Researchers Find Remains of a Hypothesized Protoplanet Under the Earth’s Mantle

Back in the 1980s, geophysicists discovered two continent-sized blobs of strange material, deep inside the Earth.

The masses – one beneath the African continent and the other under the Pacific Ocean – perplexed scientists at the time, as they seemed to be made of a different mix of elements than the Earth’s mantle that surrounded them.

Now, research published in Nature points to a remarkable new explanation: the masses could be the remnants of an ancient planet that collided with the young Earth in an impact that also spawned the Moon.

The strange world beneath the Earth’s surface

These two strange blobs are known scientifically as large low-velocity provinces (LLVPs) and exist in the lowermost mantle of the Earth. Their name comes from how they were discovered; scientists measuring seismic waves travelling through the Earth detected strange regions where seismic shear waves moved at much slower velocities than predicted.

How a seismic wave travels through a material is dependent on that material’s composition. Scientists believe that these two LLVPs contain an unusually high level of iron, which would make these regions denser than their surroundings. Such high-density regions would explain the slowing of the seismic waves.

But the central question still needed to be explained: where did these LLVPs come from?

In 2019, Qian Yuan, a post-doctoral scholar in geophysics at the California Institute of Technology (Caltech), was attending a seminar on planet formation given by Mikhail Zolotov, a professor at Arizona State University.

In the seminar, Zolotov presented what is known as the “giant-impact hypothesis,” which explains the formation of the Moon is an after-effect of a giant impact between Earth and a smaller planet known as Theia.

During the seminar, Qian noted how the Moon is relatively rich in iron. When Zolotov mentioned how no trace had ever been found of the impact of Theia, something clicked for Yuan.

"Right after Mikhail had said that no one knows where the impactor is now, I had a 'eureka moment' and realized that the iron-rich impactor could have transformed into mantle blobs," said Yuan.

Could this be evidence of Theia?

Since this moment, Yuan has been working with multidisciplinary collaborators to model different scenarios for Theia’s chemical composition and how it would behave when impacting Earth.

In their new study, the researchers say they have confirmed that the physics of the collision match up in a way that could explain the formation of both the Moon and these LLVPs. Some of Theia’s mantle could have become fused and incorporated into the Earth’s mantle, where it would become the unusual material blobs seen today. Other debris from the impact would have also mixed to form the Moon.

But how could two planets smashing together result in two significant masses of material lodged inside the Earth, rather than a true mixture with the rest of the newly forming Earth? The researchers’ simulations answer this too.

Much of the energy delivered in the impact between the Earth and Theia appears to have remained in the upper half of the mantle. With the lower mantle having been melted less by the energy of the impact, this could result in the formation of the two iron-rich blobs hidden deep within the Earth.

"A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient," said Paul Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at Caltech and the senior author on the new paper.

"It makes sense, therefore, to investigate next what consequences they had for Earth's earliest evolution, such as the onset of subduction before conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of the very oldest surviving terrestrial minerals."

Reference: Yuan Q, Li M, Desch SJ, et al. Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature. 2023;623(7985):95-99. doi: 10.1038/s41586-023-06589-1

This article is a rework of a press release issued by the California Institute of Technology. Material has been edited for length and content.