When Neil Armstrong took his famous first steps on the Moon, we believed it was an arid body with not even a hint of water. As we got better at exploring the final frontier and our technology improved, we found the Moon isn’t quite the arid satellite we thought it was.

Rock samples collected during the Apollo missions hint the interior of the Moon contains anywhere between 10 to 300 parts per million water. It’s barely any, but it does raise interesting questions. Namely, where did the water come from? That’s what a new study looked at, and it points the finger at asteroids.

How can scientists even figure this out? They compare the chemical and isotopic composition of the lunar samples with samples from comets and asteroids. Specifically, the ratio of hydrogen to deuterium (a heavier form of hydrogen). Comets and asteroids have particular ratios, and by figuring out the ratio from the moon samples – scientists can get a better handle on where it originated from.

After careful analysis, the scientists found the moon’s water most likely came from asteroids. More than 80% of the water in the moon’s interior originated from asteroids. The rest came from comets.

“It is perhaps surprising that asteroids were responsible for the water on the Moon, since they are mostly made up of rock and metal,” said Professor Sara Russell, Head of Mineral and Planetary Sciences at the Natural History Museum in London and co-author of the study. “Small amounts of water can be contained within the minerals, but they contain much less water than comets, which are mostly ice and dust.”

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“But the chemical compounds of the comets we studied did not match the chemicals in the lunar samples,” Russell added.

sara russell holding meteorite

Prof Russell holding a meteorite. Credit: NHM

As for when the moon received its water? The study points to a period about 4.5 billion to 4.3 billion years ago. Right around the time the Moon formed from an impact between a Mars-sized body called Theia and Earth. The team of scientists says the water was delivered while the moon was surrounded by a magma ocean. Before the moon’s surface cooled and formed the crust we see at night.

This recent study could also help explain where some of Earth’s water came from. Because the newly formed moon and Earth were closer together, what was happening on one was likely happening to the other.

“We are now able to say that water in the Earth and Moon shares a common origin, but there is still much left to do,” said lead author Dr. Jessica Barnes

There’s still much we don’t know about our close neighbor. While we probe the far reaches of our solar system, we still haven’t even come close to exploring all of the moon.

“There exists every possibility that rocks from the unexplored regions may provide further clues to the origin of lunar water, its characteristics and distribution,” says Barnes.

“Sending future missions to prospect for lunar volatiles could also help us determine whether water-ice on the Moon’s surface could be a useful resource for other space exploration activities.”

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