Theia: Ancient planet buried inside of Earth is likely a relic of the moon's formation

Ancient planet named Theia is buried deep inside of the Earth

In a stunning revelation, Caltech researchers have uncovered evidence suggesting that two massive blobs located deep near Earth’s core are the remains of an ancient planet, possibly named Theia, that collided with our world billions of years ago.

Unusual structures 

Back in the 1980s, geophysicists stumbled upon two unusual, continent-sized structures deep within the Earth, beneath the African continent and the Pacific Ocean. 

Each of these structures, twice the size of the Moon, showcased different elemental proportions than their surrounding mantle. They were named “large low-velocity provinces” or LLVPs due to the slowed seismic waves that passed through them, a result of their unusually high iron content.

Seismic images reveal Theia

“Seismic images of Earth’s interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle,” explained the study authors.  

“The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle.”

Giant-impact hypothesis 

The origin of these mysterious LLVPs has been the subject of debate for decades. However, the recent study suggests that these structures are remnants of Theia, a planet believed to have violently crashed into Earth.

This collision, according to the giant-impact hypothesis, was so immense that it led to the formation of our Moon

An intriguing point to note is that while evidence of Theia’s existence has been proposed for years, no traces of this celestial body have been found in the asteroid belt or meteorites.

The new research implies that Theia was largely absorbed into the young Earth, eventually forming the LLVPs, while the residual debris from the colossal impact gave birth to the Moon.

Focus of the Theia study

The basis of this discovery traces back to a seminar in 2019 where Qian Yuan, a geophysicist at Caltech, had a “eureka moment” after hearing about the giant-impact hypothesis and the iron-rich Moon. Yuan speculated that the iron-rich impactor might have transformed into the LLVPs.

To validate this theory, Yuan and his interdisciplinary team conducted simulations examining Theia’s potential chemical composition and the dynamics of its impact with Earth. 

Physics of the Theia collision 

The simulations confirmed that the physics of the collision could have indeed led to the creation of both the LLVPs and the Moon. Notably, parts of Theia’s mantle might have merged with Earth’s mantle, eventually forming the two distinct blobs we see today, while other debris formed the Moon.

“Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth’s mantle after the Moon-forming giant impact,” wrote the researchers. 

“Our canonical giant-impact simulations show that a fraction of Theia’s mantle could have been delivered to proto-Earth’s solid lower mantle.”

Merging mantles 

But why did Theia’s remnants form two separate blobs instead of blending uniformly with Earth’s forming structure? The team’s simulations suggest that much of the energy from Theia’s impact was concentrated in Earth’s upper mantle, ensuring the lower mantle remained cooler than previously thought. 

This cooler lower mantle allowed Theia’s iron-rich material to clump together at the core-mantle boundary, reminiscent of the unmelted blobs in a switched-off lava lamp.

Study implications 

Professor Paul Asimow highlighted the implications of this discovery: “A logical consequence of the idea that the LLVPs are remnants of Theia is that they are very ancient.”

“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.”

The researchers concluded that because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

Theories of the Moon’s origins 

The formation of the Moon has been a subject of extensive study and debate over the years. Several theories have been proposed, with the giant-impact hypothesis being the most widely accepted. Here’s a brief overview of the prominent theories.

Theia and the giant-impact hypothesis

The most widely accepted theory today proposes that a Mars-sized body, often referred to as Theia (discussed above), collided with the early Earth around 4.5 billion years ago.

The debris from this collision eventually coalesced under gravity to form the Moon.

Recent simulations, such as those in the Caltech study, and evidence from lunar and Earth rock samples support this theory.

Fission theory

The fission theory of the Moon’s origin suggests that the Moon formed from the Earth itself. According to this theory, the Moon was once part of the Earth and separated from it early in the planet’s history. This event is theorized to have occurred when the Earth was a young, rapidly spinning planet. The idea posits that the Earth’s rotational speed was so high that a piece of it broke away due to centrifugal forces.

Advocates of the fission theory argue that the Pacific Basin might be the scar of this colossal event, the place from which the Moon fissioned away. The concept was initially proposed by George Darwin, the son of Charles Darwin, in the 19th century. He suggested that the Moon was flung out of the Earth’s oceanic crust due to a centrifugal force which exceeded the force of gravity.

The fission theory attempts to explain why the Moon’s composition is strikingly similar to that of the Earth’s crust. However, this theory has lost favor over time because it does not adequately account for the angular momentum of the Earth-Moon system nor does it provide a satisfactory explanation for the substantial energy required to eject a large mass from the Earth to form the Moon.

Modern science has largely replaced the fission theory with the giant impact hypothesis, which posits that the Moon formed out of the debris left over from a collision between the Earth and a Mars-sized body called Theia. This hypothesis is currently the most widely accepted explanation for the Moon’s origin, as it better fits the geological and isotopic evidence found through the study of lunar rocks and Earth’s crust.

Capture theory

The capture theory proposes that the Moon was formed elsewhere in the solar system and was captured by Earth’s gravity.

This hypothesis is less accepted because capturing such a large object without the aid of significant atmospheric drag or other bodies to dampen its motion would be unlikely.

Double planet hypothesis

This idea suggests that the Earth and Moon formed together as a double system from the primordial accretion disk of the Sun. However, the differences in the composition of the Earth and Moon pose challenges to this theory.

Multiple impact hypothesis

This theory proposes that multiple impacts on the early Earth by several smaller bodies led to the creation of many moonlets which then coalesced to form the Moon.

While the giant-impact hypothesis remains the most favored due to its ability to explain many of the physical and chemical properties of the Earth-Moon system, researchers continue to study and refine our understanding of the Moon’s origins.

The study is published in the journal Nature.

Video Credit: Edward Garner

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