Where Rust Shouldn't Be

Rust is common on Earth. It shouldn’t exist on the moon. But somehow, it does. Rust is just the common name for iron oxide. Iron that has bonded to oxygen. The moon, however, has no oxygen. Yet lunar rovers have recovered grains of iron oxide in lunar soil. How could they have gotten there? One possibility is from Earth. Or rather, from our atmosphere. The magnetic field of our planet, called its magnetosphere, surrounds Earth like an invisible force field. The sun produces a solar wind of charged particles that flows outward into the solar system. When it encounters our magnetosphere, it deflects around Earth. The charged particles flow around us and deep into space, in a so-called magnetotail that stretches three million miles behind Earth. As the charged particles pass, they strip off some oxygen ions from the upper reaches of our atmosphere, and carry them into the long magnetotail. Each month during the full moon, Earth is between the moon and the Sun, which may shelter the moon from solar wind, and bathe it in oxygen from Earth’s magnetotail. Scientists believe that when these oxygen ions strike the moon’s surface, they bond with iron in the lunar soil to form, yes, tiny particles of rust. This is still a hypothesis. But it’s important because it highlights how little we know about our closest celestial neighbor… where humans might soon spend a bit more time.