Photometric properties of lunar regolith revealed by the Yutu-2 rover

Context. The surface composition of the Moon has mainly determined based on the visible and near-infrared spectra achieved from orbits and/or landing sites, and the spectroscopic analysis is based on photometric properties of the topmost lunar regolith. However, the lack of a ground truth for the photometric parameters of the undisturbed lunar surface has limited accurate applications of spectral observations. Aims. Here we report the photometric properties of the small-scale (i.e., centimeter level) undisturbed lunar regolith around the Chang’E-4 landing site, determined from a series of photometric experiments conducted by the rover Yutu-2. Methods. The simplified Hapke model was used to derive the photometric properties. The micro-topographic effect on the spectral measurements was corrected for the first time in the in situ photometric investigations on the Moon, which improves the accuracy of the derived photometric parameters. Results. The single-scattering albedo w and two parameters (b, c) of the Henyey-Greenstein phase function were derived, and they show a wavelength dependence. The regolith at the Chang’E-4 landing site exhibits strong forward scattering according to the retrieved c values, and the higher asymmetry parameter indicates that the regolith here is more strongly forward scattering than the Apollo lunar soil samples. The derived photometric parameters can serve as ground truth and can be used in the radiative transfer modeling analysis of the orbital remote-sensing data