Scientists have long puzzled over why rust exists on the Moon. Now, new research points to an unexpected culprit much closer to home: Earth itself. The findings raise fresh questions about how our planet and its natural satellite have been linked for billions of years.
How is oxygen from Earth linked to lunar rust?
A recent study suggests oxygen leaking from Earth's atmosphere may be behind the presence of hematite, a type of rust, near the Moon's poles. The research, published in Geophysical Research Letters, found that this oxygen could transform lunar iron into hematite (Fe2O3).
"We conducted a series of oxygen and hydrogen irradiation experiments to simulate lunar surface irradiation processes," said planetary scientist Xiandi Zeng of Macau University of Science and Technology. "For the first time, our experiments demonstrate both the formation and reduction of hematite minerals," Zeng said.
Hematite forms through oxidation, a chemical reaction requiring oxygen. But the Moon lacks a proper atmosphere and has little free oxygen. It is also constantly exposed to hydrogen from the solar wind, which acts as a reducing agent and should prevent rust from forming.
How does Earth's magnetotail contribute to lunar oxidation?
Researchers believe the answer lies in Earth's magnetosphere, which trails behind the planet under solar wind pressure. During the full Moon phase, this magnetotail directs oxygen particles from Earth toward the lunar surface. For about five days each month, the Moon is shielded from most solar wind and instead receives a steady flow of oxygen ions.
To test this scenario, scientists fired oxygen ions at iron-rich minerals similar to those found on the Moon, such as pyroxene, olivine, ilmenite, troilite and iron meteorites. They discovered that metallic iron, ilmenite and troilite oxidised easily, forming hematite. In contrast, silicates like pyroxene and olivine showed no such transformation, indicating that the process is selective.
"Our experimental results provide strong evidence that hematite can form on the lunar surface through oxygen ion irradiation," the team wrote. "Earth wind, the primary source of energetic oxygen ions on the Moon, acts as the oxidant, driving the oxidation of various minerals, including metallic iron and iron-bearing oxides and sulfides abundant in lunar regolith."
Can the solar wind reverse this rusting process?
To see if hydrogen from the solar wind could undo the oxidation, researchers exposed hematite to hydrogen ion beams. They found that only high-energy beams similar to Earth's oxygen wind reversed the process. The solar wind, with its lower energy, could not.
This explains why hematite is concentrated near the Moon’s poles, where Earth’s magnetotail directs oxygen ions while deflecting many hydrogen ions. It also clarifies why hematite is often found near water. During experiments, water formed as a by-product when hydrogen stripped oxygen from hematite, suggesting lunar water in these regions may result from reduction, not external delivery.
The study hints that hematite might even store a chemical record of Earth’s atmospheric history, potentially dating back to the Great Oxidation Event around 2.4 billion years ago.
"The formation of hematite (and potentially magnetite) via Earth wind irradiation underscores the material exchange between Earth and the Moon, which may have persisted for more than 4 billion years due to interactions between their coupled magnetospheres," the researchers noted.
With recent missions like Chandrayaan-3 landing near the lunar south pole and China’s Chang’E-7 set to explore the region, scientists hope to gather more data to understand this long and complex Earth-Moon connection.
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