The moon has rust, thanks to the Earth

When planetary geologist Shuai Li realized there were minerals containing iron oxide on the moon’s surface, his first thought was, How did they get there? “Finding hematite is exciting,” the University of Hawaii at Manoa researcher says, but he and his colleagues puzzled over its origin for a year. Iron oxide—better known as rust—is common on Earth, but solar wind’s fire hose of protons reduces most species on the moon’s surface, and oxygen is typically in short supply there. That all changes when Earth passes between the moon and the sun. Then, the moon moves into the tail of Earth’s protective magnetic field, and oxygen from Earth’s atmosphere can reach the moon’s surface (Sci. Adv. 2020, DOI: 10.1126/sciadv.aba1940). Li’s key insight was noticing from spectroscopy data collected by the Chandrayaan-1 orbiter that hematite was concentrated at the moon’s poles on the side that faces Earth. Terrestrial oxygen doesn’t reach the moon’s far side, and the solar wind can reduce any hematite that forms at lower latitudes, where it hits the moon’s surface more directly. But Li says that while he and his collaborators are confident in their model, they won’t be able to prove it unless a future moon mission brings back hematite samples.