Lunar opposition effect as inferred from Chandrayaan-1 M3 data

The shadow-hiding and coherent backscattering enhancement mechanisms are considered to be the major contributors to the brightness opposition effect of the Moon. However, the actual proportions of the mechanisms at different phase angles still remain not well determined. In order to assess the lunar phase function across small phase angles, we utilize imaging spectrometer data acquired by the Moon Mineralogy Mapper (M-3) onboard the Chandrayaan-1 spacecraft. We calculated phase functions of apparent reflectance and color ratios in the wavelength range 541-2976nm for several mare and highland areas. As inferred from changes in the wavelength dependence of the phase curves, the shadow-hiding effect is a major component of the brightness opposition surge at phase angle>2 degrees. The coherent backscattering enhancement may contribute some to the opposition effect at phase angles<2 degrees. We found nonmonotonic behavior of color-ratio phase curves that reveal the minimum at similar to 2-4 degrees. Using lunar observations, this is the first reliable evidence of the colorimetric opposition effect found earlier for lunar samples.