Strange bubbles in the deep mantle are the remains of an ancient planet

Large low-velocity provinces (LLVPs) in the deep Earth’s mantle may be relics of thean mantle material. Credit: Hongping Deng and Hangzhou Sphere Studio

A Caltech-led study suggests two massive, iron-rich structures deep in the Earth’s mantle are the remains of the ancient planet Thea, which collided with Earth and formed the Moon. The discovery answers long-standing questions about the moon’s origin and the fate of Thea.

In the 1980s, geophysicists made a startling discovery: two continent-sized blobs of unusual material were found deep in the Earth’s core, one beneath the African continent and one beneath the Pacific Ocean. Each bubble is twice the size of the Moon and made of different proportions of elements than the surrounding crust.

The appearance of large low-velocity provinces

Where do these strange bubbles—formally known as large low-velocity provinces (LLVPs)—come from? A new study led by Caltech researchers suggests they are the remains of an ancient planet that violently collided with Earth billions of years ago in the same giant impact that formed our Moon.

The thesis, published in the journal Nature On November 1, another planetary science also proposes an answer to the mystery. Researchers have long assumed that the Moon formed as a result of a giant impact between Earth and a smaller planet called Theia, but no trace of Theia has been found in the asteroid belt or in meteorites. The new study suggests that most of the Thea was absorbed into the young Earth, forming LLVPs, while the rest of the impact debris coalesced with the Moon.


Visualization of Earth with large dense “bubbles” near the Earth’s core. These bubbles were discovered in the 1980s. Now, researchers propose that they are actually the remains of an ancient planet, Thea, that collided with Earth to form the Moon. Credit: Edward Carnero

Research Methodology and Findings

Paul Asimov (MS ’93, PhD ’97), Eleanor and John R. The research was led by OK Earl Postdoctoral Scholar Research Associate Qian Yuan in the laboratories of both McMillan professors of geology and geochemistry. and Michael Gurnis, John E. and Hazel S. Smits Professor of Geophysics and the Clarence R. Allen is the Leadership Chair, Director of Caltech’s Seismic Laboratory, and Director of the Schmidt Academy for Software Engineering at Caltech.

Scientists first discovered LLVPs by measuring seismic waves traveling through the Earth. Seismic waves travel at different speeds through different materials, and in the 1980s, the first hints of large-scale three-dimensional variations deep within the Earth’s structure emerged. Deeper in the mantle, the seismic waveform is dominated by the signatures of two large structures near the Earth’s core that researchers believe contain unusually large amounts of iron. This high iron content means that because the regions are denser than their surroundings, seismic waves traveling through them are slower, leading to the name “large low-velocity provinces”.

A geophysicist by training, Yuan attended a 2019 seminar on planet formation given by Michael Zolotov, a professor at Arizona State University. Zolotov proposed the giant impact hypothesis, while Qian noted that the moon is rich in iron. Zolotov added that no trace of an impact that should have hit Earth was found.

“I had a ‘eureka moment’ right after Michael told me that no one knew where the impactor was now, and realized that the iron-rich impact might have turned into mantle bubbles,” says Yuan.


A detailed simulation of Thea hitting Earth. Although the collision was violent, it was not energetic enough to melt Earth’s lower mantle—meaning that the remnants of Thea could be preserved from mixing homogeneously with Earth’s material. Credit: Hongping Deng

Yuan worked with a variety of collaborators to design different scenarios for the chemical composition of Thea and its impact with Earth. The simulations confirmed that the physics of the collision could have led to the formation of both the LLVP and the Moon. Some of Thea’s mantle may have been attached to Earth’s own, where it eventually crystallized together, forming two distinct blobs detectable today at Earth’s core-mantle boundary; Other debris from the collision mixed together to form the moon.

Implications and future research

Because of such a violent impact, why did Thea’s material stick to two separate blobs instead of merging with the rest of the planet? The researchers’ simulations showed that most of the energy delivered by Thea’s impact was in the upper half of the crust, a cooler part of Earth’s lower mantle than estimated by previous, lower-resolution impact models. Because the lower mantle was not completely melted by the impact, bubbles of iron-rich material from Theia remained largely intact as they sifted down to the base of the mantle, like the colorful masses of paraffin wax in an extinguished lava lamp. If the lower mantle had been hotter (that is, it had received more energy from the impact), it would have mixed more thoroughly with iron-rich materials, like the colors in mixing paints.

The next steps will be to investigate how the initial state of the heterogeneous material of Thea deep within the Earth may have affected our planet’s internal processes.

“The logical consequence of the idea that LLVPs are remnants of Thea is that they are very old,” Asimov says. “Therefore, it makes sense to next investigate what consequences they had for early Earth evolution, such as subduction that began before the conditions were suitable for modern-style plate tectonics, the formation of the first continents, and the origin of primitive terrestrial minerals.”


New research answers two long-standing mysteries in planetary science: What are the giant mysterious “bubbles” near Earth’s core, and what happened to the planet that collided with Earth to form the Moon? A new study from Caltech suggests that remnants of that ancient planet are still inside Earth, explaining the appearance of “bubbles” near the core-mantle boundary.

Reference: Qian Yuan, Mingming Li, Steven J. Desch, Pyeongkwan Ko, Hongping Deng, Edward J. Carnero, Travis SJ. Gabriel, Jacob A. “Moon-forming impact as a source of Earth’s mantle anomalies” by Gekerris, Yoshinori Miyazaki, Vincent AK and Paul D. Asimov, 32 Oct. 2023, Nature.
DOI: 10.1038/s41586-023-06589-1

Qian Yuan is the first author. In addition to Yuan and Asimo, additional Caltech co-author Yoshinori Miyazaki is a Stanback Postdoctoral Scholar Research Associate in Comparative Planetary Evolution. Additional co-authors are Mingming Li, Steven Desch, and Edward Carnero (PhD ’94) of Arizona State University (ASU); Byeongkwan Ko of ASU and Michigan State University; Hongping Deng of the Chinese Academy of Sciences; Travis Gabriel of the US Geological Survey; of Jacob Kegerreis NASAAIIMS Research Center; and Vincent Ake of Durham University. Funding was provided by the National Science Foundation, the OK Earl Postdoctoral Fellowship at Caltech, the US Geological Survey, NASA, and the Caltech Center for Comparative Planetary Evolution.

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