The maturity index values of this Letter. (a) The npFe⁰ (wt%) of in situ spectra from the CE5 and CE6 landing sites, respectively. (b) The OMAT versus the npFe⁰ at the CE5 and CE6 landing sites. (c) The maturity index I_s/FeO of in situ spectra from the CE5 and CE6 landing sites. (d) The relationship between the maturity index I_s/FeO and OMAT of CE5 and CE6 in situ spectra. The npFe⁰ and I_s/FeO of in situ spectra of CE5 were derived from X. Lu et al. (2023). The npFe⁰ and I_s/FeO results for each site are presented in Table A3. In panels (a) and (c), all values represent averages calculated from in situ spectra (without distinguishing between sampled and non-sampled areas, or between pre- and post-sampling stages). Subsequent analyses of maturity stratification in this study utilize the results from the sampled areas, which include: CE5-Pre sampled (0009, 0010, 0012, and 0017), CE5-Post sampled (0014, 0015, and 0016), and CE6-Pre sampled (0003).