The Coherent Backscattering Opposition Effect (CBOE): Search for Wavelength Dependent Changes in the Shape of the Phase Curve

We undertook low phase angle measurements (0.05 < θ < 5 deg) of the opposition effect, of thirteen discrete particle sizes at wavelengths of λ =0.633 and 0.543 μ m to search for changes in the HWHM of the phase curve. These changes are predicted theoretically and observed experimentally in widely spaced particles in suspension (Stephen and Cwilich, 1986; Van Albada et al., 1987.) The materials exhibit opposition brightnesses of ~20% at 0.05 degrees. The slope of the phase curve and the circular polarization ratio increase with decreasing phase angle, consistent with the opposition surge being due to coherent backscattering (CBOE) (Nelson et al., Icarus, in press, 2000). CBOE predicts that the HWHM of the phase curve will vary with λ , as λ /2πD, where D is the transport mean free path in the medium. Since the samples were observed at two different wavelengths, we would expect that the observed HWHM would be smaller by about 15% at 0.543 μ m compared to 0.633 μ m. We do not observe this. There may be several possible explanations. One is that the particles, while well sorted, nevertheless have a variance about a mean size and therefore the change in HWHM is not as sharp as might be expected were all the particles the same size. Another is that our particles are more closely packed than those studied in the experimental observations of particles in suspension. This result may explain the negative Clementine search for CBOE in the lunar regolith, which found a small wavelength dependence of the phase curve of the same lunar regions when observed at different wavelengths (Buratti et al., 1996). The particle size variance in the lunar soil is greater than our samples. Measuring the change in phase curve HWHM with λ may not be a good test for CBOE in a planetary regolith. This work was supported by a NASA PG&G grant.