Meteorite: a "gift" from space

When comet Swift-Tuttle met the earth with scattered debris, the annual Perseid meteor shower arrived as scheduled. Watching the stars rain across the night sky, some people reminisce, some people make a wish, and some people worry about the falling of meteors.

Meteor is an astronomical phenomenon. There are many small celestial bodies floating in space, including asteroids, comets and their fragments. When small celestial bodies (97% are fragments of asteroids) are captured by the earth's gravity and enter the atmosphere, the heat generated by friction with the atmosphere will make them burn. Burning celestial bodies are shining in the night sky. This is the meteor we see. If these fragments don't burn out and pass through the atmosphere and fall to the earth, they are meteorites.

The probability of being hit by a meteorite is lower than winning the lottery.

When we go to the suburbs far away from the city, we can often see meteors streaking across the night sky. Apart from romantic elements, these are alien substances that have entered the earth. According to scientists' estimation, about 220,000 tons of extraterrestrial matter enters the earth's atmosphere every year in space, so it can be said that the earth is threatened by meteorite impacts all the time.

Fortunately, the earth has an atmosphere. The atmosphere is the protector of the earth. Most celestial debris was burned when it passed through the earth's atmosphere. Only a few larger fragments can pass through the atmosphere smoothly and fall to the earth. Moreover, after burning, large fragments often melt or break into small pieces and fall to the earth, greatly reducing the damage to the earth.

The atmosphere blocks most of the extraterrestrial matter, but this small part should not be underestimated when it falls to the earth. According to expert statistics, about 44 tons of meteorites fall on the earth every day. According to the number of times, in the past half century, 72 1 meteorite falls occurred on average every year in the world. Among them, 1979 and 1988 meteorites fell frequently, with more than 3000 times.

Such a high-frequency fall may seem a bit scary, but in fact, the probability of human being being hit by a meteorite is extremely small. Canadian scientists calculated the damage of meteorites to people and buildings according to the per capita area. Of the world's 6 billion people, every 10 year, only 1 person is directly killed by a meteorite with a mass of not less than100g. And only 16 houses are damaged by meteorites every year. Judging from this probability, the probability of being hit by a meteorite is much lower than the probability of winning the 5 million prize in the two-color ball.

1in March, 976, a rare meteorite shower landing occurred in Jilin province, China. A meteorite weighing several tons passed through the atmosphere and exploded and caught fire. Heavy rains fell in Yongji County and Jiaohe County of Jilin City, affecting an area of more than 500 kilometers. At least hundreds of thousands of people witnessed the burning meteorite. Miraculously, this meteorite fall did not cause one death.

20 13 February, Chelyabinsk, Russia, a meteorite weighing 18 to 30 tons burned and exploded, passing through the atmosphere, producing a lot of debris and forming the meteorite shower. According to media reports, the energy released by meteorite shower this time is about 20 times that of the atomic bomb dropped on Hiroshima in World War II, resulting in more than 1200 injuries. But none of the injured people were directly injured, and most of them were injured by buildings smashed by meteorites. No one has been directly hit in such a high-damage and large-scale meteorite shower, which shows how low the probability of being directly hit by a meteorite is.

Antarctica and desert are the most abundant areas of meteorites.

Due to the spherical characteristics of the earth, the probability that a meteorite falls from the sky and reaches all parts of the surface is random and equal. But the ocean area on the earth accounts for 70%, so according to the probability, 70% of meteorites may fall into the sea, and the remaining 30% will fall to land. On land, most of them are covered by mountains and forests, and the weathering of meteorites in humid environment makes it extremely difficult to find meteorites.

Although it is difficult to find meteorites, there are some places where the probability of finding meteorites is extremely high, that is, meteorite-rich areas. According to the information in the database of the International Meteorite Association, as of August, 20 17, 56,663 meteorites have been found and named effectively in the world. Among them, there are about 37,000 Antarctic meteorites. Among the non-Antarctic meteorites, there are about 10000 in the Sahara desert in North Africa and about 4000 in Oman in Arabian Peninsula. It can be seen that about 65% of the known meteorites in the world are found in the Antarctic ice sheet area and 25% in the desert area.

The secret of the enrichment of Antarctic meteorites began in 1969 when the Japanese discovered nine different types of meteorites in Lan Bing, Antarctica. Scientists have found that these nine meteorites have different types and different landing times. They don't belong to the same meteor shower, but a magical force brought them here from different places. This is the power of the Antarctic ice sheet.

The Antarctic ice sheet is thick in the middle and thin around. Ice flows to the surrounding ocean under the action of gravity. Frozen meteorites that land on the ice sheet flow with the ice flow. If they are blocked by the mountain, the ice will keep piling up in front of the mountain. When the ice melts, meteorites will grow out of it like crops. Therefore, the mountains around Antarctica are potential meteorite-rich areas. Moreover, dark brown meteorites are very conspicuous on the light blue ice in Antarctica and are easy to be found and collected.

At the same time, the cold and clean natural conditions in Antarctica make it a natural "ice bank", and the meteorites falling on the ice sheet are frozen and not easy to weather, and have been well preserved. Antarctic meteorites have the longest age (that is, the time they are preserved after falling on the earth's surface), which can generally reach hundreds of thousands of years, hundreds of times higher than other continental meteorites. Some polar meteorites have been discovered for 5 million years.

Although there are many Antarctic meteorites, due to the constraints of climate and environmental protection, the scale and time of Antarctic scientific research are strictly limited. Searching for meteorites in Antarctica is much more professional and difficult than in other areas. At present, there are about 1 1452 Antarctic meteorites discovered in China (data 20 12), ranking third in the world.

In desert areas, after the strong erosion of wind and sand, meteorites accumulated for a long time will gradually surface. In addition, these areas have a single background, transparent vision, dry climate, sparse vegetation and sparsely populated areas, which are areas with high efficiency for human beings to find meteorites. The Sahara region of Africa, the desert region of China and Xinjiang, the Arabian Peninsula and the desert region of southwest Australia are all rich areas of desert meteorites.

Compared with the Antarctic, the cost of searching for meteorites in the desert is much lower, not only the search area is larger, but also folk meteorite lovers can participate. In fact, most desert meteorites are collected by folk meteorite hunters, especially in the deserts of northwest Africa and Arabian Peninsula. The recycling and trading of meteorites has become an industry.

Every meteorite is unique.

Meteorites can be divided into three categories according to their composition: iron meteorites, stone meteorites and stone iron meteorites. Iron meteorites contain more than 95% iron and nickel metals, also known as iron-nickel meteorites. Stone meteorites are mainly or completely composed of silicate minerals. Stone-iron meteorite is a transitional meteorite between the first two kinds of meteorites, and its iron-nickel metal and silicate mineral content are roughly the same.

Among the meteorites falling to the earth, the largest number is stone meteorites, accounting for more than 92% of the total number of meteorites. Stone iron meteorites are rare, accounting for only 2%-4% of the total number of meteorites. The number of falling iron meteorites is also very small, accounting for 4.6% of the total number of meteorites, but iron meteorites are easier to find than the first two meteorites, and their capture rate is as high as 40%. This is because iron meteorites have high recognition in the wild; Moreover, it is not as easy to weather as a stone meteorite and lasts for a long time; Coupled with professional metal detectors, the collection of iron meteorites is much easier than other meteorites.

▲ There is an olive meteorite in the stone iron meteorite, which is no different from other ordinary meteorites in appearance, but after cutting, you can see pieces of golden olivine embedded in metal. When the light passes through, the olive meteorite is like a glowing honeycomb. Photography/photos

In addition, according to whether the meteorite matrix has undergone melting differentiation, meteorites falling on the earth can be divided into three categories: alien meteorites, incomplete alien meteorites and unclassified alien meteorites. Differentiated meteorites have undergone melting differentiation, and their parent star has a core structure; Undifferentiated meteorites have not undergone melting differentiation, and their parent star does not have a core structure; Incomplete classification of alien meteorites means partial melting differentiation.

The chondrite is the representative of undifferentiated meteorites, so it is named because it contains chondrites. The parent asteroid of chondrite has not experienced melting differentiation, and there is no obvious core and mantle structure. Its physical and chemical composition has been kept in the initial state, which can be said to be a specimen of the early evolution of the solar system. It is through the study of chondrites that scientists know that the solar system (including the earth) was formed about 4.6 billion years ago.

In addition to the meteorites mentioned above, there is a very special meteorite on the earth, that is, glass meteorite. Experts and scholars have argued for a long time about whether glass meteorites are meteorites. This is because glass meteorites are not from outer space. It was formed when the rocks on the earth melted when the alien matter hit the earth at high speed and was thrown into the air for rapid solidification.

Glass meteorites are different from other meteorites. Other meteorites fall randomly on the earth, and there is no fixed occurrence stratum and area, while glass meteorites have obvious occurrence stratum and geographical division. So far, four groups of glass meteorites have been found on the earth, which are distributed in four scattered areas: North America, Moldavia Stone, Ivory Coast and Australia-Southeast Asia.

Australia-Southeast Asia free-range area is one of the largest free-range areas, ranging from Shaanxi and Inner Mongolia in the northwest of China to southern Australia, with Leizhou Peninsula, Maoming, Wuzhou and Hainan Island in China as the main producing areas. Most of the glass meteorites scattered in Australia-Southeast Asia are ink-colored, and the ancients mistakenly thought that this meteorite was caused by lightning, so it was also called "Leigong Ink". According to experts' research, the glass meteorites in the southeast coast of China, Qiongzhou Peninsula and Hainan Island are the youngest glass meteorites in the world, which were formed about 7 1 10,000 years ago.

▲ There is a crater in Baisha Li Autonomous County, Hainan Province. Scientists have scientifically estimated the size of the "whisperd" that hit the white sand land, and think it is a meteorite with a diameter of 380 meters. The close shot in the picture is two dark ink marks of Lei Gong, and the foreground is a crater. Photography/Feng Mubo

Meteorite is the key to explore the mysteries of the universe.

Meteorite is a "gift" from space and a window for human beings to explore space. Because so far, apart from the moon and Mars, humans have not been able to directly obtain samples of other celestial bodies in the solar system. Moreover, compared with sampling on the surface of the landing planet, the scope of meteorites is wider, and studying the landing of meteorites on the surface has become the most convenient and lowest-cost way for human beings to explore the universe.

According to research, most meteorites that fall on the earth come from the asteroid belt between Mars and Jupiter, such as iron meteorites. It is found that almost all the iron meteorites found on the earth come from more than a dozen ancient asteroids. The impact of celestial bodies shattered these asteroids, and the core of the star was crushed and cooled to form high-purity iron-nickel alloy, and some metal fragments fell to the surface to become iron meteorites. It can be said that the iron meteorite is the metal core of the asteroid matrix. In addition, the internal structure of the parent asteroids of these iron meteorites is similar to that of the Dachuan Earth, so the study of iron meteorites plays an important role in human cognition of the core.

1989 The Xinjiang iron meteorite (arman Ti) found in the eastern part of Altay, Xinjiang, weighing 28 tons, is the fourth largest iron meteorite in the world. In 2004, Ullas Terrace, an iron meteorite weighing 430 kg, was discovered in this area. In 20 1 1 year, scientists discovered a 5-ton iron meteorite-Wu in the canyon in the northwest of Altay. Through research and comparison, it is found that these three iron meteorites are all formed by the explosion and scattering of the same parent star.

In addition to asteroids, there are a few meteorites from the moon and Mars. Moon meteorites and Martian meteorites are also meteorites that humans can really determine their mothers at present. At present, there are 360 moon meteorites (including paired meteorites) and 227 Mars meteorites (including paired meteorites) named by the International Meteorology Association, among which the most famous Mars meteorites are ALH 8400 1 and NWA 7034.

ALH 8400 1 was collected in Antarctica, and it was always mistaken for other asteroid meteorites after its discovery. It was not until the researchers noticed that its trace elements were related to Martian meteorites that it was determined that it was a Martian meteorite. NWA 7034, commonly known as "Black Beauty", was discovered in the arid desert of Morocco in 20 1 1 year. It is a kind of surface breccia with water content as high as 0.7%, which is the highest water content breccia found at present, and has important scientific significance for studying the evolution of ancient environment of Mars.

The study of meteorites is an important way to understand the origin and evolution of the solar system. Where did the earth and human beings come from? For this ultimate philosophical problem, meteorites will tell wonderful stories if they can talk.