Opposition effect from clementine data and mechanisms of backscatter

An analysis of Clementine data obtained from a UVVIS cam era and simulating laboratory photometric and polarimetric measurements is presented with the use of a new photometric three-parameter 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 degrees-1 degrees is approximately 10%. The Clementine data also show a flattening of phase-dependent brightness at angles less than 0.25 degrees 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 degrees-50 degrees) 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 degrees 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. (C) 1999 Academic Press.