Moon’s Formation Brought Water to Earth

German Planetologists showed for the first time, that water came to Earth with the formation of the Moon some 4.4 billion years ago.

A giant impact between the proto-Earth and a Mars-sized impactor named Theia is the best current theory for the formation of the Moon. Photo: NASA

The Moon was formed when Earth was hit by a body about the size of Mars, also called Theia. Until now, scientists had assumed that Theia originated in the inner solar system near the Earth. Researchers can now show that Theia comes from the outer solar system, and it delivered large quantities of water to Earth.

The Earth formed in the 'dry' inner solar system, and so it is somewhat surprising that there is water on Earth. To understand why this the case, we have to go back in time when the solar system was formed about 4.5 billion years ago.

From earlier studies, we know that the solar system became structured such that the 'dry' materials were separated from the 'wet' materials: the so-called 'carbonaceous' meteorites, which are relatively rich in water, come from the outer solar system, whereas the drier 'non-carbonaceous' meteorites come from the inner solar system.

While previous studies have shown that carbonaceous materials were likely responsible for delivering the water to Earth, it was unknown when and how this carbonaceous material—and thus the water—came to Earth.

"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.

The measurements show that the molybdenum isotopic composition of the Earth lies between those of the 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 the Earth's molybdenum is located in the core.

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.

Scientists show that most of the molybdenum in Earth's mantle was supplied by the protoplanet Theia, whose collision with Earth 4.4 billion years ago led to the formation of the Moon. 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. (Tasnim News Agency)

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