  The study of meter and sub-meter scale geological features, especially
boulders and boulder fields, on the surface of airless bodies can provide
insight into the evolution of the regolith and the contribution of various
processes to its formation. Prior studies have examined the photometric
properties of the lunar regolith surrounding young craters using image ratios.
We extend this methodology to extracting surface properties, in particular the
roughness characteristics, exclusive to boulder fields and the boulders that
constitute them around impact craters. In this study, rock-rich regions on the
Moon are investigated using photometric roughness by employing a normalised
logarithmic phase ratio difference metric to measure and compare the slope of
the phase curve (reflectance versus phase angle) of a rock-rich field to a
rock-free field. We compare the photometric roughness of rock-rich fields on
simulated images with the photometric roughness of rock-rich fields on LROC NAC
images (0.5m/pixel). Using this technique, we determine that rock-rich surfaces
are not necessarily photometrically rougher than rock-free areas. Additionally,
we find the roughness of resolved rock fields to indicate the presence of
diverse sub-mm scale rock roughness (microtopography) and, possibly, variable
rock single scattering albedo. These latter properties are likely controlled by
rock petrology and material response to weathering and erosion. Spatial
clustering of photometrically smooth and rough boulder fields in the downrange
and uprange of two craters is observed, reflecting ejecta asymmetry and
possibly indicating asymmetric modification of ejecta rock surfaces during the
impact excavation process.
