The effects of spatial scales on the in situ photometric analysis of the Chang’e-4 landing region

Context. Visible and near-infrared reflectance spectroscopy is widely used to determine the surface composition and maturity of the Moon, but it may be influenced by the observation geometry. In situ observation is crucial for understanding the angular scattering behaviour of the lunar surface and can help to link laboratory measurements with orbital remote sensing. The Chang’e-4 (CE-4) rover has been exploring on the lunar surface for five years and has collected in situ spectral data covering a wide range of measurement angles. Aims. Our aim is to investigate the impact of different spatial scales (from several centimetres to tens of centimetres) on the photometric analysis results of the CE-4 landing region and the phase reddening effects. Methods. We constructed four sets of spectrophotometric data with different spatial scales (~2 cm, ~5.5 cm, ~10.5 cm, and ~15 cm × 21 cm) based on the spectral data acquired by the CE-4 imaging spectrometer and employed the Hapke model for photometric inversion analysis. Results. For the four different spatial scales, the derived phase function parameters lie outside the ‘hockey stick’ area and the photometric roughness parameter is smaller than 10°. These parameters show clear variation trends as the spatial scale increases. The reflectance phase curves show a distinct bowl-like shape with a turning point near 90° of phase angle. The colour ratios (ratios of reflectance at two different wavelengths) manifest significant phase-angle dependence, which may affect the estimation of surface maturity. The colour ratio phase curves exhibit a distinct arch shape when involving visible bands (<745 nm), whereas it appears as a monotonically increasing pattern when both bands are in the near-infrared wavelength range (R945/R745). Conclusions. Varied spatial scales have no significant effect on the phase curve shape and phase reddening effects, but do influence the photometric parameters derived using the Hapke model. This study could serve as a reference to link the discrepancies between laboratory and orbital research.