Amy Shell Snyder Hale

Low phase angle laboratory studies of the opposition effect: search for wavelength dependence

Abstract We measured the reflectance change with phase angle (0.05° 0.543 μm . We searched for changes with respect to wavelength in size of the half-width at half-maximum (HWHM) of the phase curve for each particle size. These changes in HWHM are predicted theoretically and observed experimentally in scattering measurements of widely spaced spherical particles in suspension. We find that at both wavelengths the materials exhibit opposition brightness surges of ∼30% at 0.05° relative to the brightness at 5°. At both wavelengths, and for all particle sizes, the slope of the phase curve and the circular polarization ratio both increase with decreasing phase angle, consistent with the opposition surge being due to the coherent backscattering opposition effect (CBOE). Theoretical models of CBOE predict that the HWHM of the phase curve will vary with λ, as λ/2πL, where L is the transport mean free path in the medium. Our samples were observed at two different wavelengths, therefore theory predicts that HWHM would be smaller by about 15% at 0.543 μm compared to 0.633 μm . We do not observe this. There may be several possible explanations. One possibility 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 exactly of the same size. Another possible explanation is that our particles are more closely packed and less spherical than those studied in the experimental observations of particles in suspension and hence, while our experiment more closely approximates a planetary regolith, it produces a different result from similar measurements of widely spaced spherical particles in suspension. Our result may explain the unsuccessful Clementine search for CBOE in the lunar regolith which found only a very small wavelength dependence of the phase curve of the same lunar regions when observed at different wavelengths, a result not consistent with theoretical models of CBOE. One explanation may be that the particle size variation in the lunar soil is greater than it was in our samples. Furthermore, our samples, which are better sorted than the lunar regolith, are less well sorted than the theoretical models. This suggests that measuring the change in phase curve HWHM with λ may not be a good test for CBOE in a planetary regolith unless the measurements include a large range of wavelength.