Ubiquitous and progressively increasing ferric iron content on the lunar surfaces revealed by the Chang’e-5 sample

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. Lunar high-concentration ferric ion (Fe3+/∑Fe > 40%) and ~63% of nanophase metallic iron (npFe0) are produced via charge disproportionation of ferrous iron from micrometeoroid impacts, as observed in the Chang’e-5 sample. This ongoing process would lead to a continuously increasing abundance of Fe3+ in the lunar regolith.