Strange Spots on the Earth's Mantle Could Be Traces of the Impact of Planet Theia 4.5 Billion Years Ago

Recent work sheds light on the probable impact of a body called Theia on the Earth. The event would not only have left its mark on the interior of our planet but would also have created the Moon.

Theia Tierra
New research now suggests that Theia has not completely disappeared: huge chunks of it may still exist inside our planet. Credit: Pavel Gabzdyl /

For decades, scientists have been puzzled by two large, mysterious spots in Earth's mantle. These rock formations measure thousands of kilometers and are slightly denser than their surroundings, suggesting that they are made of a different material than the rest of the mantle.

In a new study published Nov. 1 in Nature, a computer simulation model suggests that these strange spots have a dramatic origin: They are the result of a gigantic collision 4.5 billion years ago between the early Earth and another young planet, the same collision that is believed to have formed the Moon.

The model estimates that this violent encounter caused material from the impacting planet, called Theia, to become embedded in the lower half of Earth's mantle. The collision also caused some of Theia's remains to be launched into orbit, which eventually merged to form the Moon.

The idea that the mantle anomalies are remnants of Theia is not new, says Robin Canup, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. "But this article is, in my opinion, the first that takes this idea seriously."

Some of Theia's material sank into Earth's mantle

A giant collision between the young Earth and a smaller protoplanet has long been the predominant theory for the formation of the Moon. This origin would explain characteristics such as the absence on the Moon of many volatile compounds, which would have vaporized during the collision with the Earth.

Such a huge impact in the early stages of Earth's formation should have left some traces. Yuan and his colleagues wondered at the work whether those traces might include the strange regions of the Earth's mantle, the layer between the crust and the core. Scientists call these formations “large low-velocity provinces” because seismic waves travel more slowly through them than through the rest of the mantle.

Theia Tierra
The protoplanet Theia, which was about the size of Mars, collided with proto-Earth 4.5 billion years ago. Artistic image - Credit: Hernán Cañellas.

The researchers performed computer simulations of the interaction between Theia's mantle and Earth's mantle from the time of the collision to the present. This showed that some Theia material initially sank to the bottom of Earth's mantle and that more Theia material accumulated there over time, forming the blobs.

The authors have been working on the topic for some time, and in this latest work, they have expanded their models. They discovered that the energy from the planetary collision would have partially melted the Earth's mantle, which would then have had two layers: a molten upper layer and a mostly solid lower layer.

The molten top layer would have swirled some of Theia's material into Earth's material. But other material from Theia would have sunk through the molten part of the mantle and lodged in the layer below. Over time two separate spots would have formed. Meanwhile, other material from Theia was launched into orbit and formed the Moon .

The hypothesis can be taken seriously and supports further research

The model is not irrefutable proof that the mantle anomalies are remnants of Theia, but Yuan and his colleagues "have shown that (the hypothesis) can be taken seriously," Canup says. "It's not just a throwaway idea, which is pretty much what I thought it was before this job." The next step will be to validate the models by comparing rock samples from the mantle with some from the Moon.

Maxim Ballmer, a geodynamist at University College London, isn't sure this idea of mantle anomalies holds up. The model "definitely needs to be tested," says Ballmer. "But I think it's a worthwhile idea. "

Yuan hopes this and subsequent research can shed light on what has made Earth so unique, factors that could include the collision with Theia. This study, says Ballmer, suggests that "this giant impact (created) some heterogeneities on Earth that may last billions of years. "

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