Soul-searching question: How do scientists know that the Earth has a core and mantle?

The innermost part of the Earth is called the core, which is divided into a solid inner core and a liquid outer core. The outer core is surrounded by the mantle, and above the mantle is the crust, on which we humans live. This is basically scientific common sense today.

The radius of the Earth is more than 6,300 kilometers thick. Compared to the Earth, the crust is very thin, with a thickness of about 5 to 70 kilometers. However, it should be noted that the deepest point humans have drilled so far, such as some oil and gas exploration drilling, is only about 14 kilometers, and it does not penetrate the crust. The deepest point humans have ever reached is the Mponeng gold mine in South Africa, which is only about 4,300 meters deep.

Since humans have never been able to penetrate the Earth's core, or even the Earth's crust, how do we know about the Earth's internal structure?

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This question has troubled people for a long time. At first, some people believed that the earth was hollow. For example, Halley, the famous scientist who discovered Halley's Comet, believed that the earth was a planet with three shells, the outermost shell was 500 miles deep, each shell was concentric, and they were separated from each other by the atmosphere, and each shell rotated at a different speed. The great mathematician Euler believed that there were no multiple shells inside the earth, but a small sun with a diameter of 600 miles to illuminate the inner civilization.

But speculation is speculation, no one can provide scientific evidence about the internal structure of the earth. While people were still discussing the topic of the hollow earth theory, the famous meteorologist and seismologist Mohorovich indirectly discovered the secret of the underground through the study of "seismic waves".

Seismic waves have different propagation speeds in different media. Just like light waves, they will be refracted and reflected when passing through different media. Such medium interfaces are called "wave velocity discontinuities". Simply put, the speed of seismic waves will be different when passing through different wave velocity discontinuities. Scientists can record seismic waves through instruments. When seismic waves pass through a medium with a faster wave velocity, they are recorded earlier by the seismograph; conversely, they are recorded later. Therefore, we can infer the structure of the underground medium based on this difference in time.

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In 1911, the Zagreb Meteorological Observatory where Mohorovic worked recorded a destructive earthquake in the Kuleba Valley of the Balkan Peninsula. He noticed that some seismic waves arrived at the observatory earlier than expected. After research, he found that the speed of seismic waves changed significantly when they propagated about 50 kilometers underground. The speed of longitudinal waves suddenly increased from about 7.0 kilometers per second to about 8.1 kilometers per second, and the speed of transverse waves suddenly increased from 4.2 kilometers per second to 4.4 kilometers per second. So Mohorovic believed that there was a discontinuity 50 kilometers inside the earth.

So far, we have identified two main first-order wave velocity discontinuities inside the earth through seismic waves. They are the Mohorovic discontinuity (Moho for short) and the Gutenberg discontinuity. The part above the Moho is called the crust, the part between the Gutenberg discontinuity and the Moho is called the mantle, and the part from the Gutenberg discontinuity to the center of the earth is called the core.

Source: CCTV Science and Education

Why is the hollow earth theory most likely wrong? Because seismic waves have basically no speed in the air, but so far, scientists have not found a large area of ​​space underground where the speed of seismic waves is extremely low. In addition, due to the high temperature and high pressure environment underground, it is also difficult to find a material that can support a huge hollow underground. Stories about exploring the center of the earth, such as Journey to the Center of the Earth, can only exist in science fiction novels.

You know, the temperature at the center of the earth is more than 6,000 degrees Celsius, and the pressure can reach 360,000 megapascals. Human beings simply cannot survive under such conditions. Therefore, for humans at present, entering the earth is much more difficult than going to the sky.

Source: CCTV Science and Education

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