Multispectral photometry of the moon and absolute calibration of the clementine UV/Vis camera

We present a multispectral photometric study of the Moon between solar phas e angles of 0 and 85 degrees, Using Clementine images obtained between 0.4 and 1.0 mu m, we produce a comprehensive study of the lunar surface contain ing the following results: (1) empirical photometric functions for the spec tral range and viewing and illumination geometries mentioned, (2) photometr ic modeling that derives the physical properties of the upper regolith and includes a detailed study of the causes for the lunar opposition surge, (3) an absolute calibration of the Clementine UV/Vis camera. The calibration p rocedure given on the Clementine calibration web site produces reflectances relative to a halon standard and further appear significantly higher than those seen in groundbased observations. By comparing Clementine observation s with prior groundbased observations of 15 sites on the Moon we have deter mined a good absolute calibration of the Clementine UV/Vis camera. A correc tion factor of 0.532 has been determined to convert the web site (www.plane tary.brown.edu/clementine/calibration.html) reflectances to absolute values . From the calibrated data, we calculate empirical phase functions useful f or performing photometric corrections to observations of the Moon between s olar phase angles of 0 and 85 degrees and in the spectral range 0.4 to 1.0 mu m Finally the calibrated data is used to fit a version of Hapke's photom etric model modified to incorporate a new formulation, developed in this pa per, of the lunar opposition surge which includes coherent backscatter. Rec ent studies of the lunar opposition effect have yielded contradictory resul ts as to the mechanism responsible: shadow hiding, coherent backscatter, or both. We find that most of the surge can be explained by shadow hiding wit h a halfwidth of similar to 8 degrees. However, for the brightest regions ( the highlands at 0.75-1.0 mu m) a small additional. narrow component (halfw idth of <2 degrees) of total amplitude similar to 1/6 to 1/4 that of the sh adow hiding surge is observed, which may be attributed to coherent backscat ter. Interestingly, no evidence for the narrow component is seen in the mar ia or in the highlands at 0.415 mu m. A natural explanation for this is tha t these regions are too dark to exhibit enough multiple scattering for the effects of coherent backscatter to be seen. Finally, because the Moon is th e only celestial body for which we have "ground truth" measurements, our re sults provide an important test for the robustness of photometric models of remote sensing observations. (C) 1999 Academic Press.