There are only 200 meteorites on the earth, but as many as 300,000 on the moon. Why?

The moon and the earth are closer to the space position, but there are only more than 200 earth meteor pits that are currently observed, and there are as many as 300,000 lunar meteorites. Why is this? In order to quickly find the meteorite pit, we locked the "DNA" of these meteorites ...

Produced: Gezhi Tao Dao Dao Dao Dao Tao

The following content is Chen Ming, a researcher at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

Hello everyone! I am Chen Ming, from the Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences, and is engaged in planet geological research. Today, I want to share with you some topics related to the earth meteor pit.

Why are there so few Chinese meteorite ponds?

Since the formation of our earth, it has been hit by countless small celestial bodies. When these small celestial bodies outside the ground pass through the atmosphere and hit the surface at a high speed, it can cause explosion and eventually form a crater.

▲ Left: Extra little celestial body

Middle: Planet impact

Right: explosion into pit

If the small celestial body is severely decelerated after entering the atmosphere, it finally falls to the earth. Even if it hit a small pothot, it is generally not called a meteorite pit. Therefore, the meteorcin must be formed by the explosion.

▲ Moon Apollo AS16 (1972)

Speaking of meteorite pits, everyone may be more familiar with the moon meteorite pit. The meteorite pit on the surface of the moon is dense and layered, and the meteorite pits of different sizes are covered with the entire surface of the moon.

Earth and the moon constitute a monthly system, that is, the position of their two in space is close. It stands to reason that the density of meteoritic pits on the earth should be closer to the moon, but there are only more than 200 meteorites on the earth that are currently observed, and as many as 300,000 meteorites that have been observed on the moon, up to 1,000 meters of meteoritic pits are up to 300,000 Essence

So what is the reason for this obvious difference? Now scientists are already very clear, because the geological history and evolution of the earth and the moon are different. As early as 3 billion years ago, the strong geological activities of the moon were basically stopped. We can call it a "dead" planet.

And our earth is different. Since the formation of the earth, strong geological activities have not stopped, which is related to its internal structure. We know that the earth has a nucleus, a mantle, and a crust, and they are constantly moving.

The strong geological activity allows the construction of sectors on the surface of the earth, mountain -building movement, volcanic eruption, glacier movement, etc.; In addition, there are our common weathering and erosion phenomena, deposition of depression, etc. , Erosion, even buried. Because of this, the number of meteorites retained on the surface of the earth is relatively scarce.

▲ Tenourmer 1.9km, 1.57ma

Kara-Kul 52km, 5MA

Australia Gosses Bluff 22km, 142MA

South Africa VREDEFORT 160 KM, 2000MA

We can look at the situation of the meteorite pit that currently found on the earth. The shape of some meteorite pits is perfect, round, and ring -shaped. But this kind of meteorite is often formed by a small meteorite impact. How small is the so -called "small"? For example, the diameter is less than a few kilometers. And these meteorite pits are very young, like "yesterday" that it was formed.

The formation of many meteorites on the planet has a long history, and now it is found that the oldest is 2.6 billion years. After the years on the earth, most of our cavities were severely eroded and destroyed.

As can be seen from the figure above, some meteorite pits seem to be round, but it is faint and not obvious. After all, it has been strongly destroyed. This is the basic state of our earth meteor pit. Therefore, most of the meteorite pit does not look like a pit.

▲ The meteorite pit distribution on the earth

The red dots in this picture show the distribution of meteoritic pits that are now confirmed on the earth. Where is the meteorite pit in the earth? North America, South America, Northern Europe, Africa, and Australia. The areas where these meteorcings are located are relatively stable places in crustal activities, that is, the stable area of ​​the crust.

It can also be seen in this distributed map that there are relatively few meteorites in Asia. Geological activities in Asia are relatively fierce, which is closely related to the camping pit we found. If you look at the Chinese area, there are only two meteorites on our territory.

▲ The area of ​​mountain building accounts for 3/5 of the country's total land area

We can analyze from two aspects. The first is that there are mountain -making belts in China. The Emmihara and Qinghai -Tibet Plateau are mountain -making belts. The southeast coast is a mountain zone. Tianshan to Daxing'an Ridge is also a mountain belt. There are also the Central China area. The area of ​​our Chinese mountain belt is nearly 60%of the national land area, that is, even if these areas have formed meteorites, it is easily eroded or eroded. Therefore, the original meteorite pit was basically eroded, or it was difficult to distinguish it.

▲ The area of ​​the first Cambrian of the front Cambrian on the surface accounted for only 7.8 %

Secondly, the rock formation in China is generally younger. More than 90%of the Chinese land is a rock formation formed after the universe, and the show is the geological stage within 540 million years. What is the problem? The meteorite pit formed in the long history of the earth was basically damaged or buried. For example, the youngest desert area and loess area of ​​China's fourth year reached hundreds of thousands of square kilometers. Therefore, China's geographical and geological conditions are not conducive to the preservation of meteorite ponds, and of course it is not conducive to the discovery of meteorite ponds. Therefore, it is difficult to find meteorite pit in China. This is why China has only two meteorite pit so far to obtain international certification.

Five evidence of the meteorite pit

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Affected by geological effects, there are many geological phenomena on the surface of the earth, such as volcanic mouth, collapse, back sloping, and oblique. These complex geological phenomena may cause ring -shaped terrain. How to find meteorites in this complex earth geology? This has raised a very important problem for scientists.

After more than 100 years of research, there have been relatively clear answers: Scientists mainly judge whether it is a meteorite pit according to the following five aspects.

▲ ring geological structure

First, is it a ring structure. When the meteorite was just formed, it must be a ring -shaped structure. However, there are also many things on the earth. The crater and collapse are round. These situations may blur our sight. At the same time, the surface of the earth will be strongly eroded and eroded. Even if some meteorite pits have just begun to have a ring structure, it is difficult to find it after being eroded. But the ring structure is still a very important basis for judgment.

▲ Corllars and lava

Second, there are corner arms and lava. We know that the meteorite pit is a pit formed on the surface by ultra -high -speed impact on the outside celestial body. The rocks in this place will be crushed after being strongly hit, so we will go to the corner gravel formed by the crushed stones. However, under the geological action of the earth itself, a large amount of corner can be formed. So you ca n’t see a bunch of corner clams, you think it is a meteorite pit, and it has other causes.

▲ Meteorite residue body

Third, whether there is a meteorite. Someone asked, how can there be a meteorite pit without meteorite? In fact, this is a question of our Chinese usage. The meteorite pit is generally called the "impact pit" academically. In fact, there is not necessarily meteorite in the meteorite pit.

When we find meteorites in one place, we must first judge whether it was generated before the impact, did it come during the impact, or did it fall after the impact or after the pit? In fact, due to the role of the earth's atmosphere, the small meteorite may be residial after falling, but through the impact with the surface, it will have a severe explosion. For example, a 6 -meter -diameter meteor pit, the energy generated by the explosion is equal to a atomic bomb; a 1.2 -kilometer meteor pit, the explosion equivalent is equal to the energy of hundreds of atomic bombs. Under such energy, it is difficult to find meteorites, and most of them have been gasified. Therefore, in general, there is no need to go to the meteorite for most of the meteorites with a diameter of more than 1.2 kilometers.

Fourth, meteorite elements and geomyogenic abnormalities of isotopes. Meteorites can bring some special elements, but we also noticed that some chemical elements on the earth also produce similar phenomena. Some elements and isotochemical earth chemical abnormalities can cause us to make errors in distinguishing.

Fifth, impact deterioration. What is impact deterioration? The pressure brought by the Xiaotian body impact on the surface can reach millions of atmospheric pressure. The high temperature and high pressure generated by the impact can cause physical and chemical changes in rocks in a large area. This change will cause damage in rocks and minerals. Even if the shape of the meteorite is gone, these injuries may be retained.

In fact, in these five types of standards, the first four types of standards generally have the multi -solution of the cause, so it is not possible to see these things as meteoritic pits. However, the fifth type of impact deterioration is only possible in the meteorite pit, so it is a "DNA evidence" for meteorite pits. We can find it to determine whether it is a meteorite pit.

Find China's meteorite

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So how do we find? Let ’s take two meteorite pits as an example to introduce you.

This is the Xiuyan meteorite pit, located in Liaoning, China. This is a meteorite pit with a diameter of 1.8 kilometers, forming 53,000 years ago.

Look, the shape of this meteorite pit is not round? Yuan, but its form is more like a pentagon in aviation photos, which means that it has been strongly eroded, although it has only a little more than 50,000 years old. So judging according to the form, it gives us an impression that looks like a meteorite pit. We once worked hard to find meteorites, but could not be found.

So how to judge? The only way we have is to find stones in the mountains and find those stones that have been hit strongly. Due to the strong weathering on the surface, we needed to drill under the surface to extract stones. In the end, we found some strong impact on metamorphic rocks.

▲ Packed Clamp Gravation

This photo is displayed by drilling, a hit corner gravel extracted from four or five hundred meters below the surface. Through modern microstructure analysis technology, such as electronic microscope, we found some evidence of mineral physical and chemical changes caused by shock waves. ▲ Ultra -high -pressure minerals

The first is called ultra -high -pressure mineral. What is ultra -high -pressure mineral? Under certain temperature and pressure conditions, the surface of the surface is stable; if the conditions of pressure and temperature change, it will become another high -density mineral.

For example, at ultra -high pressure, a silica will form a mineral called Ke quartz. This mineral can only appear only hundreds of kilometers deep in the earth. Now it appears on the surface, indicating that the surface is over -pressure. Therefore, ultra -high -pressure minerals are the basis for judging the meteorite pit.

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Second, under the strong effect of shock waves, special deformation will occur, such as generating surface -like deformation pages. This page is very special. Like a knife cut, this phenomenon cannot be found on ordinary earth rocks. Only by shock wave can this phenomenon.

▲ Mineral shock glass

The third is the glassization of mineral crystals. The shock wave effect can make the atomic dot matrix in the crystal instantly disrupted, and the shape of the mineral can be saved, but the inside is no longer the crystal structure, and it becomes a vitreous.

The above three criteria are the three types of diagnostic evidence of the international scientific community that judge the meteorite pit. As long as one of them can be found, it is certain that it is the meteorite pit. If you can't find these diagnostic evidence, even if you have a beautiful meteorite form, or even if you find meteorites, you have to demonstrate. This is academic rules.

The same situation is the same situation, it is a crater that we just confirmed in 2021. This meteorite pit is located in the southeast of Xiaoxing'an Ridge, China, and is located in a forest cover area. Its diameter is 1.85 kilometers, which is slightly larger than the Xiuyan meteorite pit. No meteorite is found in this meteorite pit.

The base rocks in this area are granite. We still collect some evidence of impacting metamorphic granite by collecting and drilling on the surface.

▲ Left: Ultra -high pressure mineral

Upper right: mineral shock surface

Middle right: Form deformation page theory

The bottom right: mineral shock glass

The picture on the left shows the changes that occur under the effect of hundreds of thousands of atmospheric pressure and thousands of degrees of temperature and pressure. In the impact of metamorphic granite, we found that ultra -high -pressure minerals, mineral shock surface deformation pages and mineral shock glass were found, and several sets of meteoritic pit diagnostic evidence was found internationally, which was confirmed internationally.

Why study meteoritic ponds?

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Finally, let's talk about the research significance of the meteorite pit.

The meteorite pit is a cosmic geological wonders, and the meteorite on the earth is the same geological phenomenon, formation mechanism, and characteristics. Then we can stand on the earth and watch other planets, and we can experience it in the realm.

Therefore, meteorite pit is a very good science and science education base. In fact, many foreign meteorites have been developed into museums. According to the assessment of international colleagues, the two meteorites in China are also the ideal location of natural museums.

In addition, the meteorite pit is the legacy of the planet impact, which can provide important information for us to study the geological characteristics of the universe and the evolution of the earth.

The impact will produce a special environmental effect. If it is a huge impact, it may be connected with the extinction of certain creatures, such as dinosaurs. The picture above shows a small asteroid impact of 65 million years ago in the Gulf of Mexico. This incident caused dramatic global climate and environment at that time, which eventually led to dinosaurs.

There are other biological extinction incidents on the earth. Related analysis may also be related to the environmental effects caused by the huge impact of the planet.

In addition, there are mineral resources. Some meteorite pits have important economic value, such as gold, diamond, nickel ore and other important economic mineral resources. Many of them are produced from meteorite pits.

That's my report today, thank you for your listening! Thanks!

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