In Situ Photometric Experiment of Lunar Regolith With Visible and Near-Infrared Imaging Spectrometer On Board the Yutu-2 Lunar Rover

China's Chang'E-4 (CE-4) mission successfully landed in Von Karman crater within South Pole-Aitken basin of the Moon. The Visible and Near-Infrared Imaging Spectrometer (VNIS) on board Yutu-2 rover investigated the photometric properties of lunar regolith. Seven VNIS measurements were conducted on a small lunar surface with a diameter <5 m by the rover rotating at the center, with the phase angles from 39.6 to 97.1 degrees obtained in the similar observational geometry of solar altitude and observation angle. The phase function, which varies in different wavelength, is derived using a third-order polynomial fitting, in combination with the calibration and comparison of orbital/in situ VNIS data at the Chang'E-4 landing site and the same regions. After the photometrical correction of the spectra with the phase function, the derived FeO contents and optical maturity parameters of the regolith reduce much of their deviations, which is consistent with the homogeneity of the regolith and hence demonstrates the significance of the photometric correction on the VNIS spectra. Plain Language Summary For mineral identification and mapping with optical data, the photometric correction is important and challenging. The rover Yutu-2 conducted experiments on the lunar surface to measure how the spectra change with phase angles. We derived a photometric function using seven measurements obtained by the Visible and Near-Infrared Imaging Spectrometer on board Yutu-2. The photometric correction of reflectance spectra is critical for estimations of the FeO content and the spectral maturity, which relate to geologic interpretations. The photometric function is also important for the calibration, validation, and comparison of different observations acquired by the same/different instruments.