Near-infrared Hapke Photometric Analysis of the Moon’s Regolith with Passive Radiometry from the Lunar Orbiter Laser Altimeter (LOLA)

Abstract Measurements of the Moon’s surface reflectance across a broad range of wavelengths and geometries can provide vital clues about the processes acting on its regolith. In the near-infrared (1064 nm), the Lunar Orbiter Laser Altimeter (LOLA) operates as a passive radiometer when its laser is turned off, adding a wavelength to the capabilities of the Lunar Reconnaissance Orbiter for narrowband photometric studies. We analyze LOLA passive radiometry spanning 2013 December through 2024 April, using the Hapke photometric model to infer regolith properties. We find that the highlands have a more backscattering single-particle phase function than the maria, with optically immature highlands exhibiting the strongest backscattering. In the maria, scattering becomes more symmetrical between backward and forward scattering with increasing TiO 2 abundance, which may be related to the reflectance peak of ilmenite near 1 μ m. Looking in detail at the Reiner Gamma swirl, we find that the contrasts in on/off-swirl photometric behavior can be explained by differences primarily in single-scattering albedo, and to a lesser extent the single-particle phase function, without any difference in surface roughness or porosity. The stronger influence of single-particle properties relative to surface textural properties is consistent with the solar wind standoff and electrostatic dust transport hypotheses for swirl formation, but not with the cometary impact hypothesis.