The dark asteroid (101955) Bennu studied by NASAtextquoteright s OSIRIS-REx mission has a boulder-rich and apparently dust-poor surface, providing a natural laboratory to investigate the role of single-scattering processes in rough particulate media. Our goal is to define optical roughness and other scattering parameters that may be useful for the laboratory preparation of sample analogs, interpretation of imaging data, and analysis of the sample that will be returned to Earth. We rely on a semi-numerical statistical model aided by digital terrain model (DTM) shadow ray-tracing to obtain scattering parameters at the smallest surface element allowed by the DTM (facets of textasciitilde{}10 cm). Using a Markov Chain Monte Carlo technique, we solved the inversion problem on all four-band images of the OSIRIS-REx missiontextquoteright s top four candidate sample sites, for which high-precision laser altimetry DTMs are available. We reconstructed the emph{a posteriori} probability distribution for each parameter and distinguished primary and secondary solutions. Through the photometric image correction, we found that a mixing of low and average roughness slope best describes Bennus surface for up to $90^{circ}$ phase angle. We detected a low non-zero specular ratio, perhaps indicating exposed sub-centimeter mono-crystalline inclusions on the surface. We report an average roughness RMS slope of $27_{-5}^{circ+1}$, a specular ratio of $2.6_{-0.8}^{+0.1}%$, an approx. single-scattering albedo of $4.64_{-0.09}^{+0.08}%$ at 550 nm, and two solutions for the back-scatter asymmetric factor, $xi^{(1)}=-0.360pm0.030$ and $xi^{(2)}=-0.444pm0.020$, for all four sites altogether.
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