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M. K. Shepard (Bloomsburg University), R. E. Arvidson (Washington University)
We present polarization-phase curve measurements and analysis for three basalt samples: one fresh, one coated with oxidized clay, and one coated with a dark desert varnish. Each sample displays a prominent opposition surge, countering the widely held notion that only particulate surfaces, and not bare rock, will display this phenomenon. We therefore suggest that an opposition surge requires a surface that is only "optically" porous. These observations also suggest the absence of an opposition surge in planetary observations may be due to something other than a bare rock surface. The photopolarimetric properties of the samples are most comparable to C-class asteroids, and thus provide alternatives to the traditional view that asteroid surfaces are either coarse-grained regoliths or rocky surfaces dusted with fines. Fresnel reflection, whether from smooth coatings, polished surfaces, or individual grains plays an important role in the polarization properties of these surfaces, and may contribute to their opposition surge.