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An international, multidisciplinary research team recently discovered that a massive anomaly deep within the Earth’s interior may be a remnant of the collision about 4.5 billion years ago that formed the Moon. This research provides important new insights not only into Earth’s internal structure, but also into its long-term evolution and the composition of the inner solar system.
The research is based on computational fluid dynamics methods developed by Professor Dr. Deng Hongping of the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences was published as a featured article in Nature on November 2. The formation of the Moon has been an ongoing mystery for generations of scientists. The prevailing theory is that during the late stages of Earth’s growth, about 4.5 billion years ago, a massive collision, known as a “giant impact,” occurred between the primordial Earth (Gaia) and a Mars-sized protoplanet known as Theia. . It is believed that the moon was formed from the debris of this collision.
Numerical simulations have shown that the Moon likely inherited material primarily from Theia, while Gaia is only slightly contaminated with Theia material due to its much greater mass.
Because Gaia and Theia were relatively independent formations and composed of different materials, theory suggested that the Moon — which is dominated by Theian matter, and the Earth, which is dominated by Gaian matter — would have different compositions. However, very precise isotope measurements later revealed that the compositions of the Earth and the Moon were remarkably similar, calling into question the traditional theory of the Moon’s formation.
Although several sophisticated models of the giant impact have been proposed, all have been tested. To further improve the theory of moon formation, Professor Ding began researching moon 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 mixing of materials.
Using this new approach and performing several simulations of a giant impact, Professor Ding found that the early Earth showed post-impact mantle layers, where the upper and lower mantle had different compositions and states. The upper mantle contained a magma ocean, created by extensive mixing of material from Gaia and Theia, while the lower mantle remained largely solid and retained the composition of Gaia’s materials.
“Previous research has focused too much on the structure of the debris disk (the front of the Moon) and overlooked the impact of the giant impact on the early Earth,” Ding said. After discussions with geophysicists at the Swiss Federal Institute of Technology in Zurich, Professor Ding and his collaborators realized that these layers in the mantle may have persisted to the present day, consistent with global seismic reflectors in the middle of the mantle (about 1,000 kilometers underground). The earth’s surface).
According to a previous study by Professor Ding, the entire lower mantle of the Earth was still dominated by Gaian material before the collision, which has a different elemental composition (including a higher silicon content) than the upper mantle. “Our findings refute the traditional idea that a giant impact led to the homogenization of the early Earth,” the professor said. “Instead, the giant collision that formed the Moon appears to be the origin of early mantle heterogeneity and represents the starting point for Earth’s geological evolution over 4.5 billion years.”
Another example of mantle heterogeneity are two anomalous regions, called large low-velocity provinces (LLVPs), that extend thousands of kilometers at the base of the Earth’s mantle. One is located under the African tectonic plate and the other is under the Pacific tectonic plate. When seismic waves pass through these areas, the wave speed decreases significantly.
LLVPs have important implications for mantle evolution, supercontinent separation and assembly, and Earth’s plate tectonic structures. However, their origins remained a mystery. doctor. Yuan Qian of Caltech and collaborators suggested that the LLVPs may have formed from a small amount of Thean that entered Gaia’s lower mantle. They then invited Professor Ding to investigate the distribution and state of Thean material deep within the Earth after the giant impact.
Through an in-depth analysis of previous simulations of the giant impact and by running new simulations at higher resolution, the research team found that a significant amount of Theian mantle material, about 2% of the Earth’s mass, had entered Gaia’s lower mantle.
Professor Deng then invited the astrophysicist Dr. Jacob Kegeris to confirm this conclusion using conventional SPH (smooth particle hydrodynamics) methods. The research team also calculated that this Theian mantle material, similar to moon rocks, is rich in iron, making it denser than the surrounding Gaian material. As a result, they quickly sank to the bottom of the mantle, forming two prominent LLVP regions during the course of long-term mantle convection. These LLVPs have remained stable over 4.5 billion years of geological evolution.
Heterogeneities in the deep mantle, both in the mid-mantle reflectors and in the LLVPs at the base, indicate that the Earth’s interior is far from being a uniform and “boring” system. In fact, small amounts of deep heterogeneity can be brought to the surface by mantle plumes, cylindrical upward convection currents generated by mantle convection, such as those likely formed in Hawaii and Iceland.
For example, geochemists studying the isotope ratios of rare gases in samples of Icelandic basalt have found that these samples contain components different from typical surface materials. These components are remnants of heterogeneities in the deep mantle dating back more than 4.5 billion years, and serve as a key to understanding the original state of the Earth and even the formation of nearby planets.
doctor. Yuan: “By carefully analyzing a larger number of rock samples, combined with more accurate giant impact models and Earth evolution models, we can infer the physical composition and orbital dynamics of the primordial Earth, Gaia, and Theia. This allows us to trace the entire history of the solar system’s interior mapping origins.” ” Professor Deng sees a broader role for current research. “This research also provides inspiration for understanding the formation and habitability of exoplanets outside our solar system.”
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