Chang'e-6 samples reveal first evidence of impact-formed hematite and maghemite on the Moon

(Press-News.org) A joint research team from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) and Shandong University has for the first time identified crystalline hematite (α-Fe2O3) and maghemite (γ-Fe2O3) formed by a major impact event in lunar soil samples retrieved by China's Chang'e-6 mission from the South Pole–Aitken (SPA) Basin. This finding, published in Science Advances on November 14, provides direct sample-based evidence of highly oxidized materials on the lunar surface.

Redox reactions are a fundamental component of planetary formation and evolution. Nevertheless, scientific studies have shown that neither the oxygen fugacity of the lunar interior nor the lunar surface environment favors oxidation. Consistent with this, multivalent iron on the Moon primarily exists in its ferrous (Fe2+) and metallic (Fe0) states, suggesting an overall reduced state. However, with further lunar exploration, recent orbital remote sensing studies using visible-near-infrared spectroscopy have suggested the widespread presence of hematite in the Moon's high-latitude regions.

Furthermore, earlier research on Chang'e-5 samples first revealed impact-generated sub-micron magnetite (Fe3O4) and evidence of Fe3+ in impact glasses. These results indicate that localized oxidizing environments on the Moon existed during lunar surface modification processes driven by external impacts. Despite this research progress, though, conclusive mineralogical evidence for strongly oxidizing minerals like hematite on the Moon had remained elusive. Additionally, the extent of oxidation processes and the prevalence of characteristic oxidized minerals on the lunar surface have long been topics of intense debate.

The SPA Basin, one of the largest and oldest impact basins in the Solar System, with extremely complex impact scales and frequencies, offers an ideal natural laboratory for studying oxidation reactions on the lunar surface. The successful return of soil samples from the SPA Basin by the 2024 Chang'e-6 mission offered an opportunity to search for highly oxidized substances formed during major impact events. The research team identified micron-sized hematite grains in the Chang'e-6 lunar soil for the first time. Through a combination of micro-area electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy, they confirmed the crystal structure and unique occurrence characteristics of these hematite particles, verifying that the minerals are primary lunar components rather than terrestrial contaminants.

The study proposes that hematite formation is closely linked to major impact events in lunar history. The extreme temperatures generated by large impacts would have vaporized surface materials, creating a transient high-oxygen-fugacity vapor-phase environment. At the same time, this process would have caused desulfurization of troilite; the released iron ions were then oxidized in the high-fugacity environment and underwent vapor-phase deposition, forming micron-sized crystalline hematite. This hematite coexists with  magnetic magnetite and maghemite.

Notably, the origin of widespread magnetic anomalies on the lunar surface, including those in the northwestern SPA Basin, remains poorly explained. Given the close correlation between oxidation processes and the formation of magnetic carrier minerals, this study provides key sample-based evidence to clarify the carriers and evolutionary history of these lunar magnetic anomalies.

This research challenges the long-held belief that the lunar surface is entirely reduced. It also offers crucial clues for deciphering the evolution of lunar magnetic anomalies and the mechanisms underlying large impact events, thereby advancing our understanding of lunar evolution.

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