Key Takeaways:

  • Earth’s water likely arrived with the Moon-forming collision 4.4 billion years ago.
  • Theia, the Mars-sized object that hit Earth, wasn’t local, but came from the outer solar system (“exoplanet”).
  • Molybdenum isotopes reveal Earth’s water came from outer solar system’s “carbonaceous” material, delivered by Theia.
  • The collision with Theia likely provided all the water we have on Earth.
  • Both a large Moon and water were crucial for life’s development on our planet.

The Earth is special in our solar system because it is the only terrestrial planet with a sizable moon that balances the Earth’s axis and a lot of water on it. For life to develop on Earth, both were necessary. For the first time, planetary scientists at the University of Münster in Germany have demonstrated that water arrived on Earth along with the Moon’s formation some 4.4 billion years ago. The Moon was created when Theia, a body roughly the size of Mars, collided with Earth. Scientists had previously believed that Theia originated close to Earth in the inner solar system.

Nonetheless, Münster researchers have now demonstrated that Theia is an exoplanet that brought a great deal of water to Earth. The results are published in the current issue of Nature Astronomy.

From the outer into the inner solar system

Being in the ‘dry’ inner solar system during its formation, the presence of water on Earth is somewhat surprising. We must travel back in time to the solar system’s formation, some 4.5 billion years ago, to understand why this is the case.

Previous research has indicated that the solar system evolved to separate ‘wet’ and ‘dry’ materials. The outer solar system is home to the drier ‘non-carbonaceous’ meteorites, while the inner solar system is home to the relatively water-rich ‘carbonaceous’ meteorites. It was unknown when and how this carbonaceous material—and consequently the water—came to Earth, despite earlier research indicating that carbonaceous materials were probably responsible for bringing the water to Earth.

“We have addressed this question with molybdenum isotopes. 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.

The Münster researchers’ measurements indicate that some of Earth’s molybdenum originated in the outer solar system, as the isotopic composition of Earth’s molybdenum lies between that of carbonaceous and non-carbonaceous meteorites. Since molybdenum is an element that loves iron, its chemical properties are important in this context because the majority of molybdenum on Earth is found in the planet’s core.

According to Dr. Christoph Burkhardt, the study’s second author, “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.” The scientists’ results therefore show, for the first time, that carbonaceous material from the outer solar system arrived on Earth late.

But the scientists are going one step further. These demonstrate that the protoplanet Theia, whose collision with Earth 4.4 billion years ago produced the Moon, provided the majority of the molybdenum in the Earth’s mantle. However, since a large part of the molybdenum in Earth’s mantle originates from the outer solar system, this means that Theia itself also originated from the outer solar system.

According to the scientists, the collision provided sufficient carbonaceous material to account for the entire amount of water on Earth. “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.

Simply put, life on Earth most likely wouldn’t exist without the Moon, according to University of Münster planetaryology professor Thorsten Kleine.

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