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R. M. Nelson (Jet Propulsion Laboratory), B. W. Hapke (University of Pittsburgh), W. D. Smythe, A. S. Hale (Jet Propulsion Labotatory), J. L. Piatek (University of Pittsburgh)
Experiments find that the Opposition Effect (OE) in particulate materials is due to two processes, Shadow Hiding (SHOE) and Coherent Backscattering (CBOE)(Nelson, et al. 2000; 2002). SHOE arises because, as phase angle approaches zero, shadows cast by regolith grains on other grains become invisible to the observer. CBOE results from constructive interference beween rays traveling the same path but in opposite directions. We measured the angular scattering properties of 9 mixtures of Aluminum Oxide and Boron Carbide powders of the same particle diameter (25 microns). The reflectance of the materials ranged from 7-91%. Along with the reflectance phase curve we measured the circular polarization ratio, CPR-the ratio of the intensity of the light returned with the same helicity as the incident light to that with the opposite helicity. An increase in CPR with decreasing phase angle indicates increased multiple scattering and is consistent with CBOE. It might be expected that materials of higher albedo would exhibit increased multiple scattering and that CBOE would increase as albedo increases. Remarkably, we find the highest albedo samples did not have the strongest CBOE opposition peaks. Instead, the maximum CBOE contribution was for the samples with reflectance between 15 and 40%. We derived a theoretical model which reproduces the data quite satisfactorily. This model shows that the reflectance where we find the CBOE amplitude to be a maximum is where the contribution of second order scattering is largest relative to the other orders. Hence, for closely packed media the maximum contribution of CBOE does not occur in materials of highest albedo but where the relative contribution of second order scattering is largest. Nelson, et al. 2000. Icarus, 147, 545-558. Nelson, et al., 2002, Planetary and Space Science, 50, 849-856. This work was done at JPL supported by NASA's PGG program.
The author(s) of this abstract have provided an email address for comments about the abstract: robert.m.nelson@jpl.nasa.gov
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.