Photometry of rock-rich surfaces on airless bodies.

Introduction and Methods:  Our understanding of the response of boulders to space weathering, micrometeorite abrasion, thermal fatigue, and consequently their evolution into regolith can be improved by characterizing the surface roughness of the uppermost layer of boulders. In the first phase of our study [1] we characterize the surface roughness of boulder fields photometrically by using the phase ratio methodology applied to orbital image data. In the second phase of our study (in-progress) we focus on characterizing the sub-mm scale topography and roughness of naturally fresh surfaces of meteorite samples. The photometric roughness of boulder fields on the lunar surface is studied by employing a normalized logarithmic phase ratio difference (NLPRD) metric, described in [1], 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 Lunar Reconnaissance Orbiter Narrow Angle Camera (LROC NAC) images sampled around an Unnamed crater at Hertzsprung S.

Results and Discussion: The NLPRD is normalized to a rock-free reference surface, assuming the roughness of the regolith within the boulderfield is comparable to the roughness of the regolith at the rock-free reference regions, the higher roughness of the boulder-fields implies the presence of rocks with diverse sub-mm scale roughness and, possibly, variable single scattering albedo. In figure 1b, the spread in NLPRD values for different rock morphologies, is exceeded by the spread in  NLPRD of the NAC-resolved boulderfields. We find spatial clustering of photometrically smooth and rough boulderfields in the downrange and up-range respectively of the Unnamed crater at Hertzsprung S, reflecting ejecta asymmetry (in agreement with [2]) and possibly indicating asymmetric modification of ejecta rock surfaces during impact excavation process. Our results imply that rock physical properties at the start of the surface exposure period are a function of petrology as well as the (shock) effects imparted upon ejecta rock formation and excavation. The work-in progress deals with supplementing our findings with investigation of the sub-mm scale topography and roughness of meteorite and lunar samples. To study the sub-mm scale roughness of these samples we produce high-resolution DTMs at the µm scale using a non-contact optical profilometer. A sample high-resolution DTM of lunar breccia NWA11273 is shown in figure 2.

Figure 3 shows that variations of the mean slope with spatial scale exists within different meteorites types. Next, we will investigate the scale-dependent rock micro-texture of various samples (i.e., ordinary and carbonaceous chondrites, lunar basalts and breccias as well as meteorites from the HED clan), and provide typical values of surface roughness that will inform photometric modelling of rock surfaces.

References: [1] Marshal, R. M., Rüsch, O., Wöhler, C., Wohlfarth, K., & Velichko, S. (2022). Icarus, 115419 [2] Velichko, S., Korokhin, V., Velikodsky, Y., Kaydash, V., Shkuratov, Y., & Videen, G. (2020). PSS, 193.