Shulou(Shulou.com)11/24 Report--

In the 1980s, scientists discovered two unusual structures in the deep mantle of the earth through the analysis of seismic waves. The two structures are about the size of the moon, one under the Pacific plate and the other under the opposite African continent. These two giant structures are presumed to contain more iron and are significantly denser than the surrounding mantle. The question of why there are two "iron bumps" in the mantle, which should be evenly distributed, has perplexed scientists for many years.

Since the invention of the telescope, people know far more about outer space than about the interior of the earth. After all, there is nothing in the way of looking up, just a little farther away, but it is not so easy to look down, and there seems to be no other way to see it but to drill.

Some people may think, "if you just drill a hole, how hard can it be?"

I have to say that scientists underestimated the difficulty of this task at first. As early as the last century, the former Soviet Union carried out a series of scientific drilling work. Since 1970, it has taken nearly a decade for people to break through the 10000-meter mark. Then it took about 10 years to drill to a depth of 12262 meters, and since then there has been no breakthrough, a record that remains to this day. However, this depth has not even drilled through the outermost crust of the earth, and it is really realized that "it is easy for heaven to enter the earth".

Although the thick crust prevents us from peeping into the mysteries of the deep strata, as a medium, the rock can carry vibration. So although seeing is not believing, through the analysis of seismic waves, we can at least have a general understanding of the interior of the earth. Later, sure enough, some anomalies were detected from the seismic waves.

In the 1980s, scientists found that seismic waves seemed to encounter resistance in some parts of the lower mantle and spread more slowly. Through the calculation, the resistance is mainly concentrated under the Pacific plate and the African plate. There seem to be two unusually large "patches" in these two places, which account for 8% of the entire mantle, so they are called "large low velocity bodies (LLVPs)".

Scientists have put forward several hypotheses about the origin of these two giant "plaques". First of all, these structures in the depths of the mantle must be very old in terms of time, perhaps dating back to when the earth first formed billions of years ago. At that time, the earth was still in a molten state, and the surface was covered with magma. Since the surface is lava, it is conceivable that the mantle at this time is a sea of magma. So the scientists' initial idea was that the liquid mantle was different from the surrounding area because it was mixed with some impurities. Then, as it cooled slowly, the impurities in the mantle solidified before they were fully mixed, thus forming today's "patches".

Another more mainstream view is that the formation of these two "patches" may be related to the movement of the earth's plates. In this view, these large structures do not appear so early, they were formed hundreds of millions of years ago with the drift and impact of crustal plates. When plates collide with each other, some of the crust is squeezed and sunk to the bottom of the mantle, followed by the formation of later "patches".

There is also a view that these two structures may have emerged long after the formation of the Earth, but since they appear in the lowest layer of the mantle, next to the core, they may be caused by iron in the core seeping into the mantle.

In addition to the above three guesses, there has been a bolder guess about the origin of these two "patches" in recent years-they may be relics of the extraterrestrial object Theia! Yes, it is the ancient celestial body in the "big collision" hypothesis about the birth of the moon.

About 4.5 billion years ago, about 100 million years after the formation of the solar system, a planet the size of Mars collided with the original Earth. The collision not only tilted the earth's axis of rotation, but also produced debris that directly contributed to the birth of the moon, so the planet was called "Thea" after the mother of the moon goddess.

Although there is a lot of evidence to support the "big collision" hypothesis, it still faces some challenges, the biggest of which is why the moon and Earth are strikingly similar in terms of isotopes.

Because of the influence of the solar wind, there must be some differences in the element content of different celestial bodies. The material that formed the moon is generally believed to be mainly from Theia, so the composition of the moon should be very different from that of the earth. But then all the isotope tests showed that the earth and the moon seemed to be homologous.

In order to close the loopholes in the theory, scientists later put forward many improved versions of the "big collision" hypothesis, but there is a lack of practical evidence at present. In recent years, however, some scientists have realized that the two puzzling "patches" previously found in the mantle may be the first evidence of the "big collision" hypothesis.

The latest theory is that after the collision between Theia and the early Earth, the high temperature caused by the impact directly melted Theia. So the material of Theia is fully mixed with the upper mantle of the earth in a molten state, but the lower mantle is still mainly solid, retaining the composition of the primitive earth. Then some of the mixture was thrown out, and some of it broke away from the Earth's Roche limit and slowly formed the moon, while the other part fell back to Earth. In other words, the moon may not be made up entirely of fragments of Theia as thought, but from a mixture of Earth matter.

More crucially, although Theia was melted and mixed with mantle material during the collision, it was so dense that some of the Theia material was "precipitated" to the bottom of the magmatic flow of the mantle and sealed in solid state, thus forming a huge "patch" structure. To some extent, these two large low-speed body "patches" are not actually the product of primitive Earth, but from the remains of the planet Theia.

In addition to the logic of this theory, the researchers also gave some evidence. For example, through the chemical analysis of the material ejected from the mantle to the surface layer, the elements there are indeed different from other mantles. In addition, according to the theoretical model, the researchers simulated the mantle movement by computer, and the simulation results did produce two "patches" similar to the reality.

In other words, these "patches" are strange, but they should not have any effect on us human beings, right? In fact, although they seem to lie under the deep mantle in the interior of the earth, their existence has a far-reaching impact on the whole planet.

For example, we know that most of the volcanoes in the world are distributed at the boundary of the plate. But oddly enough, there are some very active volcanoes that are not in such places. For example, the North Pacific Ocean, where the Hawaiian Islands are located, has many active volcanoes, and even many islands are formed because of undersea volcanoes. But it is located in the middle of the Pacific plate, far from the junction of the plate, how can there be so many active volcanoes?

Originally, people only know that the volcanoes here are due to the existence of a vertical upward "hot pipe" called the "mantle plume" in the mantle, along which magma from the depths of the earth emerges. It was later found that these "pipes" seemed to be located right on the edge of a large low-speed body in the Pacific Ocean. Whether the existence of "patches" leads to the emergence of "pipes", or the geological movement here is already quite intense, thus pushing the "patches" to their current position, it seems that there should be some connection between them.

However, "extraordinary views require extraordinary evidence," and follow-up researchers need to look for more evidence to support their views. For example, in addition to Earth-based exploration, sampling the interior of the moon is also crucial to solving the ultimate mystery. Finally, although the study is aimed at the earth-moon system, it is not only to understand the origin of the moon, including the earth, but also to provide clues for the future search for habitable exoplanets.

Reference:

[1] https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole

[2] https://en.wikipedia.org/wiki/Large_low-shear-velocity_provinces

[3] https://en.wikipedia.org/wiki/Theia_(planet)

[4] https://en.wikipedia.org/wiki/Giant-impact_hypothesis

[5] https://www.cas.cn/syky/202310/t20231030_4983318.shtml

[6] https://www.caltech.edu/about/news/the-remains-of-an-ancient-planet-lie-deep-within-earth

[7] https://www.sci.news/space/theia-earth-mantle-12412.html

[8] https://www.youtube.com/watch?ab_channel=caltech&v=k4dQW_fUgik

[9] https://www.bbc.com/future/article/20220510-why-are-there-continent-sized-blobs-in-the-deep-earth

[10] https://www.hou.usra.edu/meetings/lpsc2021/pdf/1980.pdf

[11] https://www.nature.com/articles/s41586-023-06589-1

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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