Bidirectional reflectance distribution function measurements of characterized Apollo regolith samples using the visible oxford space environment goniometer

A laboratory study was performed using the Visible Oxford Space Environment Goniometer in which the broadband (350–1250 nm) bidirectional reflectance distribution functions (BRDFs) of two representative Apollo regolith samples were measured, for two surface roughness profiles, across a range of viewing angles—reflectance: 0–70°, in steps of 5°; incidence: 15°, 30°, 45°, and 60°; and azimuthal: 0°, 45°, 90°, 135°, and 180°. The BRDF datasets were fitted using the Hapke BRDF model to (1) provide a method of comparison to other photometric studies of the lunar regolith and (2) to produce Hapke parameter values which can be used to extrapolate the BRDF to all angles. Importantly, the surface profiles of the samples were characterized using an Alicona 3D® instrument, allowing two of the free parameters within the Hapke model, φ and $\overline{\theta }$, which represent porosity and surface roughness, respectively, to be constrained. The study determined that, for $\overline{\theta }$, the 500–1000 μm size-scale is the most relevant for the BRDF. Thus, it deduced the following “best fit” Hapke parameters for each of the samples: Apollo 11 rough—$w$ = 0.315 ± 0.021, $b$ = 0.261 ± 0.007, and $h_S$ = 0.039 ± 0.005 (with $\overline{\theta }$ = 21.28° and φ = 0.41 ± 0.02); Apollo 11 smooth—$w$ = 0.281 ± 0.028, $b$ = 0.238 ± 0.008, and $h_S$ = 0.032 ± 0.006 (with $\overline{\theta }$ = 13.80° and φ = 0.60 ± 0.02); Apollo 16 rough—$w$ = 0.485 ± 0.155, $b$ = 0.155 ± 0.083, and $h_S$ = 0.135 ± 0.007 (with $\overline{\theta }$ = 21.69° and φ = 0.55 ± 0.02); Apollo 16 smooth—$w$ = 0.388 ± 0.057, $b$ = 0.063 ± 0.033, and $h_S$ = 0.221 ± 0.011 (with $\overline{\theta }$ = 14.27° and φ = 0.40 ± 0.02). Finally, updated hemispheric albedo functions were determined for the samples, which can be used to set laboratory measured visible scattering functions within thermal models.