Recently, a joint research team from Institute of Geochemistry of Chinese Academy of Sciences, Shandong University and Yunnan University, analyzing lunar soil samples returned by the Chang'e-6 mission from the South Pole–Aitken (SPA) Basin, has for the first time discovered crystalline hematite (α-Fe₂O₃) and maghemite (γ-Fe₂O₃) formed as a result of a major impact event. This discovery provides direct sample evidence for the presence of highly oxidized materials on the lunar surface. This breakthrough finding not only reveals that the lunar surface is not as entirely reduced as long believed, but also provides crucial clues for deciphering the origins of lunar magnetic anomalies and the mechanisms behind large impact events.
Redox reactions are a crucial component of planetary formation and evolution. Unlike Earth, the Moon lacks a protective atmosphere and lost a significant portion of its water and other volatiles during the period of heavy impacts. This resulted in a low intrinsic oxygen fugacity within the Moon, where iron primarily exists in the ferrous (Fe²⁺) or metallic (Fe⁰) state, indicating an overall reduced environment. However, recent advances in lunar exploration have challenged this view. Remote sensing studies using visible and near-infrared spectroscopy in recent years suggested the possible widespread presence of hematite at high lunar latitudes. Furthermore, research on Chang'e-5 samples reported the first discovery of impact-generated sub-micron magnetite (Fe₃O₄) and evidence for the presence of Fe³⁺ in impact glasses. These findings strongly indicate the existence of localized oxidizing environments on the lunar surface, primarily associated with external impact processes. Despite this, direct mineralogical evidence for strong oxides on the Moon, such as hematite, remained elusive, and the prevalence of oxidation processes and related diagnostic minerals on the lunar surface has been highly debated.
Fig.1. Morphology, composition, and crystal structure of the studied iron oxide mineral in the Chang'e-6 sample.(Image by IGCAS)
The study suggests that the formation of hematite is most likely intimately linked to large-scale impact events in lunar history. The high temperatures generated by a major impact can vaporize lunar surface materials, creating a transient gaseous environment with high oxygen fugacity. Simultaneously, this process causes desulfurization of troilite. The iron ions are then oxidized in this high oxygen fugacity environment and subsequently form crystalline micron-sized hematite grains through vapor deposition, co-forming alongside magnetic minerals like magnetite and maghemite. The presence of hematite reveals a novel oxidation mechanism on the Moon. Additionally, the origin of lunar magnetic anomalies, a distinctive geological feature widely observed on the lunar surface, has remained poorly understood. Given the close link between oxidation and the formation of magnetic minerals, this study provides the first evidence highlighting the critical role of large impact events in magnetizing the surfaces of airless bodies like the Moon.
Fig.2. Graphical depiction of the formation scenario of ferric oxides in Chang'e-6 lunar sample
These research findings have been officially published in the international multidisciplinary journal Science Advances, under the title "Discovery of crystalline Fe₂O₃ in returned lunar soils". This research was supported by the China National Space Administration lunar sample (No. CE6C0300YJFM00301), and received funding from the National Natural Science Foundation of China, the National Key R&D Program of China, and the China Postdoctoral Science Foundation.
Contact：
LI Yang
Institute of Geochemistry, Chinese Academy of Sciences
Email: liyang@mail.gyig.ac.cn
(By Prof. LI Yang’s group)
