What is a meteorite (what is a meteorite?)
On June 30, 1908, a meteorite hit Siberia, causing damage in a radius of 40 kilometers. To commemorate this day, many countries now designate June 30th as “Meteor Observation Day” to promote the popularization of science. So what is a meteorite? Will meteorites still hit the earth?
Figure 1. Meteorites hit the Earth.
1. The concept of meteorites
Meteorites are geological samples of asteroids, planets and their satellites in the solar system. They preserve the information of all processes of solar nebula condensation, planetesimal accumulation and melting differentiation. These samples are of great significance for studying the formation and evolution of the solar system, the magma evolution of planets and asteroids, the origin of life on Earth, and the composition of deep matter.
2. Photo of meteorite GRV 022115 collected by China Polar Research Institute
According to statistics, most meteoroids will disintegrate when they enter the atmosphere. It is estimated that about 500 meteors, ranging from marbles to basketballs, still fall to the ground every year. However, usually only 5 to 10 meteors are spotted falling each year, and scientists know about them and find them. So things are rare and expensive, and meteorites are rare rocks that are worth collecting.
Figure 3, Meteorite
Second, crater
The crater is the direct evidence that the meteorite hit the earth, and it is the most common geological structure on the surface of a solid planet. Meteor craters are pits or ring-shaped geological structures formed when asteroids, meteoroids or comets hit the surface of solid planets at super high speed, and are widely developed on the surface of terrestrial planets and satellites in the solar system.
Figure 4, Meteor Crater, Arizona, USA
Meteor craters are irreplaceable in determining the surface age of stars, catastrophic events in Earth’s geological history, impact metamorphism of minerals and rocks, etc. role. The meteorite impact process is characterized by high temperature and high pressure, which can leave unique traces in mineral rocks, thus providing a basis for judging impact craters.
Figure 5. Backscattered images show the structure and composition of impacted molten veins (Si Jiaxin, 2020)
3. Will large meteorites still hit the earth?
From the photos sent back by high-powered telescopes and spacecraft, we can know that the surface of the moon has been smashed beyond recognition by meteorites and is full of craters. Venus, Mercury, and Mars are also heavily cratered. The Earth is much larger than these planets, so the probability of the Earth being hit by a meteorite should be much higher.
Figure 6. Craters on the surface of the moon.
At present, about 10,000 to 80,000 small meteorites hit the earth every year. After passing through the atmosphere, a considerable part of the meteorite that hit the earth will be melted by friction with the atmosphere. About 75% of the unmelted meteorites fall into the sea, and about 15% of meteorites hit deserts or uninhabited areas. Among them, 8% of the meteorites that rushed into residential areas were burned up due to the protective friction of the atmosphere, and only 2% of the meteorites may be noticed by humans.
Figure 7. The atmosphere would melt the meteorite.
4. Where can I find meteorites?
After the meteorite rushes through the earth’s atmosphere and rubs it, although it burns out, there will still be meteorites falling near the crater, but it is rarely found in the crater with a diameter of more than 3 kilometers. This may be due to the instantaneous high temperature and high pressure and the melting of meteorite fragments when the meteorite collided with the ground. The Antarctic ice sheet has a soilless background, and the color of the meteorite contrasts sharply with the white of the ice sheet, making it easier to spot the meteorite.
Figure 8. Chinese expedition discovers Vesta meteorite in Antarctica.
Verb (abbreviation of verb) craters in China
The study of craters in my country began in the 1990s, and a number of craters have been reported, such as Baisha crater in Hainan, Taihu Lake in Jiangsu Crater Mountain, Duolun Crater Mountain in Inner Mongolia, Hailaer Crater Mountain in Inner Mongolia, etc. However, these craters have not been widely recognized by the scientific community due to the lack of key evidence of shock metamorphism.
Figure 9. Asteroid hits Earth.
At present, there are two recognized craters in China, namely Xiuyan crater in Liaoning and Yilan crater in Heilongjiang, both of which were confirmed by Chen Ming and others through timely pdfs. Xiuyan crater and Yilan crater are simple bowl-shaped craters. The former is complete in shape and about 1,800 meters in diameter. Drilling of the Xiuyan crater shows that the filling in the crater is composed of lake sediments with a thickness of 107 m in the upper part and alluvial breccia deposits with a thickness of 188 m in the lower part.
Figure 10, Xiuyan Crater
To sum up, it is a basic natural phenomenon that many debris in the solar system hit the earth during its operation. Among them, most of the meteorites from outside the solar system were sucked away by the massive sun and Jupiter. Only 2% of meteorites that hit Earth are likely to be noticed by humans.