Today, scientists can clearly describe the earth in the first tens of millions of years of the birth of the solar system: the surface of the earth was covered with magma, the earth was bombarded by meteorites from time to time, and there was no trace of oxygen in the dense atmosphere of the earth. Since then, the earth has fallen into this hellish situation several times. However, the subsequent changes of the earth and the situation of the earth's youth remain a mystery: how did the earth first form? How did the earth become a planet that can receive cattle, sheep and daisies? Where does the water that nourishes animals and plants come from? These are all hot topics in the science lab. As technology becomes more and more accurate, scientists try their best to make samples collected from volcanoes, Greenland and even comet tails speak. However, although the speed of discovery is getting faster and faster, some basic questions are still unanswered.

The origin of the earth is unknown.

The first puzzle that scientists tried to solve: the scene of the birth of the earth 4.5 billion years ago.

Now it is said that when the sun was born, the cosmic dust floating in the air mass combined into meteorites or chondrites, and then these chondrites collided countless times, and final gathering became the earth. Astronomy calls this phenomenon "accretion". But the above statement can't explain all the secrets. After analyzing the composition of more than ten tons of chondrites that fall to the earth every year, scientists found that there are some spheres made of silicate inside these chondrites. This may indicate that in the process of accretion, the interior of the meteorite itself already contains micro-planets only a few kilometers away. Therefore, the earth may not only be formed by cosmic dust around the sun.

Time required for the growth process

Until now, scientists generally believe that the accretion process lasted about 654.38 billion years. Chaussidon, a scientist at the French Center for Petrography and Geochemistry, thinks it may only take 30 million years.

Xiao Xiden said that through the observation of Mars and exoplanets, we now know that a planet is formed much faster than people think. However, there are still many controversies about this hypothesis.

The early state of the earth

If there are still many unsolved mysteries about the formation of the earth, the situation of the earth in its youth is even more a mystery. This stage refers to the 600 million years when the earth was first formed. The biggest obstacle is that at the end of this stage, the earth was bombarded by countless meteorites, and this trace can still be seen on the moon today. Described by scientists as a "late attack", it completely erased all traces of the earth's initial formation.

So, what happened before the earth?

There is a view that the earth has been in the stage of merger from formation to explosion. But another view is that at this stage, the earth has already taken its present shape, the core is surrounded by the mantle, and there may have been some continents and oceans on its surface, and there may have been life. Until the "late attack", the evolution of the earth was destroyed and the earth regressed to the stage of merger. Everything has to start over.

How is the structure of the earth formed?

How many times has the earth been bombarded by countless asteroids, and sometimes even hit by planets as big as Mars * * * to re-form the core, mantle and crust again and again?

Bilk, director of the Institute of Geophysics in Paris, believes that the earth is in a combined state due to the consequences of the drastic changes in the earth's crust. The earth is completely covered by magma with a temperature of thousands of degrees Celsius and a thickness of hundreds of kilometers. Various chemical elements are free according to their respective properties. For example, iron, a heavy metal incompatible with silicate, will flow to the center of the earth to form the core. Then, when countless asteroids ended bombing and the surface cooled again, most of the heat energy was converted into space, and the crust on the surface began to become hard. In theory, oceans and life can appear at this time, until a new round of indiscriminate bombing makes the earth fall into a state of merger again. The problem is that every bombing and collision basically erased the traces of the last time. But Bilk said that it can't be said that all traces have been erased. He and his research team proved that the earth formed a mantle 600 million years after its initial formation by analyzing neodymium found in some rocks in Greenland.

When was the earth's atmosphere formed?

One thing is certain, the atmosphere of the earth today is completely different from the original atmosphere.

Scientists believe that the earth's atmosphere at that time was similar to Venus today, and its main components were carbon dioxide, nitrogen, hydrogen, water vapor and methane. The atmospheric pressure at that time was 20 to 480 times higher than it is now. On the one hand, it is related to a large number of gases released by the mantle, which should be about 654.38+0.5 billion years after the formation of the earth; On the other hand, it is related to the release of its own gas by celestial bodies that attack the earth every day. But it is not easy to describe the history of the earth's atmosphere. The main obstacle is that at the beginning of the earth's formation, there was not enough temperature and gravity to capture all the gas components in the atmosphere, and the lighter components gradually dissipated into space.

Morera, an expert from the Institute of Geophysics, said that just as the atmosphere was bombed by asteroids and swept away by the solar wind, many things no longer existed. Therefore, the Institute of Geophysics in Paris focuses on the evolution of some rare gases such as xenon, helium, argon and neon, which have existed since the birth of the earth. Morera said that argon is very heavy and difficult to escape into space, so it has accumulated in the earth's atmosphere for 4.5 billion years. A clear study of its evolution can help us grasp the time of mantle degassing and understand the atmosphere in the first two billion years after the formation of the earth. And many major problems about the earth are concentrated in these two billion years. After that, it became clear that in the first 2.3 billion years after the formation of the earth, oxygen appeared on a large scale, and with the help of the initial photosynthesis, this phenomenon contributed to the extremely rapid development of life. Absorbed by plants and captured by silicates, the dominant carbon dioxide in the atmosphere quickly dropped from 98% to less than 1%, and the atmosphere became what we are today (today, the carbon dioxide in our atmosphere only accounts for 0.03%).

Where does the water on the earth come from?

Although this is still a difficult mystery, scientists are trying to solve it. Yavo, a researcher at the Institute of Geophysics in Paris, explained that there are about two sources of water:

On the one hand, water existed at the beginning of the earth's formation, and then it was released after the original mantle was degassed. The other part of the water comes from the universe, which was "input" to the earth by meteors, especially water-rich comets in a dense way at the early stage of the earth's formation.

In order to confirm the weight of these two origins, experts try to trace deuterium, which seems to be very abundant in comets and very rare in the mantle. Yavo said, of course, the debate about the origin of water is still inconclusive, but the data show that half of the water on the earth may come from comets.

The remaining question is, how did the ocean form and form? Falling from the sky?

Yes, from the sky. Because when the earth is in a combined state, water is hidden in the atmosphere in the form of steam. The temperature drops, water vapor condenses, and water becomes liquid, so the earth enters a continuous flood period. The sediments discovered by experts from Greenland confirmed that these oceans appeared 3.9 billion years ago, but there may be more primitive oceans, which may completely turn into steam and enter the atmosphere after the earth was bombed by meteorites. So everything has to start all over again. However, the recent discovery in Australia has caused confusion: this is the same type of zircon as silicate. Their age is determined between 4.3 billion and 4.4 billion years ago. The problem is that the formation of zircon requires sediments, that is, liquid water. Can it be said that in such an early period, that is, shortly after the formation of the earth, the ocean has been produced? This is another unsolved problem about the earth. (Paris, April 20 th)

Background information:

The World Earth Day

April 22nd is Earth Day. The theme set by China this year is "Cherish the Earth's resources, change the development mode and advocate a low-carbon life".

Earth Day originated in America. 1969 Gaylord Nelson, a senator from the Democratic Party of the United States, proposed to hold a lecture on environmental protection on the campuses of universities all over the country. Dennis hayes, a 25-year-old Harvard Law School student at that time, quickly turned Nelson's proposal into a concrete idea of launching large-scale community activities all over the United States, which was widely supported by many young students. 1On April 22nd, 970, the United States held a massive "Earth Day" for the first time. This is the first large-scale mass environmental protection movement in human history.

As the beginning of modern human environmental protection movement, "Earth Day" has promoted the establishment of environmental laws and regulations in many countries. 1On April 22nd, 990, various environmental protection publicity activities were held in more than 40 countries and more than 200 million people around the world. This activity was approved by the United Nations. Since then, April 22nd every year has been designated as "Earth Day".

The purpose of holding "Earth Day" is to arouse people's awareness of caring for the earth and protecting their homes, and to promote the coordinated development of resource exploitation and environmental protection. Since 1990s, China has held Earth Day on April 22nd every year, and determined the theme of the event according to the situation of that year. This year's theme is "Low-carbon Life", which aims to raise public awareness of the national conditions of land and resources, popularize relevant scientific and technological knowledge, and guide the whole society to actively participate in the practice of saving resources, reducing carbon emissions and promoting the transformation of economic development mode with the help of the Earth Day activity platform. (Xinhua News Agency reporter Gu Dewei)