The Moon, a celestial body long thought to be a dry, airless, and inert world, appears to be rusting. This surprising discovery involves the presence of hematite, which is a specific iron oxide mineral commonly known as rust. Rusting is a process of oxidation that requires two substances typically absent from the Moon: oxygen and water. The finding is chemically unexpected because the lunar surface is constantly bombarded by particles that should actively prevent oxidation from happening.
Confirming the Presence of Hematite
The specific form of rust detected is hematite, which has the chemical formula Fe₂O₃. This ferric mineral is the same reddish compound that forms on iron objects exposed to the elements here on Earth. The presence of hematite on the Moon’s surface is considered a paradox because the Moon’s environment is chemically reducing.
In a reducing environment, the native iron abundant in lunar rocks should remain in its pristine metallic state or be broken down, not oxidized. The evidence for this unexpected mineral came from hyperspectral reflectance data collected by the Moon Mineralogy Mapper (M³). This instrument was a U.S.-provided payload aboard the Indian Space Research Organization’s Chandrayaan-1 mission, launched in 2008.
Analysis of the light reflected off the lunar surface revealed a unique spectral signature that closely matched that of hematite, particularly at the Moon’s high latitudes. The detection of hematite, an oxidized iron-bearing mineral, fundamentally challenged established knowledge about the Moon’s surface chemistry.
How Earth Supplies the Oxygen
The source of the oxygen required for this oxidation process appears to be Earth itself. The planet’s magnetic field extends out into space, forming a long, comet-like structure called the magnetotail. The magnetotail trails behind Earth in the direction opposite the Sun, stretching far enough to occasionally envelop the Moon.
About five days each month, during the full moon phase, the Moon passes through this magnetotail. As it does, it is shielded from the solar wind, and oxygen ions from Earth’s upper atmosphere are stripped away and channeled toward the Moon’s surface. These oxygen ions are carried by the magnetic field lines over the 239,000 miles.
This intermittent flow of terrestrial oxygen provides the oxidizing agent needed to react with the iron in the lunar rocks. The discovery is supported by the observation that more hematite is concentrated on the Moon’s Earth-facing side than on the far side. The Earth’s magnetic field acts as a conduit, delivering the necessary ingredient for rust to form in an otherwise oxygen-deprived environment.
The oxygen transfer mechanism explains the supply of the oxidant. Studies using data from other missions confirmed that oxygen ions from Earth are implanted into the lunar soil during these periods. This continuous process over billions of years has allowed for the subtle, yet detectable, accumulation of the rust mineral.
The Role of Solar Wind and Trace Water
The formation of hematite on the Moon requires not only oxygen but also a brief window that allows the reaction to proceed without interference. This window is created by the very structure that delivers the oxygen: the Earth’s magnetotail. The solar wind, a constant stream of charged particles from the Sun, is rich in hydrogen, which is a powerful reducing agent.
Hydrogen actively works against oxidation by adding electrons, effectively reversing the rusting process. When the Moon is outside the magnetotail, the solar wind bombards the surface, continuously preventing the iron from oxidizing. The magnetotail blocks this hydrogen-rich solar wind for several days each month, creating a temporary shielding event.
During this shielding period, the Earth-supplied oxygen can interact with the lunar surface without the interference of the reducing hydrogen. The final piece of the puzzle is the water, which is a traditional requirement for rust but is scarce on the Moon. Trace amounts of water or hydroxyl groups are known to exist on the Moon.
This trace water is found either as ice in permanently shadowed craters at the poles or as molecules embedded in the lunar soil by solar wind or micrometeorite impacts. This water is thought to act as a chemical catalyst, facilitating the reaction between the lunar iron and the newly introduced oxygen. The complex interplay between Earth’s magnetic field, the solar wind, and trace amounts of water provides the unique set of circumstances necessary for the Moon to slowly rust.