Earth is the only planet in the solar system with a large amount of water and a relatively large moon that stabilizes its axis. Both facts were essential for Earth to develop life. Planetologists from the University of Münster (Germany) were able to show for the first time that water arrived on Earth with the formation of the moon 4.4 billion years ago.

The moon was formed when the Earth was hit by a Mars-sized body, also called Theia. Until now, scientists assumed that Theia originated in the inner solar system near Earth. However, researchers from Münster can show that Theia comes from the outer solar system and delivered large amounts of water to Earth. The results were published in the current issue of NatureAstronomy.

From the outside to the inside of the solar system

The Earth formed in the inner "dry" solar system, and so it is somewhat surprising that there is water on Earth. To understand why this is the case, we have to go back in time when the solar system was formed, about 4.5 billion years ago. From previous studies, we know that the solar system was structured in such a way that the "dry" materials were separated from the "wet" materials: theso-called "carbonaceous" meteorites, which are relatively rich in water, come from the outer solar system, while "non-carbonaceous" meteorites come from the inner solar system.

While previous studies have shown that carbonaceous materials were likely responsible for providing water to Earth, it was not known when and how that carbonaceous material - water - arrived on Earth. "We use molybdenum isotopes to answer that question."

Molybdenum isotopes allow us to clearly distinguish carbonaceous and non-carbonaceous material and, as such, represent a "genetic imprint" of the outer and inner solar system material," explains Dr. Gerrit Budde from the Institute of Planetology in Münster and lead author of the study.

Measurements by the Münster researchers show that the isotopic composition of Earth's molybdenum is among those of carbonaceous and non-carbonaceous meteorites, demonstrating that some of Earth's molybdenum originated in the outer solar system. In this context, the chemical properties of molybdenum play a key role because, as it is an iron-loving element, most of themolybdenum of the Earth is located in the core. "The molybdenum that is accessible today in the Earth's mantle therefore originated in the last stages of the Earth's formation, while the molybdenum of earlier phases is entirely in the core," explains Dr. Christoph Burkhardt, second author of the study. The scientists' results therefore show for the first time that this carbonaceous material of theouter solar system arrived late on Earth.

But scientists are going one step further

They show that most of the molybdenum in Earth's mantle was supplied by the proto-planet Theia, whose collision with Earth 4.4 billion years ago led to the formation of the Moon. However, since much of the molybdenum in Earth's mantle originated from the outer solar system, that means Theia itself also originated in the outer solar system. According to the scientists, the collisionprovided enough carbonaceous material to account for the total amount of water on Earth.

"Our approach is unique because, for the first time, it allows us to link the origin of water on Earth with the formation of the moon. To put it simply, without the moon there would probably be no life on Earth," says Thorsten Kleine, professor of planetology at the University of Münster.[Phys]

Gerrit Budde et al, Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth, Nature Astronomy (2019).DOI: 10.1038/s41550-019-0779-y