New analysis of lunar soil retrieved by China’s Chang’e-6 mission reveals the first direct evidence of crystalline hematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃) formed by a major impact event. This discovery, published in Science Advances, challenges the long-held assumption that the Moon’s surface is predominantly in a reduced state and provides crucial insights into the planet’s evolution.
The Lunar Oxidation Puzzle
For decades, scientists believed the Moon’s environment and interior lacked the conditions necessary for significant oxidation. Iron on the Moon was expected to exist primarily in its ferrous (Fe²⁺) or metallic (Fe⁰) forms. However, recent orbital studies suggested the presence of hematite in high-latitude regions, creating a scientific debate. Prior research on samples from the Chang’e-5 mission found impact-generated magnetite (Fe₃O₄), hinting at localized oxidizing environments during surface modification.
Despite these findings, conclusive mineralogical proof of strongly oxidizing minerals like hematite remained elusive. The extent of oxidation processes and the prevalence of oxidized minerals on the lunar surface remained unclear.
Chang’e-6 Samples Reveal New Evidence
The Chang’e-6 mission, which successfully returned soil samples from the South Pole–Aitken (SPA) Basin, provided the opportunity to search for highly oxidized substances formed during major impact events. The SPA Basin, one of the largest and oldest impact craters in the solar system, offers an ideal natural laboratory for studying oxidation reactions.
Researchers identified micron-sized hematite grains in the Chang’e-6 lunar soil. Using micro-area electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy, they confirmed the crystal structure and unique characteristics of these hematite particles, verifying that they are primary lunar components rather than terrestrial contaminants.
How Impacts Drive Oxidation
The study proposes that hematite formation is closely linked to major impact events in lunar history. Extreme temperatures generated by large impacts vaporize surface materials, creating a transient, high-oxygen-fugacity environment. This process also causes the desulfurization of troilite, releasing iron ions that are then oxidized in the high-fugacity environment and undergo vapor-phase deposition, forming micron-sized crystalline hematite. This hematite coexists with magnetic magnetite and maghemite.
Implications for Lunar Magnetism
The origin of widespread magnetic anomalies on the lunar surface, including those in the northwestern SPA Basin, remains poorly explained. Given the 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 advances our understanding of lunar evolution by challenging the long-held belief that the lunar surface is entirely reduced. The findings offer crucial clues for deciphering the evolution of lunar magnetic anomalies and the mechanisms underlying large impact events
