Mysterious blobs of dense rock could be remnants of the ancient planet Theia, study finds 

Blobs of rock lurking deep inside the planet could be the last remaining pieces of Theia, an early world that collided with Earth billions of years ago, a new study claims.

The moon is thought to have formed from a ‘giant impact’ early in the evolution of the solar system when a Mars-sized planet called Theia collided with the Earth. 

Deep within the Earth are a number of strange, huge blobs of dense rock called large low-shear-velocity provinces (LLSVPs) and a team from Arizona State University believe these massive regions may be parts of that long-destroyed proto-world.

One of these LLSVPs is buried under Africa and another deep below the Pacific Ocean, both so large they are involved with the weakening of Earth’s magnetic field.

Qian Yuan, author of this theory, suggests the blobs, which ‘straddle the Earth’s core like headphones’ are denser and chemically different to the rock surrounding them.

This artist’s concept shows a celestial body about the size of our moon slamming into a body the size of Mercury in a scenario that could be similar to Theia colliding with Earth

ASU scientists believe Theia and the proto-Earth came together to form not just the moon but also a new, larger core, with Theia leaving ‘blobs’ of rocks in the Earth’s mantle


Large low-shear-velocity provinces are large regions of the Earth’s mantle that straddle the core ‘like headphones’.

They have only been detected by seismic waves, but are believed to be denser than the surrounding rock.

They stretch up to 600 miles high into the 1,800 mile deep mantle, spreading thousands of miles across on either side of the world.

One sits deep below the Pacific ocean, the other buried under Africa.

The mantle itself makes up 84% of the total volume of the Earth, with these regions combined making up 8% of the volume of the mantle. 

Recent theories suggest that they are the remnants of the proto-planet Theia that is thought to have collided with the Earth 4.45 billion years ago.

It had a denser core and mantle, which sank to the bottom of Earth’s mantle. 

Some theories suggest other remnants of ancient proto-planets also sit within the mantle, shown as pockets of denser rock. 

Yuan said Theia’s mantle was denser than that of the Earth, and so when the world’s collided parts of the smaller planet sank deep inside the mantle of our world. 

The mantle is a mostly-solid 1,800 mile thick interior of the Earth that lies between the core and the crust. It makes up 84 per cent of the total volume of the Earth.

The ‘Great Impact’ theory aligns well with some of the physical aspects of the Earth-moon system, according to the team, but no proof of Theia has been found. 

This event happened between 20 and a 100 million years after the solar system itself first began to form around the Sun – about 4.5 billion years ago.

Some supporters of the theory have suggested that the cores of the two primordial worlds ‘fused into one’, possibly releasing chemicals needed to support life.

The latest theory by Yuan, a PhD student at Arizona State, draws in the LLSVP blobs to the Earth/Theia merger concept – suggesting they are evidence of the collision. 

These blobs are up to 620 miles tall and thousands of miles wide, making up the ‘largest single thing’ inside the mantle of the Earth, Yuan explained.

Several theories try to explain their existence, including the idea they crystallised out of the depths of Earth’s early magma ocean, or actual parts of the mantle from proto-planet Theia that collided with the Earth, as Yuan and his colleagues suggest. 

Yuan suggests that the blobs are ancient fragments of Theia’s iron-rich and highly dense mantle which sank into Earth’s own mantle when they came together. 

‘We demonstrate that Theia’s mantle may be several percent denser than Earth’s mantle,’ he said in a presentation to the Lunar and Planetary Science Conference.

His modelling revealed that Theia’s rocks would be up to 3.5 per cent denser than those found in the Earth’s mantle, allowing them to sink through the viscous rock. 

Adding that this ‘enables the Theia mantle materials to sink to the Earth’s lowermost mantle and accumulate into thermochemical piles that may cause the seismically-observed LLSVPs.’ 

Over time these rocks would end up as piles near the Earth’s core, which matches with the location of LLSVPs, he said, adding that only an impactor the size of the Theia could deliver rocks the size of the ones near the core.  

Previous research has shown that the chemical signatures coming from the LLSVPs are ‘at least as primitive’ as the Theia impact – dating to early in the solar system. 

Qian Yuan, author of this theory, suggests the blobs, which 'straddle the Earth's core like headphones' are denser and chemically different to the rock surrounding them

Qian Yuan, author of this theory, suggests the blobs, which ‘straddle the Earth’s core like headphones’ are denser and chemically different to the rock surrounding them

The moon is thought to have formed from a 'giant impact' early in the evolution of the solar system when a Mars-sized planet called Theia collided with the Earth

The moon is thought to have formed from a ‘giant impact’ early in the evolution of the solar system when a Mars-sized planet called Theia collided with the Earth


Crust: To a depth of up to 70km, this is the outermost layer of the Earth, covering both ocean and land areas.

Mantle: Going down to 2,890km with the lower mantle, this is the planet’s thickest layer and made of silicate rocks richer in iron and magnesium than the crust overhead.

Outer core: Running from a depth of 2,890- 5,150km, this region is made of liquid iron and nickel with trace lighter elements.

Inner core: Going down to a depth of 6,370km at the very centre of planet Earth, this region is thought to be made of solid iron and nickel.

‘Therefore, the primitive materials may be from the LLSVPs, which is well explained if the LLSVPs preserve Theia mantle materials that are older than the Giant Impact,’ Yuan explained in the presentation.

The early solar system was a chaotic and violent place. Debris would slam into growing planets, smaller planetesimals were just forming and coming together from smaller rocks and sometimes these would completely destroy forming worlds. 

In the case of Theia, it appears to have combined with Earth, possibly created a cloud of debris that went on to form our moon.

The LLSVPs have only been detected through monitoring seismic waves, which allows scientists to get a picture of their size and density.

This allows them to also predict the type of material that would be contained within the rock, with scientists finding a number of small, dense pockets of rock within the mantle that don’t match the surrounding material.

This suggests if Theia really is at the bottom of the mantle, it may not be alone, with remnants of other proto-planets that once hit the young Earth buried alongside it. 

‘Theia, in fact, might be just one grave in a planetary cemetery,’ said Jennifer Jenkins, a seismologist at Durham University, speaking to Science. 

Yuan and colleagues presented their findings to the 52nd Lunar and Planetary Science Conference.


About 4.45 billion years ago, 150 million years after the solar system formed, Earth was hit by a Mars-size object called Theia.

The collision created the moon, but debate has raged exactly what happened during this event – and a mystery has persisted on why the moon and Earth are so similar in their composition.

The impact of Theia with Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon.

This cloud would have been composed of some Earth material, explaining the similarity between Earth and the moon, and other material.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

But one mystery has persisted, revealed by rocks the Apollo astronauts brought back from the moon – why are the moon and Earth so similar in their composition?

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon.

Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon.

Or Theia could have, coincidentally, been isotopically similar to young Earth.

A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact.