The Moon's Water Actually Came From Asteroids, Not Comets

Asteroids delivered water to the moon during its early evolution, similar to what they did on Earth.

by Kastalia Medrano
Getty Images

A study released today in Nature Communications details how we’ve been getting the early history of the moon all wrong. While we previously thought the moon’s water was delivered by comets, it would seem that we should have been crediting asteroids in providing water during the beginning of the moon’s evolution, around 4.5 billion years ago.

Scientists collaborating from all over the world analyzed samples from both comets and meteors (as well as the lunar surface) and found that more than 80 percent of water inside the moon can be traced back to asteroids; it was asteroids, not comets, that were overwhelmingly hitting the moon during its early of geological history (the first 500 million years or so).

The deuterium-enriched water comprised the other 20 percent did come from comets. This was before the moon had formed its hard outer crust; instead, it had a “magma ocean,” meaning the asteroid material — including water — could make a deeper impact into the moon’s interior. The same thing was happening on Earth around the same time, according to the study.

Because the moon didn't have a fully formed crust, but a "magma ocean," the material from the asteroids was able to burrow more deeply

Lunar and Planetary Institute/David A. Kring

The study’s authors note that early water composition appears similar not just on the Earth and the moon, but on Mars as well, “suggesting that the same types of accreting objects as those modelled here for the Moon delivered a vast majority of the water to the rocky planets in the inner Solar System.” The mantles of all three celestial bodies demonstrate similar a composition of H-isotopes, the unifying factor identified in the research.

From this, the authors conclude that from around 4.5 billion years ago to 3.9 billion years ago, the entire inner solar system was actually receiving the same composition of asteroid and comet material. So this tells us not just how water originated on the moon, but how water originated in our inner solar system as a whole, which is really very cool.

Here’s the full study:

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