Earth’s early history is marked by massive collisions with other objects, including planetesimals. One of the defining events in our planet’s history, the formation of the Moon, likely resulted from one of these catastrophic collisions when a Mars-sized protoplanet crashed into Earth. That’s the Giant Impact Hypothesis, and it explains how the collision produced a torus of debris rotating around the Earth that eventually coalesced into our only natural satellite.
New research strengthens the idea that Theia left some of its remains inside Earth.
The Giant Impact that created the Moon occurred in the Hadean eon. The Hadean is the first of Earth’s four eons and spans from the Earth’s formation about 4.5 billion years ago up to about 4.03 billion years ago when it was succeeded by the Archaean eon.
Earth was a magma ocean for about the first 50 million years. It began cooling during the Hadean, but the mantle was still much more viscous than it is today. Residual heat from its formation and the higher level of radiogenic heating kept the mantle in a more fluid state. There was more water in the mantle at that time, too, adding to the mantle’s fluidity.
That’s important because when objects slammed into Earth, they were able to sink deeper into the mantle.
Back in the 1980s, scientists made a remarkable discovery. Two gigantic, continent-sized blobs were embedded deep in the Earth. One is under Africa, and one is under the Pacific Ocean. They’re called LLSVPs, or Large Low-Shear-Velocity Provinces, and they have unusually high iron levels. The iron concentration changes the speed of seismic waves that travel through them, leading to their discovery.
Both the LLSVPs extend for thousands of kilometres horizontally and extend up to 1000 km upwards from the boundary between the Earth’s core and its mantle. They contain about 8% of the Earth’s mantle volume and about 6% of the Earth’s total volume.
For decades, their origins were a mystery. Scientists wondered if they could be the remains of Theia, the protoplanet that slammed into Earth, resulting in the Moon. But convincing evidence was elusive.
Now, new research points convincingly at the Giant Impact as the LLSVP’s source. The new paper is “Moon-forming impactor as a source of Earth’s basal mantle anomalies.” It’s in the journal Nature, and the lead author is Qian Yuan, a Postdoctoral Scholar Research Associate at Caltech’s Seismological Laboratory.
Yuan is a geophysicist, but when he attended a seminar on planet formation by Mikhail Zolotov, a professor at Arizona State University, in 2019, a light went on. Zolotov was explaining the Giant Impact Hypothesis. Two things happened simultaneously: Qian noted that the Moon is relatively iron-rich, and Zolotov explained that no trace of Theia has ever been found.
“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,” says Yuan.
Yuan isn’t the first scientist to consider this possibility. But, scientific methods and tools improve over time. Yuan worked with other researchers from multiple disciplines to perform multiple simulations of the Giant Impact with different chemical compositions for Theia and its impact on the Hadean Earth.
According to their work, this is what happened.
When Theia slammed into Earth, it released an enormous amount of energy. It sprayed molten material from both planets into orbit around Earth. Some escaped, much of it coalesced into the Moon, and some of it remained inside Earth’s molten form.
But much of the energy delivered by the collision remained in Earth’s upper regions, never penetrating to the core. This is where Yuan’s simulations differ from previous efforts. They’re more detailed and higher resolution. Previous efforts failed to show that the energy never penetrated the core, leading to uncertain conclusions.
But if Earth’s core was effectively blocked off from the impact energy, it remained much cooler. That also means that the Earth’s lower mantle wasn’t heated to the degree previous research showed. So, the material from Theia, called Theia Mantle Material (TMM) that remained inside the Earth didn’t dissolve completely into the mantle. Instead, it formed the two recognizable clumps that form both of Earth’s LLSVPs.
If the mantle had been warmer, meaning it had received more energy from the impact, the Earth’s mantle material and the TMM would’ve mixed together more thoroughly. But they didn’t mix, and the higher iron content in the TMM makes the LLSVPs visible in seismic probing because all that iron slows down the seismic waves.
The result is what geophysicists call thermochemical piles.
“Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth’s core and survive to the present day,” the authors write in their paper. “The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact.”
This all leads to another fascinating line of inquiry. How did this material influence the rest of Earth’s history? Its plate tectonics, climate, even the course of evolution?
“A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient,” said study co-author Paul Asimow. “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.”