Bi-directional reflectance, polarization, and spectroscopic measurements of the Chang'e-5 regolith sample

Context . Optical measurement is a powerful tool for retrieving the regolith physical properties of the lunar surface. Aims . It has been a long-standing question in planetary remote sensing whether laboratory measurements are consistent with remote sensing measurements. The sample return mission of Chang’e-5 (CE5) has provided an opportunity to answer this question. Methods . For this work we performed photometric, linear and circular polarimetric, and reflectance spectroscopic measurements of the Chang’e-5 surface scooped regolith (soil) sample CE5C0600. Results . Our results show that the CE5 regolith exhibits both a strong opposition effect and a pronounced forward-scattering lobe, and a moderate contrast between its minimum and maximum reflectance values compared to the in situ measurements of Chang’e-3 and −4. A slight monotonic phase reddening trend is observed with increasing phase angle, while no clear colorimetric opposition effect was found at small phase angles The regolith shows maximum linear polarization at large phase angles, ∼16 at 633 and ∼21% at 532 nm, slightly higher than the values reported in ground-based observations and laboratory measurements on the Apollo and Luna samples. The circular polarization ratio increases as the phase angle decreases, consistent with previous observations of the Apollo samples. Compared with orbital, in situ,and ground-based observations of the CE5 landing site, the laboratory-measured regolith exhibits higher reflectance but a very similar spectral slope, suggesting a higher degree of compaction in the Earth environment. Conclusions . Studies of lunar samples with varying porosities and space weathering degrees are needed to better understand their behavior under in situ condition, enabling their use as a reliable ground truth for current and future Lunar missions.