The Moon is rusting – and it’s Earth’s fault.

Scientists have found that oxygen particles blown from Earth to the Moon can turn lunar minerals into hematite, also known as rust¹. The discovery adds to researchers’ growing understanding of the deep interconnection between Earth and the Moon — and shows how the Moon keeps a geological record of those interactions, says Ziliang Jin, a planetary scientist at Macau University of Science and Technology in China. He and his colleagues reported their findings earlier this month in Geophysical Research Letters.

Most of the time, both Earth and the Moon are bathed in a stream of charged particles emanating from the Sun. But for around five days each month, Earth passes between the Sun and the Moon, blocking most of the flood of solar particles. During that time, the Moon is exposed mainly to particles that had been part of Earth’s atmosphere before blowing into space — a phenomenon known as Earth wind.

That wind contains ions of various elements, including hydrogen, oxygen and nitrogen. When those charged particles hit the Moon, they can implant themselves into the upper layers of lunar soil² and trigger chemical reactions.

In 2020, scientists reported that India’s Chandrayaan-1 mission had spotted hematite near the Moon’s poles³. Hematite is an iron-rich mineral that can form when rocks react with water and oxygen. But the Moon’s chemical environment isn’t conducive to the presence of oxygen, meaning that the oxygen for the hematite might have arrived from somewhere else. The 2020 paper’s authors proposed that it might have arrived in the Earth wind.

Experimental support

Jin and his colleagues decided to test that idea in the laboratory. They simulated the Earth wind by accelerating hydrogen and oxygen ions to high energies. They then sent the ions whizzing into single crystals of iron-rich minerals that are known to exist on the Moon.