The Collision that Created the Moon Might Have Also Brought Water to the Early Earth

Scientists at the University of Munster have discovered that Earth got its water from a collision with Theia. Theia was the ancient body that collided with Earth and formed the Moon. Their discovery shows that Earth’s water is much more ancient than previously thought.

The standing theory for the formation of the Moon involves an ancient body called Theia. About 4.4 billion years ago, Theia collided with Earth. The collision created a massive debris ring, and the Moon formed from that debris.

Standing theory also says that Earth gathered its water over time, after the collision with Theia, with comets and asteroids delivering the water. But the new study from the University of Munster presents evidence that supports a different source for Earth’s water: Theia itself.

“Our approach is unique because, for the first time, it allows us to associate the origin of water on Earth with the formation of the Moon.”

Thorsten Kleine, Professor of Planetology at the University of Münster.

Scientists have long thought that Theia was a body from the inner solar system, since it was rocky in nature. But the new study says that’s not the case. Instead, Theia had its origins in the outer Solar System.

Key to understanding these events is the idea of the wet and dry parts of our Solar System. The Solar System was formed about 4.5 billion years ago, and we know that the way it was structured led to a dry inner region and a wet outer region. Earth is a little bit of a mystery, because it formed in the dry region, closer to the Sun, yet it has an abundance of water. So studies like this one, which try to understand how Earth got its water, are important.

Much or our understanding of Earth’s water come from two types of meteorites: carbonaceous meteorites, which are rich in water, and non-carbonaceous meteorites, which are drier. And carbonaceous meteorites come from the outer Solar System, while the drier non-carbonaceous meteorites come from the inner Solar System. Got all that?

There’s lots of evidence that Earth’s water was delivered by the wet carbonaceous meteorites from the outer Solar System, but when and how that happened has never been certain. This study brings some certainty to the issue.

“We have used molybdenum isotopes to answer this question.”

Dr. Gerrit Budde, lead author, Institute of Planetology in Munster.

The study is called “Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth,” and it’s published in the journal Nature Astronomy. As the title makes clear, it’s all about isotopes of molybdenum, and the difference between the molybdenum in the Earth’s core, and the molybdenum in Earth’s mantle.

“We have used molybdenum isotopes to answer this question. The molybdenum isotopes allow us to clearly distinguish carbonaceous and non-carbonaceous material, and as such represent a ‘genetic fingerprint’ of material from the outer and inner solar system,” explains Dr. Gerrit Budde of the Institute of Planetology in Münster and lead author of the study.

Why molybdenum? Because it has a very helpful property when it comes to answering the question of the origin of Earth’s water. Molybdenum is very iron-friendly, meaning most of it exists in the Earth’s core, which is largely iron.

The core is ancient, because the Earth was a molten ball in its early days and heavier elements like iron migrated to form the core. Since molybdenum loves iron, molybdenum went to the core too. But there’s also molybdenum in the Earth’s crust, which must have been delivered to Earth after it cooled, or else it would have migrated to the core too. So the Earth has two populations of molybdenum, and they’re each different isotopes.

And that late-to-the-party molybdenum in the Earth’s mantle must have come from bodies that crashed into Earth later on in its formation. “The molybdenum which is accessible today in the Earth’s mantle, therefore, originates from the late stages of Earth’s formation, while the molybdenum from earlier phases is entirely in the core,” explains Dr. Christoph Burkhardt, second author of the study.

What’s these results make clear, for the first time, is that carbonaceous material from the outer, wet area of the Solar System arrived on Earth late.

But the paper goes further than that. Since the molybdenum in the mantle had to have come from the outer Solar System, due to it being a different isotope, that means that Theia also had to come from the outer Solar System. The scientists behind this research show that the collision with Theia provided enough carbonaceous material to account for the majority of Earth’s water.

“Our approach is unique because, for the first time, it allows us to associate the origin of water on Earth with the formation of the Moon. To put it simply, without the Moon there probably would be no life on Earth,” says Thorsten Kleine, Professor of Planetology at the University of Münster.