Although ferric iron indisputably exists on the highly reducing surface of the Moon, its formation mechanism and evolution are still under debate. Here we show that micrometeorite impact-induced charge disproportionation of iron could have produced the large amounts of ferric iron (average Fe3+/∑Fe > 0.4) in agglutinate melts returned by China’s Chang’e-5 mission. The charge disproportionation reaction synchronously generated nanophase metallic iron (npFe0), and quantitative analyses of iron valence indicate that it is a dominant pathway for formation of npFe0 within the lunar agglutinate glass. The discovery of the charge disproportionation reaction in the agglutinates suggests that much more Fe3+ could be present on the Moon than previously thought, and that its abundance is progressively increasing with micrometeoroid impacts.
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We thank all the team members of China’s Chang’e Lunar Exploration Project for their hard work in returning lunar samples and CNSA for providing the lunar sample CE5C0400YJFM00408. We thank M. Chen for the helpful discussion on the early version of the scientific story. This study was supported by the Special Lunar Project of Chinese Academy of Sciences (ZDBS-SSW-JSC007-11) to Y.-G.X., the Director’s Fund of Guangzhou Institute of Geochemistry, CAS (2022SZJJZD-03) to J.Z., H.X, Y.Y. and X.L and the Youth Innovation Promotion Association CAS (2021353) to H.X. This is contribution No. IS-3264 from GIGCAS.
The authors declare no competing interests.
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Extended Data Fig. 1 Melt splashes on the surface of a CE5 agglutinate particle.
The brushed melts on the glass suggest repeated micrometeoroid impacts experienced by the agglutinate paticle.
Extended Data Fig. 2 Microscopic chemical compositions of the CE5 agglutinate glass.
HAADF-STEM image (a), elemental EDS maps (b–h), and selected area EDS spectra (i) of the same area shown in Fig. 1c,d of the main text.
Extended Data Fig. 3 Peak position and shape comparation of intensity normalized Fe L2,3 EELS spectra of metallic Fe0, olivine, and hematite.
The Fe0 and Fe2+ have a typical Fe L3 peak positions at 708.1–708.9 eV while the Fe3+ has a typical Fe L3 peak position at 709.9 eV. The normalized intensity of the peak tail of Fe0 at 730 eV is 2 times higher than those of Fe2+ and Fe3+. The full width at half maximum (FWHM) of Fe0 L3 peak (3.9 eV) is larger than those of Fe2+ and Fe3+ L3 peaks (~3.4 eV). The green arrow indicates the high-tail feature of Fe L-edge for metallic Fe0, while the orange arrow indicates the relative low-tail features of Fe L-edges for both olivine and hematite.
Extended Data Fig. 4 Iron valence distribution of the analyzed zone No. 2 from the CE5 agglutinate glass.
HAADF-STEM image (a), EELS Fe valent map (b), Fe3+/Fe ratios map (c), and the frequency and cumulative distributions (d) of the Fe3+/∑Fe ratios of a randomly chosen zone in the CE5 agglutinate.
Extended Data Fig. 5 Iron valence distribution of the analyzed zone No. 3 from the CE5 agglutinate glass.
HAADF-STEM image (a), EELS Fe valent map (b), Fe3+/∑Fe ratios map (c), and the frequency and cumulative distributions (d) of the Fe3+/∑Fe ratios of another randomly chosen zone in the CE5 agglutinate.
Extended Data Fig. 6 The relationship between the particle size and melting temperature (Tm) of iron nanospheres.
The bulk melting temperature (Tmb =1536ºC) is indicated by the blue dashed line.The black dotted line and arrow indicate estimated melting temperature (1524ºC) for the largest npFe0 in the studied agglutinate glass.
Extended Data Fig. 7 Schematic illustration of the micrometeorite impact-induced ferrous charge disproportionation reaction.
The topographic base was adapted with permission from Ground Research and Application System of China’s Lunar and Planetary Exploration Program (http://moon.bao.ac.cn)44.
Supplementary Table 1.
3D-view movie of Fe valence state dispersion in agglutinate glass.
Supplementary Data 1
Abundance calculation details of various valent iron species in agglutinate glass.
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Xian, H., Zhu, J., Yang, Y. et al. Ubiquitous and progressively increasing ferric iron content on the lunar surfaces revealed by the Chang’e-5 sample. Nat Astron 7, 280–286 (2023). https://doi.org/10.1038/s41550-022-01855-0