Chang’e-6 samples reveal first evidence of hematite and maghemite formed by impact on the moon

A joint research team from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) and Shandong University identified crystalline hematite (α-Fe) for the first time₂Oh₃) and maghemite (γ-Fe₂Oh₃) formed by a major impact event on lunar soil samples recovered by the Chinese Chang’e-6 mission in the South Pole-Aitken (SPA) basin. This discovery, published in Scientific advances on November 14, provides direct, sample-based evidence of the presence of highly oxidized materials on the lunar surface.

Redox reactions and the mystery of lunar oxidation

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 exists primarily in ferrous form (Fe²⁺) and metallic (Fe⁰), suggesting an overall reduced state. However, with continued lunar exploration, recent orbital remote sensing studies using visible-near infrared spectroscopy have suggested the widespread presence of hematite in high latitude regions of the Moon.

Additionally, previous research on Chang’e-5 samples revealed impact-generated submicron magnetite (Fe) for the first time.₃Oh₄) and evidence of Fe³⁺ in impact glasses. These results indicate that localized oxidizing environments on the Moon existed during lunar surface modification processes caused by external impacts.

Despite these advances in research, conclusive mineralogical evidence for the existence of strongly oxidizing minerals like hematite on the Moon had remained elusive. Additionally, the extent of oxidation processes and the prevalence of characteristic oxide minerals on the lunar surface have long been the subject of intense debate.

SPA Basin samples reveal new evidence

The SPA Basin, one of the largest and oldest impact basins in the Solar System, with extremely complex impact scales and frequencies, provides 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 provided an opportunity to search for highly oxidized substances formed during major impact events.

The research team identified micrometer-sized hematite grains in the lunar soil of Chang’e-6 for the first time. Using 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. Extreme temperatures generated by large impacts would have vaporized surface materials, creating a transient vapor phase environment with high oxygen fugacity. At the same time, this process would have caused desulfurization of the troilite; the released iron ions were then oxidized in a high fugacity environment and underwent vapor deposition, forming micron-sized crystalline hematite. This hematite coexists with magnetic magnetite and maghemite.

Implications for lunar magnetism and evolution

Notably, the origin of widespread magnetic anomalies on the lunar surface, including those in the northwest 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 to decipher the evolution of lunar magnetic anomalies and the mechanisms underlying large-impact events, thereby advancing our understanding of lunar evolution.