Possible Remnant of Moon-Forming Collision Found as Massive Anomaly in Earth’s Mantle
A cross-disciplinary international research team has recently unearthed a substantial anomaly residing deep within the Earth’s interior. This anomaly is potentially a remnant from the colossal collision approximately 4.5 billion years ago that played a pivotal role in the moon’s formation. This groundbreaking research not only enhances our understanding of Earth’s inner structure but also sheds light on its long-term evolution and the genesis of the inner solar system.
Published as the featured cover story in Nature on November 2, this study harnessed innovative computational fluid dynamics techniques pioneered by Professor Deng Hongping at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences.
For generations, scientists have grappled with the enigma of how the moon came into existence. The prevailing theory postulates a significant cosmic event known as the “giant impact,” which transpired during the late stages of Earth’s formation around 4.5 billion years ago. This event involved a massive collision between the early Earth, often referred to as Gaia, and a proto-planet of Mars’s size named Theia. The moon is thought to have originated from the debris generated by this cataclysmic encounter.
Numerical simulations have suggested that the moon primarily inherited its material from Theia, while Gaia, owing to its considerably greater mass, only experienced a modest contamination with Theian material. Given that Gaia and Theia were distinct celestial bodies composed of dissimilar materials, conventional wisdom dictated that the moon, predominantly comprised of Theian material, and the Earth, largely composed of Gaian material, should exhibit distinctive compositions. However, meticulous isotope measurements have subsequently revealed a striking similarity in the compositions of the Earth and the moon, challenging the established theory of moon formation.
Although several advanced models of the giant impact have been put forward over time, they have all encountered significant challenges.
In an effort to refine the theory of lunar formation, Professor Deng initiated research on the moon’s origin in 2017. His focus was on the development of a novel computational fluid dynamics technique known as Meshless Finite Mass (MFM), designed to excel at accurately simulating turbulence and material mixing.
Through the use of this innovative approach and numerous simulations of the giant impact, Professor Deng uncovered that the early Earth underwent mantle stratification following the collision. This stratification resulted in the upper and lower mantle possessing distinct compositions and states. Specifically, the upper mantle featured a magma ocean formed through the thorough mixing of materials from both Gaia and Theia, while the lower mantle remained predominantly solid and retained the material composition of Gaia.
Deng commented, “Previous research had overly emphasized the structure of the debris disk (the precursor to the moon) and had neglected the impact of the giant collision on the early Earth.”
After consultations with geophysicists from the Swiss Federal Institute of Technology in Zurich, Professor Deng and his collaborators realized that this mantle stratification might have persisted to the present day, corresponding to the global seismic reflectors located in the mid-mantle, approximately 1,000 kilometers below the Earth’s surface.
More specifically, it is conceivable that the entire lower mantle of the Earth still predominantly consists of pre-impact Gaian material, which exhibits a distinct elemental composition, including a higher silicon content, compared to the upper mantle, as outlined in Professor Deng’s earlier study.
“Our findings challenge the conventional notion that the giant impact led to the homogenization of the early Earth,” Professor Deng noted. “Instead, the moon-forming giant impact appears to have initiated the early mantle’s heterogeneity, marking the beginning of Earth’s geological evolution over the course of 4.5 billion years.”
Another manifestation of heterogeneity in the Earth’s mantle is found in two peculiar regions known as Large Low Velocity Provinces (LLVPs), extending for thousands of kilometers at the base of the mantle. One is situated beneath the African tectonic plate, while the other lies beneath the Pacific tectonic plate. These areas exhibit a significant reduction in seismic wave velocity.
LLVPs have profound implications for the evolution of the mantle, the formation and breakup of supercontinents, and the structure of the Earth’s tectonic plates. However, their origins have remained a mystery.
Dr. Yuan Qian from the California Institute of Technology, in collaboration with others, proposed the notion that LLVPs might have originated from a small quantity of Theian material that entered Gaia’s lower mantle. This prompted Professor Deng to investigate the distribution and condition of Theian material within the deep Earth following the giant impact.
Through an in-depth analysis of previous giant impact simulations and the execution of high-precision new simulations, the research team determined that a significant portion of Theian mantle material, roughly 2% of Earth’s mass, made its way into Gaia’s lower mantle.
To corroborate this conclusion, Professor Deng invited computational astrophysicist Dr. Jacob Kegerreis to employ traditional Smoothed Particle Hydrodynamics (SPH) methods.
The research team further calculated that this Theian mantle material, similar to lunar rocks, is enriched with iron, making it denser than the surrounding Gaian material. Consequently, it rapidly sank to the bottom of the mantle and, over the course of long-term mantle convection, gave rise to the two prominent LLVP regions. These LLVPs have remained stable throughout 4.5 billion years of geological evolution.
The presence of heterogeneity in the deep mantle, whether in the mid-mantle reflectors or the LLVPs at the base, signifies that the Earth’s interior is far from being a uniform and uninteresting system. In fact, small amounts of deep-seated heterogeneity can be brought to the surface by mantle plumes, which are cylindrical upwelling thermal currents generated by mantle convection, such as those thought to have formed Hawaii and Iceland.
For instance, geochemists examining isotope ratios of rare gases in Icelandic basalt samples have revealed that these samples contain components distinct from typical surface materials. These components are remnants of heterogeneity in the deep mantle dating back over 4.5 billion years and provide crucial insights into the Earth’s initial state and even the formation of nearby planets.
According to Dr. Yuan, “Through precise analysis of a broader range of rock samples, combined with more sophisticated giant impact models and models of Earth’s evolution, we can infer the material composition and orbital dynamics of the primordial Earth, Gaia, and Theia. This enables us to constrain the entire history of the formation of the inner solar system.”
Professor Deng envisions an even broader role for the present study. “This research also serves as an inspiration for comprehending the formation and potential habitability of exoplanets beyond our solar system.”
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