Variation trends of the spectral ratio toward phase angle and spectral reflectance for the spectra of CE5 lunar soil. (a)–(c) are spectral ratios of R950/R750, R1600/R700, and R750/R450 versus phase angles, respectively. (d) and (e) are R950/R750 versus R750 and R1600/R700 versus R700, respectively. The spectral ratios of JSC-1 were derived from Johnson et al. (2013). The spectra of CE5 in situ data were derived from Lin et al. (2022). The division of the maturity range is based on the relationship between the spectral ratio and I_s/FeO of the Apollo returned samples. The spectra of the Apollo lunar soil samples are from the Lunar Soil Characterization Consortium (LSCC) database. The LSCC spectra are all measured under standard geometry (i = 30°, e = 0°, g = 30°).