Mysterious Lumps in Earth’s Mantle May Be Remains of the Crash That Formed the Moon

Around 4.5 billion years ago, a nascent Earth collided with a protoplanet called Theia, an object roughly the size of Mars. Debris from the crash flew into orbit around the Earth and accrued to form the moon, according to the most widely accepted theory.

Now, researchers say remnants of this impact are lodged deep within our planet, according to a new paper published Wednesday in the journal Nature. Two continent-sized lumps of material in Earth’s lower mantle have baffled scientists for decades, but the new computer modeling suggests they could be left over from Theia’s mantle.

The theory that these lumps originated with the moon-forming collision is not new, Robin Canup, a planetary scientist at the Southwest Research Institute who did not contribute to the findings, says to Nature News’ Anil Oza. “But this paper is the first in my mind to really take that notion seriously,” she tells the publication.

Scientists first discovered the giant blobs of material in the 1980s—one beneath Africa and one beneath the Pacific Ocean, according to a statement from Arizona State University (ASU). By measuring seismic waves traveling through Earth, researchers can glean insights into what lies below.

“In a similar way to ultrasound or MRI machines, we study recordings of the waves after they have traveled through structures or reflected off of them to identify anomalies,” Edward Garnero, a co-author of the new study and a geophysicist at ASU, tells Inverse’s Doris Elín Urrutia. “In this way, ‘pictures’ of the interior are computed, just like X-ray images.”

When seismic waves pass through these two dense structures, they move more slowly, indicating these spots have different compositions from the surrounding mantle. Previously, researchers had theorized they are remnants of sunken tectonic plates, according to Science News’ Sid Perkins.

In the new study, the researchers tested whether these lumps could be from Theia. Evidence from the Apollo missions supports the collision theory of the moon’s formation: Rock samples suggest the moon was molten at first and was covered in a magma ocean for at least tens of millions of years, a sign that it formed from a high-energy impact, per NASA.

“This moon-forming giant impact is maybe one of the most important factors for why Earth is so different from any other rocky planet we’ve found,” Qian Yuan, a co-author of the study and geophysicist at the California Institute of Technology, tells New Scientist’s Leah Crane. “This impact changed the atmosphere, changed the crust, changed the mantle, changed the core—so it was really probably the most important event in Earth’s history.”

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The researchers used computer models to simulate this momentous collision. The simulations showed that the high-energy crash would have melted the upper part of Earth’s mantle, allowing Theia’s dense remnants to get pulled through the molten top layer and settle in the more solid bottom, per Nature News. Over billions of years, convection in Earth’s mantle would have gathered some of Theia’s pieces into the two mysterious lumps on the surface of the core. Other material from Theia’s mantle would have mixed into Earth’s, per the paper.

Still, some researchers aren’t convinced that any Theian chunks could have remained distinct from Earth’s mantle.

“In our simulations, the mantle of Theia and Earth’s mantle tend to be well-mixed,” Miki Nakajima, a planetary scientist at the University of Rochester who was not involved in the research but has studied the evolution of layers in rocky planets, tells National Geographic’s Liz Kruesi.

The new model “definitely needs to be tested,” Maxim Ballmer, a geodynamicist at University College London who did not contribute to the findings, tells Nature News. “But I do think that it’s an idea worth pursuing.”

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Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.