Chinese scholars have come up with bold hypotheses to answer this puzzle that has puzzled the academ

Mondo Education Updated on 2024-01-28

Wen Bu Jinting

Recently, Deng Hongping, an associate researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, and Yuan Qian, a postdoctoral fellow at the California Institute of Technology, published in Nature, revealing the secret of the huge anomalies deep in the earth's interior.

Deng Hongping introduced the content of the study to China Science News in one sentence: "The earth was hit by a major impact in childhood, and there may be remnants of the impactor in the interior to this day." ”

It's as simple as that?”

It's as simple as that?”

Deng Hongping gave a report at the headquarters of the European Southern Observatory (ESO) in Munich.

It has puzzled scientists for a long time: where did the moon come from?

The Moon is a satellite of the Earth and the closest celestial body to the Earth. For thousands of years, human beings have never stopped exploring this mysterious existence that is visible at the top of their heads. But where does the moon come from?This puzzle has puzzled generations of scientists.

The "big impact" hypothesis mentioned by Deng Hongping is the mainstream theory to answer this question: 4.5 billion years ago, a Mars-sized protoplanet Theia collided with the nascent Earth, and the moon gradually aggregated and grew from the debris that was knocked out. The collision, which contained about 100 million times more energy than the small celestial impact event that led to the extinction of the dinosaurs, reshaped the young Earth and may have left a deep mark on the Earth's mantle.

This hypothesis can not only explain the dynamics of the Moon and its unique material composition, but also fit in with the theory of the collisional growth of terrestrial planets.

However, through high-precision isotope analysis of Apollo lunar samples, scientists found that the composition of the surface material and the moon is highly similar, but the composition of the moon in the big impact simulation is dominated by the "extraterrestrial visitor" theia, which should be different from the composition of the earth.

The root cause of this contradiction is that the two stars cannot be fully mixed in the collision.

In 2017, while studying for a PhD in astrophysics at the University of Zurich in Switzerland, Deng Hongping learned about this problem from his colleague Christian Reinhardt and began to try to use new methods to simulate the mixing of matter in this process.

The smooth particle hydrodynamics method is widely used in large collision simulations, but it has a high numerical viscosity and is not good at simulating subsonic turbulence and the material mixing caused by it. Although this defect has become a consensus in the field of computational astrophysics, it has not attracted attention in the field of planetary science, and the optimization of old methods and the development of new methods have stalled.

Deng Hongping and others have developed a new lattice-free computational fluid dynamics method that can more effectively capture the mixture of matter brought about by weak turbulence. However, they were not able to mix Gaia and Theia evenly as they had hoped. During the impact, the material of the GAIA always tends to "float" in the upper half of the GAIA, and cannot sink down into the deep mantle of the GAIA in large quantities, and the upper half of the GAIA heats up violently, forming a molten magma ocean.

As a result, the Earth's mantle after the collision is layered: the upper mantle is a magmatic ocean, which is a mixture of Gaia and Theia, while the lower mantle is basically solid and retains mainly the material composition of Gaia.

After discussions with geophysicists at ETH Zurich, the research team came to a startling conclusion: the mantle stratification that formed 4.5 billion years ago may still have traces in the mantle, and that the layered mantle model can better explain the similarity between the composition of the surface material and the moon.

A possible model of mantle evolution. **frédéric deschamps/nature geoscience.

"If Gaia and Theia are completely mixed, it will be difficult for us to have the opportunity to distinguish between the two and study their respective properties," Deng said. And this interesting hierarchical structure, if preserved in some form, will provide important opportunities for us to study the formation of the moon and even the evolution of the earth. ”

In 2019, Deng Hongping and others submitted ** to Nature, and after more than a year of grinding review, it was unfortunately rejected. Subsequently, they submitted ** to the well-known astronomical journal The Astrophysical Journal (APJ), which was soon published.

This attempt to connect astronomy and geoscience did not attract much attention at first, but attracted another collaborator, Yuan Qian.

Yuan Qian. Interdisciplinary collaboration forms a perfect closed loop.

In the 80s of the 20th century, geoscientists discovered that there are two huge anomalous regions at the base of the mantle, and when the ** wave passes through them, the wave speed decreases significantly. As a result, these two anomalous bodies, which stretch for thousands of kilometers at the base of the mantle, are often referred to as large low-velocity bodies. Although most geoscientists believe that these two regions may have higher densities due to their relative iron richness, it is still unclear what processes formed these two anomalous regions.

In 2019, at a symposium at Arizona State University in the United States, a professor questioned the hypothesis of the "big impact" of the formation of the moon: does the impactor Theia really exist?Why hasn't anyone found evidence of the existence of Theia?

It was this question that gave Yuan Qian a flash of inspiration: "Could it be that the large low-speed body is the remnant of theia?"”

After the meeting, Yuan Qian checked the relevant materials and found that no one had put forward a similar hypothesis. Subsequently, he collaborated with Professor Mingming Li, a doctoral supervisor at Arizona State University, to test this conjecture from the perspective of geodynamics. Their results support this hypothesis, but it is necessary to assume that the Earth's mantle did not melt completely after the "Great Impact", whereas most previous correspondents** believed that the impact largely caused the total melting of the Mantle due to algorithmic and resolution issues.

It was at this time that Yuan Qian saw Deng Hongping's latest apj**, which described the stratification of the earth after the "Great Impact", which was the evidence he was looking for.

Therefore, Yuan Qian invited Deng Hongping to explore the Theia material that entered the deep part of the earth after the great collision. Prior to this, these theia materials entering the deep part of the earth had been ignored by Deng Hongping, and then the cooperation between the two was like a fish in water, perfectly forming a closed loop of research.

Surprisingly, by in-depth analysis of previous large collision simulations and new simulations with higher accuracy, the research team found that Theia material with the same mass as the large low-velocity volume enters the lower mantle of GAIA. Deng Hongping further invited Jacob Kegerreis, an expert in computational astrophysics, to confirm this conclusion with traditional smooth particle hydrodynamics methods.

The heteroplasm in the deep mantle today may be the remnant of the major collision event that led to the formation of the moon Source: Deng Hongping and Hangzhou Sifeier Technology***

In addition, the research team also calculated that these TEIA mantle materials are as rich in iron as the lunar rocks, so they are denser than the surrounding GAIA materials, so they quickly sink to the bottom of the mantle, and form two significant large low-velocity bodies under the long-term convection of the mantle, which still exist stably above the core-mantle boundary after 4.5 billion years of geological evolution.

This time, their ** was successfully received by nature and was also selected as the cover article. "Thanks to the collaboration of geophysicists, geochemists and astrophysicists, we can integrate the formation and evolution of the Earth-Moon system into the framework of terrestrial planet formation throughout the inner solar system, and even provide inspiration for understanding the formation and habitability of exoterous Earths," Deng said. ”

The atmosphere of freedom allows them to dare to think and do!

Today's people don't see the ancient times and the moon, but this month once looked like the ancients. "The collapse of the heavens and the earth 4.5 billion years ago has long been hidden in the dust and smoke, and it is difficult to find a trace. However, the imagination and computing power of scientists can transcend the limitations of time and space and lead us to explore ancient ruins buried deep underground.

Previously, Deng Hongping was mainly engaged in astrophysical research, and knew little about the evolution of the earth, but was brought into the study of the formation of the moon by his colleagues by chance.

But luckily, the experts he contacted by email were willing to seriously go to those wild ideas with this ordinary graduate student. And the later cooperation with Yuan Qian also started with an email from a stranger.

This is also the most profound part of the whole research process: when young scholars put forward ideas and seek cooperation, everyone does not feel crazy, and the free and relaxed scientific research atmosphere allows them to dare to think and do!

The publication of * is not the end. The explorations of Deng Hongping, Yuan Qian and others are still continuing, in order to further enrich these theories or verify them.

*Links:

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