherent backscatter. Buratti et al. (1996) in an earlier study of Clementine data found shadow hiding to be the dominant mechanism. More recent studies by Helfenstein et al. (1997) and Hapke et al. (1998) suggest that both mechanisms make significant contributions. Given these new results a further study of the Clementine data is warranted. We have developed a photometric model incorporating coherent backscatter into Hapke's model. We then applied this model to absolutely calibrated Clementine images of the Moon to reexamine the lunar opposition effect. We find that shadow hiding is the dominant mechanism with a half width ~8°. However, for the brightest regions (the highlands at 0.75 - 1.0 microns) a small but significant additional narrow component (half width <2°) of total amplitude ~1/6 to 1/4 of the shadow hiding component is observed which may be attributed to coherent backscatter. Interestingly, no evidence for the narrow component is seen in the mare or in the highlands at 0.415 microns. This may naturally be explained if the highlands in the near infrared are bright enough to exhibit enough multiple scattering for coherent backscatter (which only occurs in the multiply scattered light) to be seen, but the mare and the highlands in the violet are simply too dark for significant multiple scattering to occur.
Buratti et al. (1996), Icarus, 124, 490 – 499; Hapke et al. (1993), Science 260, 509 – 511; Hapke et al. (1998), Icarus, 133, 89 – 97; Helfenstein et al. (1997), Icarus, 128, 2 – 14.