The remains of another planet that collided with the Earth and formed the Moon are still buried deep inside the Earth’s interior, according to a new study.
An international, interdisciplinary team of researchers says Large Low-Velocity Provinces (LLVP) deep in our planet’s mantle could be relics of a planet that collided with Earth around 4.5 billion years ago which have since influenced our planet’s geography.
The new study, published in the journal Nature, offers important new insights not only into the Earth’s internal structure but also into its long-term evolution and the formation of the inner solar system.
The research relied on computational fluid dynamics methods pioneered by Professor Hongping Deng of the Shanghai Astronomical Observatory (SHAO), of the Chinese Academy of Sciences.
How our own Moon was formed has proved a persistent challenge for several generations of scientists.
However, prevailing theories suggest that during the late stages of the Earth’s growth – around 4.5 billion years ago – a huge collision known as the ‘giant impact’ occurred.
This impact was between primordial Earth – known as Gaia – and a proto-planet around the size of Mars, known as Theia.
The Moon is believed to have formed from the debris generated by this huge collision.
Simulations have also indicated that the Moon likely inherited material primarily from Theia, whilst Gaia, due to its bigger size, was only mildly contaminated by Theian material.
Since Gaia and Theia were relatively independent formations composed of different materials, previous theories suggested that the Moon, being dominated by Theian material, and the Earth, being dominated by Gaian material, should have distinct compositions.
However, high-precision isotope measurements later revealed that the compositions of the Earth and Moon are remarkably similar, thus challenging the conventional theory of the formation of the Moon.
And various models of the giant impact that led to its formation have all faced significant challenges.
To further refine the theory of the Moon’s formation, Deng began conducting research on its formation in 2017.
He focused on developing a new computational fluid dynamics method called Meshless Finite Mass (MFM), which excels at accurately modeling turbulence and material mixing.
Using this approach and simulations of the giant collision, Deng discovered that the early Earth exhibited mantle stratification – or layering – after the impact, with the upper and lower mantle having different compositions and states.
Specifically, the upper mantle featured a magma ocean created through a thorough mixing of material from Gaia and Theia, while the lower mantle remained largely solid and retained the material composition of Gaia.
“Previous research had placed excessive emphasis on the structure of the debris disk, the precursor to the Moon, and had overlooked the impact of the giant collision on the early Earth,” Deng explained.
“This research even provides inspiration for understanding the formation and habitability of exoplanets beyond our solar system.”
Following discussions with geophysicists from the Swiss Federal Institute of Technology in Zurich, Deng and his collaborators realized that this mantle layering may have persisted to the present day, corresponding to the global seismic reflectors in the mid-mantle, located around 1,000 km beneath the Earth’s surface.
In particular, the entire lower mantle of the Earth may still be dominated by pre-impact Gaian material, which has a different elemental composition including higher silicon content than the upper mantle, according to Deng’s previous studies.
“Our findings challenge the traditional notion that the giant impact led to the homogenization of the early Earth,” he said.
“Instead, the Moon-forming giant impact appears to be the origin of the early mantle’s heterogeneity and marks the starting point for the Earth’s geological evolution over the course of 4.5 billion years.”
Another example of the Earth’s mantle diversity is two anomalous regions, called Large Low-Velocity Provinces (LLVPs), which stretch for thousands of kilometers at the base of the mantle.
One is located beneath the African tectonic plate, whilst the other is under the Pacific plate.
When seismic waves pass through these areas, wave velocity is significantly reduced.
LLVPs have significant implications for the evolution of the mantle, the separation and aggregation of supercontinents, and the Earth’s tectonic plate structures – but their origins have remained a mystery.
Dr. Qian Yuan, from the California Institute of Technology, proposed along with his collaborators that LLVPs could have evolved from a small amount of Theian material that entered Gaia’s lower mantle.
They subsequently invited Deng to explore the distribution and state of Theian material in the deep Earth after the giant impact.
Through in-depth analysis of previous giant-impact simulations, and by conducting new, higher-precision simulations, the researchers found a significant amount of Theian mantel material – around two percent of the Earth’s mass – entered the lower mantle of Gaia during the giant impact.
Deng invited computational astrophysicist Dr. Jacob Kegerreis to confirm this conclusion using traditional Smoothed Particle Hydrodynamics (SPH) methods.
The research team also calculated that this Theian mantle material – which is similar to lunar rocks – is enriched with iron, making it denser than the surrounding Gaian material.
As a result, it rapidly sank to the bottom of the mantle and, over the course of long-term mantle convection, formed two prominent LLVP regions which have remained stable throughout 4.5 billion years of geological evolution.
Diversity in the deep mantle suggests that the Earth’s interior is far from a uniform or boring system, but instead, small amounts of deep-seated heterogeneity can be brought to the surface by mantle plumes – thermal currents caused by mantle convection which are believed to have formed the islands of Hawaii and Iceland.
For example, geochemists studying isotope ratios of rare gases in samples of Icelandic basalt have discovered that these samples contain components different from typical surface materials.
These components are remnants of diversity in the deep mantle dating back more than 4.5 billion years and serve as keys to understanding Earth’s initial state and even the formation of nearby planets.
Dr. Yuan added: “Through precise analysis of a wider range of rock samples, combined with more refined giant impact models and Earth evolution models, we can infer the material composition and orbital dynamics of the primordial Earth, Gaia, and Theia.
“This allows us to constrain the entire history of the formation of the inner solar system.”
Produced in association with SWNS Talker
