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

Large low-velocity provinces (LLVPs) in Earth’s deep mantle may be remnants of Theyan mantle material. Credit: Hongping Ding and Hangzhou Field Studio

A study by the California Institute of Technology suggests that two massive, iron-rich structures deep in Earth’s mantle are the remains of Theia, an ancient planet that collided with Earth, also creating the Moon. This discovery answers long-standing questions about the origin of the Moon and the fate of Theia.

In the 1980s, geophysicists made a startling discovery: two continent-sized blobs of unusual materials had been found deep in the Earth’s center, one beneath the African continent and one beneath the Pacific Ocean. Each dot is about twice the size of the Moon and is likely composed of different proportions of elements compared to the surrounding mantle.

Large, low-speed county assets

Where did these strange blobs – officially known as Large Low-Velocity Provinces (LLVPs) – come from? A new study by researchers from the California Institute of Technology suggests they are the remains of an ancient planet that violently collided with Earth billions of years ago in the same giant impact that gave rise to our Moon.

The study was published in the journal nature On November 1, it also proposes an answer to another mystery in planetary science. Researchers have long assumed that the Moon was created following a giant collision between Earth and a smaller planet called Theia, but no trace of Theia has been found in the asteroid belt or in meteorites. This new study suggests that most of Theia was absorbed into the young Earth, forming LLVPs, while debris remaining from the collision was absorbed into the Moon.

Visualization of the Earth with large “blobs” of dense matter near the Earth’s core. These points were discovered in the 1980s. Now, researchers suggest that it is actually the remains of an ancient planet, Theia, which collided with Earth to form the Moon. Credit: Edward Garnero

Research methodology and results

The research was directed by Qian Yuan, a Oakey Earle postdoctoral research associate in the laboratories of Paul Asimo (MS ’93, PhD ’97), the Eleanor and John R. MacMillan Professor of Geology and Geochemistry; and Michael Jorness, the John E. and Hazel S. Smits Professor of Geophysics and the Clarence R. Allen Leadership Chair, director of the Caltech Seismic Laboratory, and director of the Schmidt Academy of 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 emerged of large-scale three-dimensional variations deep within the Earth’s structure. In the deeper mantle, the pattern of seismic waves is dominated by the signatures of two large structures near the Earth’s core that researchers believe have an unusually high level of iron. This high iron content means that the areas are denser than their surroundings, causing seismic waves passing through them to slow down, leading to the name “large low-velocity provinces.”

Yuan, a geophysicist by training, was attending a symposium on planet formation given by Mikhail Zolotov, a professor at Arizona State University, in 2019. Zolotov presented the giant impact hypothesis, while Qian pointed out that the moon is relatively rich in iron. Zolotov added that no trace of the vehicle had been found, which had certainly collided with the ground.

“Right after Michael said that no one knows where the impact object is now, I had a ‘eureka moment’ and realized that the iron-rich impact object could have been transformed into blobs of the mantle,” Yuan says.

Detailed simulation of Theia’s impact with Earth. Although the impact was violent, it was not powerful enough to melt Earth’s lower mantle, meaning that remnants of Theia could have been preserved, rather than homogeneously mixed with Earth’s material. Credit: Hong Bing Ding

Yuan worked with interdisciplinary collaborators to model different scenarios for the chemical composition of Theia and its impact on Earth. Simulations confirmed that the physics of the collision could lead to the formation of both LLVPs and the Moon. It is possible that some of Theia’s mantle was incorporated into Earth’s mantle, where it eventually pooled and crystallized together to form the two distinct blobs that can be detected today at the mantle-core boundary today; Other debris from the collision mixed together to form the Moon.

Implications and future research

Given this violent impact, why did Theia’s material collect in the two distinct spots instead of mixing with the rest of the forming planet? The researchers’ simulations showed that much of the energy from Theia’s impact remained in the upper half of the mantle, leaving Earth’s lower mantle cooler than previous low-resolution impact models had estimated. Because the lower mantle was not completely melted by the impact, blobs of iron-rich material from Theia remained largely intact as it sifted to the base of the mantle, like colorful clumps of paraffin wax in an extinguished lava lamp. If the lower mantle had been hotter (that is, received more energy from the collision), it would have mixed better with iron-rich materials, like colors in a pot of paint.

The next steps are to study how the early presence of Theia heterogeneous material deep within the Earth affected our planet’s internal processes, such as plate tectonics.

“The logical consequence of the idea that LLVPs are remnants of Theia is that they are very ancient,” Asimov says. “So it makes sense to then study what consequences they had on the Earth’s early evolution, such as the onset of subduction before conditions became suitable for modern plate tectonics, the formation of the first continents, and the origin of the oldest tectonic plates. Survival of Earth’s minerals.”

New research answers two long-standing mysteries of planetary science: What are the giant, mysterious “blobs” of material near Earth’s core, and what happened to the planet that collided with the Earth to create the Moon? A new study from the California Institute of Technology suggests that remnants of this ancient planet still exist within Earth, explaining the origins of the “blobs” near the core-mantle boundary.

Reference: “Moon-forming collision as a source of Earth’s basal mantle anomaly” by Qian Yuan, Mingming Li, and Stephen J. Desch, Byung-Kwan Koo, Hongpeng Deng, and Edward J. Garnero, Travis S.J. Gabriel, and Jacob A. , Vincent Ecke and Paul D. Asimov, October 32, 2023, nature.
doi: 10.1038/s41586-023-06589-1

Qian Yuan is the first author. In addition to Yuan and Asimo, an additional co-author at Caltech is Yoshinori Miyazaki, a Stanback postdoctoral researcher involved in comparative planetary evolution. Additional co-authors are Mingming Li, Stephen Desch, and Edward Garnero (PhD ’94) of Arizona State University (ASU); Byungkwan Ko of Arizona State University and Michigan State University; Hongping Ding of the Chinese Academy of Sciences; Travis Gabriel of the USGS; Jacob Kegeris NASAAmes Research Center; and Vincent Ecke of Durham University. Funding was provided by the National Science Foundation, the Aoki Earle Postdoctoral Fellowship at Caltech, the USGS, NASA, and the Caltech Center for Comparative Planetary Evolution.