Earth Wind-Driven Formation of Hematite on the Lunar Surface

The recent discovery of hematite ((Formula presented.)) in lunar high-latitude regions challenges the notion that the Moon exists in a reduced state. However, the origin of hematite remains unclear. We conducted irradiation experiments using 10 keV (Formula presented.) and (Formula presented.) ions on Fe-bearing minerals to simulate the exposure of lunar regolith to Earth wind, and 2 keV (Formula presented.) to represent solar wind irradiation. Our results demonstrate that Earth wind oxygen ions can anhydrously oxidize metallic Fe, iron sulfide, and ilmenite to hematite. High-energy hydrogen ions can reduce hematite to metallic Fe, while low-energy hydrogen ions are largely ineffective. Consequently, hematite retention depends on both the energy and the relative flux ratio of oxygen and hydrogen ions from Earth wind impacting the lunar surface. These findings provide new insights into the dynamic redox processes shaping the lunar surface as well as the effective mass transport within the Earth-Moon system.