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B. Hapke (U. of Pittsburgh), R. M. Nelson, W. D. Smythe, K. Manatt (JPL)
This report is the latest in a continuing effort to understand the opposition effect (OE) in planetary regoliths. Accepted theoretical and experimental results on media of well-separated spheres had predicted that the HWHM of the peak should be a strong function of particle size for media in which the particles are touching. However, previous measurements by us on well-sorted abrasive powders found little dependence of the HWHM on size. We hypothesized that this might be caused by departures of these particles from perfect sphericity To test this hypothesis we used the JPL long-arm goniometer to measure the phase functions of media of particles of the same size but differing in shape and internal structure. The OE's of media of perfect NaK silicate glass spheres had size-dependent HWHM's consistent with theoretical predictions. Imperfect spherical fragments made by partially crushing larger glass spheres were similar, indicating that shape was not a strongly-determining factor. However, irregular particles of the same size made by grinding chunks of glass had wider HWHM's, implying that the OE is sensitive to surface roughness and internal cracks. A medium of plaster of paris particles 74-140 micrometers in diameter consisting of sub-particles of gypsum a few micrometers in size had well-developed OE's. Evidently the most important factor controlling the HWHM of a medium is the internal structure of the particles rather than their size. This research is sponsored by grants from the Planetary Geology and Geophysics program of NASA.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.