For generations, astronomers have been puzzled by the mystery surrounding the formation of the Moon. The prevailing idea is that late in Earth’s evolution, around 4.5 billion years ago, a cataclysmic collision, referred to as the „giant impact,” occurred between the primordial Earth (Gaia) and a proto-planet known as Thea that was around the size of Mars. years ago. Debris from this impact is thought to have formed the Moon.
Although many improved models of mass impact were subsequently proposed, they all encountered difficulties.
In 2017, Professor Deng Hongping of the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences began to study the formation of the Moon to improve the Moon formation hypothesis. His work focused on developing Meshless Finite Mass (MFM), a new approach to computational fluid dynamics that excels in accurately simulating turbulence and material mixing.
Using this new approach and running multiple simulations of the giant impact, an interdisciplinary, international research team recently found that a large anomaly deep in Earth’s interior may be the remnant of a collision that formed the Moon about 4.5 billion years ago. The study provides new insights into Earth’s internal structure, its long-term evolution, and the formation of the inner Solar System.
Using this method, scientists have shown signs of stratification of early Earth’s upper and lower mantles with different compositions and levels following the impact. More precisely, the lower mantle remained mostly solid, preserving Gaia’s material composition. At the same time, the higher mantle contained a magma ocean formed by complete mixing of material from both Gaia and Thea.
After speaking with geophysicists at the Swiss Federal Institute of Technology in Zurich, Prof. Deng and collaborators found that this mantle stratification may have continued to the present day. This corresponds to global seismic reflectors in the mid-mantle about 1000 km below the Earth’s surface. In particular, Prof. According to DENG’s previous research, the entire Earth’s lower crust may be dominated by pre-impact Gaian material, which differs from the upper mantle in terms of basic composition (including high silicon content).
Professor Deng said, „Our findings challenge the traditional view that a giant impact led to the homogeneity of the early Earth. Instead, the giant impact that formed the Moon appears to be the origin of early mantle heterogeneity and the starting point for Earth’s geological evolution over 4.5 billion years.”
Another example of Earth’s mantle heterogeneity is two peculiar regions known as low-velocity provinces (LLVPs). These regions extend over thousands of kilometers at the base of the mantle. One lies beneath the tectonic plates of Africa and the other lies beneath the Pacific Ocean. Seismic waves travel at significantly lower speeds in these areas.
Earth’s tectonic plate systems, mantle evolution, and the breakup and accretion of supercontinents are all significantly affected by LLVPs. But their origin is still unknown.
According to the theory of Dr. Yuan Qian of the California Institute of Technology and colleagues, a small amount of thean material that infiltrated Gaia’s lower mantle may have formed the LLVPs.
Professor DENG was invited to investigate the distribution and position of Theian objects in the deep Earth following the massive impact.
By performing new, high-precision simulations and thoroughly reviewing previous giant impact models, the research team found that a significant fraction of thean mantle material—about two percent of Earth’s mass—entered Gaia’s lower mantle.
Traditional smooth particle hydrodynamics (SPH) methods were used to confirm the result. The scientists also determined that this thean mantle material, like lunar rocks, is enriched in iron and is denser than the surrounding Gaian material.
That is why it quickly sank to the bottom of the mantle and throughout the long-term mantle convection, formed two main LLVP regions. These LLVPs have remained stable throughout 4.5 billion years of geological evolution.
Whether in LLVPs at the base or as a mid-mantle reflector, heterogeneity in the deep mantle suggests that Earth’s interior is anything but homogeneous and „boring” in structure. In fact, mantle plumes—cylindrical upward heat currents carried to the surface by mantle convection—such as those created mostly by Iceland and Hawaii—can bring moderate amounts of deep heterogeneity to the surface.
Dr. Yuan In the states, „Through the precise analysis of a wide range of rock samples combined with highly refined giant impact models and Earth evolution models, we can infer the material composition and orbital dynamics of the primordial Earth, Gaia and Thea. This allows us to constrain the entire history of the formation of the inner Solar System.
Prof. DENG sees an even broader role for the current study: „This research inspires understanding of the formation and habitability of exoplanets beyond our solar system.”
Journal Note:
- Yuan, Q., Li, M., Desch, SJ, et al. Moon-forming impact as a source of Earth’s basement mantle anomalies. Nature 623, 95–99 (2023). DOI: 10.1038/s41586-023-06589-1
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