Photometry of LROC NAC resolved rock-rich regions on the Moon

TL;DR

This paper investigates the photometric roughness of lunar rock-rich regions using high-resolution images, revealing insights into surface properties, microtopography, and impact-related surface modifications.

Contribution

It extends photometric analysis methods to characterize boulder field roughness on the Moon, comparing simulated and real images to understand surface heterogeneity.

Findings

Rock-rich surfaces are not necessarily rougher than rock-free areas.

Photometric roughness indicates diverse sub-mm scale rock microtopography.

Spatial clustering reflects ejecta asymmetry and impact modification processes.

Abstract

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.