I. Formation and evolution of the solar system
According to the research results of astronomy and geology at present, there are two hypotheses about the formation of the solar system, namely the nebula hypothesis and the Big Bang hypothesis.
1, nebula hypothesis
This hypothesis was first put forward by Kant in 1755 and Laplace in 1796. According to this theory, the solar system was formed by the collapse of a huge molecular cloud 4.6 billion years ago.
2. The Big Bang Hypothesis
According to this theory, during the Big Bang, the explosion of a black hole led to the fission reaction of its inner core and outer shell materials in a strong explosion. In the process of fission, air masses containing a large number of fusion products are produced. The fusion products in these air masses reach a certain amount, and the volume and internal pressure of the air masses reach a certain level, so the nuclear fusion of air masses is produced. The time dimension was determined 4,567.2 million 600,000 years ago.
According to the above theory, it is inferred that the solar system was formed about 4.6 billion years ago.
Second, the analysis of the formation time of the earth
1, a direct measurement of the earth's age.
Isotope dating is the most effective method to directly determine the age of the earth, the best method to determine the age of the earth, and the standard clock for calculating the history of the earth. There are many methods for isotopic dating. The main method to determine the time dimension of billions of years is uranium (U)- lead (Pb) method. Both U238 and U235 naturally decay and release alpha particles. The half-life of U238 is 4.5 billion years, that is, 1 g U238 will remain 0.5 g after 4.5 billion years, and only 0.25 g after 9 billion years. U238 decays into U234, and then decays into isotope Pb206 through a series of by-products. Similarly, as a result of this radioactive decay, U235 decays into Pb207, Th232 decays into Pb208, and uranium-containing materials accumulate lead continuously. Zircon is a natural mineral containing this uranium (U) element in nature, so this method is often called zircon dating.
The world's best dating samples have been tested for 4.2 billion years, and rock samples were collected from Australia. But only common sense can tell us that the actual age of the earth must be greater than 4.2 billion years, but this age can be used as the lower limit of the age of the earth.
2. Indirect measurement of the Earth's age
Because the earth has been undergoing geological evolution since its formation, various volcanic activities and crustal movements are quite intense and frequent, it is almost impossible to find an undisturbed rock specimen, which makes it difficult to directly determine the age of the earth, but scientists have thought of another method to indirectly determine the age of the earth, that is, the moon, the neighbor of the earth.
The only natural satellite on the earth-the moon
Since its formation, the moon has been relatively stable, without volcanic activity and crustal movement, and has not been affected by water erosion and air elegance. Except for the meteorite impact, its original parent rock is well preserved. After Apollo successfully landed on the moon, astronauts brought back lunar rocks, which provided basic samples for determining the age of the moon. Rb-Sr isochron method shows that the oldest rocks on the surface of the moon are 4.52-4.60 billion years old and the dust is 4.6 billion years old.
Third, summary.
Combining different methods to determine the age of the earth is about 4.5-4.6 billion years, which is currently recognized as the age of the earth, but the age of the earth as a discipline is far from writing down its full stop. I believe that with the development of science and technology, the age of the earth will become more and more accurate.