The finding of rust on the Moon has prompted scientists to ask a question that seems to defy the laws of space: how long will it take for the Moon to rust? Rust is the common name for iron oxide, a reddish compound that forms when iron oxidizes. This process requires three things—iron, oxygen, and water—components that are seemingly absent from the Moon’s dry, airless, and heavily irradiated environment. The finding of hematite, a form of iron oxide, on the lunar surface suggests an unexpected chemical process is at work, connecting our planet to its satellite in a way scientists are still trying to understand.
The Fundamental Requirements for Rust
On Earth, the familiar reddish-brown decay we call rust is a chemical reaction known as oxidation. The fundamental components for this transformation are iron, free oxygen gas (\(\text{O}_2\)), and water (\(\text{H}_2\text{O}\)) or hydroxyl (\(\text{OH}\)). Iron atoms readily give up electrons in the presence of an oxidizing agent like oxygen, eventually forming ferric oxide, or \(\text{Fe}_2\text{O}_3\), which is the main component of the mineral hematite.
Water does not participate in the final chemical formula but acts as an electrolyte, significantly speeding up the transfer of electrons between the iron and oxygen. This catalytic role of water is why iron rusts so quickly in humid or wet environments. The lunar surface, however, is covered in iron-rich rock, but it is nearly a perfect vacuum with no free oxygen.
The solar wind is a constant stream of charged particles, primarily hydrogen ions, streaming from the Sun. When these hydrogen ions strike the lunar surface, they act as a “reducing agent,” the chemical opposite of an oxidizing agent. These hydrogen ions should chemically reverse any oxidation that might occur, constantly turning iron oxide back into metallic iron. This prevailing environment is what makes the presence of hematite on the Moon so puzzling.
The Unexpected Sources of Oxygen and Water on the Moon
The unlikely formation of rust on the Moon is explained by two periodic phenomena that temporarily overcome the harsh lunar environment. The first is a rare source of oxygen that originates from our own planet. The Moon spends about five days each month passing through a region of space known as Earth’s magnetotail.
Earth’s magnetic field stretches out like a windsock on the side opposite the Sun, forming this magnetotail. When the Moon travels through it, the magnetotail shields the lunar surface from the solar wind’s hydrogen ions. Residual oxygen ions escaping from Earth’s upper atmosphere are channeled along the magnetic field lines and transported to the Moon. This provides the necessary oxygen for the oxidation reaction to occur without the constant interference of the solar wind’s hydrogen.
The second factor is the presence of trace amounts of water or hydroxyl. While the Moon is dry, scientists have detected water molecules (\(\text{H}_2\text{O}\)) and hydroxyl (\(\text{OH}\)) embedded in the lunar soil, or regolith. The solar wind itself may be responsible for generating some of this water. The hydrogen ions from the wind can react with oxygen atoms already bound in lunar minerals to form hydroxyl. This hydroxyl acts as the necessary catalyst, allowing the iron and the newly delivered oxygen to react.
Current Evidence and the Rate of Lunar Oxidation
Evidence for this slow, planetary-scale rusting process came from the Moon Mineralogy Mapper (\(\text{M}3\)) instrument aboard the Indian Space Research Organisation’s Chandrayaan-1 orbiter. Analysis of the data revealed the distinct spectral signature of hematite (\(\text{Fe}_2\text{O}_3\)) at the Moon’s high latitudes. Notably, the hematite is found to be more concentrated on the nearside of the Moon, the side that always faces Earth.
This uneven distribution strongly supports the hypothesis that Earth’s channeled oxygen is the primary driver of the reaction. Because the necessary ingredients—oxygen from Earth and shielding from the solar wind—are only present during the few days the Moon is within the magnetotail each month, the oxidation process is slow. The reaction also occurs under extremely cold and dry conditions, which further limits its speed.
The rust found on the Moon today is the result of a process that has been occurring for billions of years. Though the Moon is slowly oxidizing, it is not destined to become a reddish orb like Mars, which has a distinct atmosphere and a history of liquid water. The Moon’s rust is a thin, patchy layer of mineral. The timeline for any significant, widespread change would span many more billions of years, making the Moon’s rusting a geological phenomenon.