Buried Planet: Evidence of Earth’s Collision with Theia Revealed

While scanning at a depth of 1,800 miles (2,900 kilometers) below the Earth’s surface, geologists found two huge continent-sized masses embedded in the planet’s mantle (a solid area of silicone rock sandwiched between the Earth’s crust and the molten inner core) in the 1980s. For nearly four decades, scientists have puzzled over the nature and origin of these masses, but a new study suggests these peculiar anomalies are actually the remnants of a long-destroyed planet known as Theia, which is alleged to have smashed into the Earth 4.5 billion years ago with so much force that the Moon was created from the resulting debris field.

In an article just published in Nature, a team of planetary scientists from Arizona State University and the California Institute of Technology hypothesize that the two huge masses or blobs buried deep in the Earth’s mantle are chunks of what was once the mantle of Theia. These were left behind when this wandering Mars-sized object collided with the Earth while our home planet was still in the process of forming.

• Year of Darkness: The Climate Cataclysm of 536 You’ve Never Heard Of
• Cataclysm, Mass Extinctions, and the Consequent Myths

While much of the doomed planet was liquefied during this collision, pieces from its interior could have survived the impact. Propelled forward by extreme momentum, these masses could have been driven into the ground with enough force to penetrate deep below the Earth’s surface. This would explain the presence of the two continent-sized anomalies, which were found via seismic imaging below the floor of the Pacific Ocean in one case and beneath the African landmass in the other.

Scientists believe they have found evidence that the giant Moon-forming impact between Earth and Theia left behind extraterrestrial rock under the Earth’s surface. (Hernán Cañellas/Nature)

Theia Was Here, Under Our Feet, All the Time

In support of this deep penetration hypothesis, the scientists offer the results of computer simulations that accurately recreate the results of the impact between Theia and Earth. These were based on the assumption that Theia possessed the characteristics that scientists have previously determined it must have had. Using the latest in state-of-the-art simulation technology, the researchers were able to show that a scenario like that one outlined above was a likely outcome of this catastrophic event.

Based on measurements of materials retrieved from the Moon, a body that is composed of remnants of both the Earth and ancient Theia, planetary scientists have concluded that Theia’s mantle must have been about 3.5 percent denser than the Earth’s. This is incredibly important, because the denseness of the Theian mantle is what would have allowed its surviving pieces to continue sinking deeper and deeper into the Earth’s mantle after their initial penetration, only stopping when they reached the edge of the planet’s thicker outer core.

Bits of an ancient planet called Theia may be buried more than a kilometre underground, offering an explanation for two large high-density blobs in Earth’s mantle that have been puzzling researchers for decades. https://t.co/KaqvxVuDZh

— New Scientist (@newscientist) November 1, 2023

Under the scenario proposed by the Arizona State University and California Institute of Technology scientists, the surviving pieces of Theia’s mantle would have been much smaller than the huge blobs now embedded far below the Earth’s surface. But they would have been deposited close enough to one another that when they all sank, they would have coalesced to form the much larger anomalous bodies now spotted on the deep Earth seismic images.

The two massive rocky conglomerations are larger in size than the Moon (their sister). They are also noticeably denser and hotter than the surrounding rock, which is consistent with an extraterrestrial origin.

In an interview with France 24, the French news service, geodynamics researcher and lead study author Qian Yuan noted that it was “very, very strange” that no evidence of the Theia impact had ever been found on the Earth’s surface. Long intrigued by this oddity, eventually Yuan began to wonder if surviving pieces of Theian material might be found somewhere under the ground.

It was then that he made the connection between his speculation and the two huge anomalous blobs that are in fact buried far beneath our planet’s surface. His curiosity was piqued enough to organize the new study of these underground masses. The findings of this research are consistent with the deep mantle penetration theory. “Where is the impactor? My answer is: it's in the Earth,” Yuan exclaimed.

According to Yuan, when Theia hit the Earth it was traveling at speeds in excess of six miles (9.7 kilometers) per second. This would have been fast enough to have pushed it through the crust and well down into the mantle.

The material from the Theian mantle would have been largely melted by the heat generated by the collision, but as it cooled and solidified it would have begun to sink further, ultimately reaching the border separating the Earth’s mantle from its core. Its added density would have come from its high iron content, which is reflected in the relatively high levels of iron oxide found on the Moon (the “child” that was birthed from the Earth-Theia collision).

Using high-resolution giant impact simulations, mantle convection models, mineral physics calculations, and seismic imaging the team has concluded that two massive seismically-observed basal mantle structures may represent remnants of planet Theia’s mantle rock. (Hernán Cañellas/Nature)

Theia’s Rough Landing and its Miraculous Result

In addition to creating the gigantic debris cloud that ultimately formed the Moon, the collision between Earth and Theia dramatically changed conditions on the former planet, virtually overnight. Other research suggests that it was Theia that brought water to the Earth, which if true means that without this planetary catastrophe, life on Earth may never have emerged.

• Ancient Cataclysms: 9 Catastrophes That Rocked the World
• Fire in the Sky: Cosmic Impact Obliterated Prehistoric Settlement

Interestingly, the anomalous blobs themselves have also played a role in Earth’s evolutionary history. These hot, rocky conglomerations, which in scientific terminology are known as large low-shear-velocity provinces (LLVPs), will occasionally send columns of magma surging toward the Earth’s surface, increasing interior volcano-like activity. They have also been connected to the development of the supercontinents that gave birth to the present continents, meaning they contributed to the processes that gave the Earth the appearance it has today. Theia “left something in the Earth, and that played a role in Earth's subsequent 4.5 billion years of evolution,” Yuan declared.

Given that the Moon is also essential to the survival of life on Earth (its gravitational pull stabilizes the climate by controlling the tides and the wobble of the Earth’s axis), it is clear that Theia’s arrival was actually a godsend, even if its “landing” was not exactly smooth.

Top image: Scientists believe they’ve discovered evidence of Earth’s collision with Theia nearly 4.5 billion years ago thanks to seismic imaging. Source: Hongping Deng, Hangzhou Sphere Studio, China/Nature

By Nathan Falde