Disk-integrated and disk-resolved photometry of the Moon with GaoFen-4 space observations

The GaoFen-4 (GF-4) satellite obtained high-resolution images of the entire lunar disk in a single exposure without atmospheric disturbance. These data are favorable for the study of both disk-resolved and disk-integrated photometric properties of the Moon. Using the GF-4 observations, we modeled the lunar reflectance phase functions with the Hapke model and analyzed the lunar photometric properties. The results indicate that the lunar surface exhibits pronounced backscattering, and the maria appear to be smoother compared to the highlands. The disk-resolved and disk-integrated models produced different best-fit parameters for the whole Moon, with the disk-integrated model fitting the small phase angle data better, and the disk-resolved model showing a more favorable appearance at large phase angles. By calculating the disk-integrated phase function with a disk-resolved method, the study confirms the equivalence of the two models, albeit with a small discrepancy due to an imperfect approximation of the roughness function. This discrepancy is due to the different forms of the phase function used in the models. The geometric albedo of the Moon derived from the disk-resolved model is 0.137 and the spherical albedo is 0.063 for Band 3 of GF-4 at 560.6 nm. This study observed an arched phase curve of the ratio of reflectance in two wavelengths, indicating a phenomenon of phase reddening at small phase angles and bluing at large phase angles.