A meteorite is a natural object from space that survives atmospheric entry to strike the ground. These space rocks can and frequently do rust. This deterioration is a form of chemical breakdown known as oxidation, the same process that causes iron and steel to rust on Earth. This reaction highlights the stark difference between the stable, low-oxygen environment of space and the reactive conditions found on our planet.
The Metallic Components That Cause Rusting
The susceptibility of a meteorite to rust is tied to its metallic composition, particularly high iron and nickel content. More than 95% of all meteorites contain an iron-nickel (FeNi) metal alloy that is highly reactive in a terrestrial environment. In iron meteorites, the metal makes up the bulk of the specimen, while in stony meteorites, it appears as small, scattered flecks.
The metal exists as two distinct alloys: kamacite (lower nickel) and taenite (higher nickel, typically 5% to 30%). This contrasts sharply with most industrial iron, which usually has less than 1% nickel. When these alloys react with oxygen and water, the iron converts into iron oxide, a reddish-brown substance called rust. Scientists refer to this corrosion product as “terrestrial alteration” because it results from exposure to Earth’s atmosphere.
Why Meteorites Rust Only on Earth
Rusting, or oxidation, is an electrochemical process that requires three primary components: a susceptible metal, free oxygen, and liquid water. The outer solar system environment, where meteorites spend most of their existence, is a near-perfect vacuum with very low concentrations of free oxygen and no liquid water. Consequently, the metal alloys remain pristine for billions of years in space.
Earth’s atmosphere provides the necessary ingredients for corrosion to begin. Our atmosphere is about 21% oxygen, which is readily available to react with the metallic iron. Furthermore, humidity and moisture in the air or soil supply the liquid water needed to complete the electrochemical circuit.
The corrosion process is accelerated by chloride salts within the meteorite structure. These salts, introduced either in space or from terrestrial soil, are highly hygroscopic, meaning they absorb and hold moisture from the surrounding air. This trapped water creates a localized, corrosive environment deep within the meteorite’s cracks and pores. The resulting iron and nickel chlorides continuously draw in more water, making the corrosion a self-sustaining and progressive reaction.
Preservation and Storage of Corroding Specimens
The progressive nature of rusting is a serious concern for curators and scientists, as it destroys the integrity and scientific value of the specimen. Corrosion alters the meteorite’s original chemical and isotopic signatures, which hold valuable information about the early solar system. In advanced stages, deterioration can cause the meteorite to crumble entirely.
To mitigate this damage, institutions employ rigorous preservation methods focused on eliminating moisture. The most common approach is housing specimens in controlled, low-humidity environments, often using desiccants like silica gel. For unstable or freshly cut specimens, chemical treatments are also used to stabilize the metal.
These treatments involve soaking the meteorite in anhydrous alcohol to draw out trapped moisture or applying specialized cleaners to remove visible rust spots. For long-term protection, some iron specimens are coated with a thin layer of protective material, such as specialized oil, Automatic Transmission Fluid (ATF), or microcrystalline wax. Other advanced methods include Volatile Corrosion Inhibitors (VCI), which release a vapor that forms a protective, anti-corrosive layer on the metal surface.