1. Split theory
Earth-moon split theory
[4] The theory of tidal splitting of the moon is one of the famous hypotheses in the study of the origin of the moon. The theory of tidal splitting of the moon insists that the moon is the biological daughter of the earth, that is, the moon is split from the earth. Scientists who insist on this hypothesis believe that in the early days of the earth's formation, the earth was in a molten state. Even if only the angular momentum of the earth and the moon is considered, the rotation of the earth is unstable due to tidal vibration. At that time, the earth's rotation period was only four hours, and with the tidal action of the sun, the earth's rotation period could be shortened to two hours. Therefore, it is reasonable to think that in the early days of the earth's history, the earth's rotation speed was very fast, and its rotation rate was much higher than now. If the early earth was in a molten state, there would be an expansion zone on the equatorial plane of the earth's material, so that part of the melt on the equatorial plane would be separated. In other words, this part of molten material was thrown from the equatorial region under the condition of high-speed rotation of the earth, and the thrown material condensed in the interplanetary space near the earth and condensed to form the moon. Some people who hold this hypothesis also believe that the Pacific Ocean on the earth is a "scar" left by the split of the moon. Because this hypothesis puts forward that the moon is separated from the earth, it is figuratively compared to the "mother-daughter theory". However, because this assumption is different from the basic characteristics of earth-moon system.
2. Homology theory
Earth-moon homology theory
[6] Like the capture theory, the split theory and the collision theory, the moon homology theory is also one of the famous hypotheses in the study of the origin of the moon. The homologous theory of the origin of the moon firmly believes that the moon and the earth are sisters or brothers, and they were "born" at the same time during the condensation of the solar nebula, or both the earth and the moon were formed in the same area of the nebula. Scientists who advocate this hypothesis believe that in the primitive solar nebula, the temperature and chemical composition depend on the distance from the sun. Planets in the solar system are formed by condensation and accretion of nebula substances with different chemical compositions in different areas of the nebula. The moon and the earth are close to each other in the solar nebula, and the formation process is similar, belonging to the "brothers" formed at the same time. As for the difference in composition between the earth and the moon, they explained that when planets are formed, they first condense and accrete, forming a planetary core with iron as the main component. After the metal core grows further, the remaining nonmetallic substances in the nebula condense, and the moon is composed of the remaining substances after the formation of the earth. Homologous theory tries to explain the composition differences between the Earth and the Moon and the composition of the Moon's core, mantle and shell, but its model is not consistent with the condensation process of the solar nebula and the motion characteristics of earth-moon system. Therefore, this assumption is not satisfactory.
3. Capture theory
Earth-moon capture theory
[8] The theory of lunar capture holds that the moon is the "daughter" snatched by the earth, that is, the earth and the moon are formed by substances that do not belong to the same nebula. Due to the change of the Earth-Moon orbit, in the range of radius of the earth 1 ~ 10, the alien moon was captured by the strong gravity of the earth when it flew close to the earth, and eventually became a satellite orbiting the earth. Scientists who advocate the capture theory believe that the earth and the moon are located in different parts of the solar nebula and are condensed from nebula materials with different chemical compositions. The original orbit of the moon intersects with the orbital plane of the earth at a small angle (about 5 degrees). When the moon moves near the earth, it may be captured by the earth and become an earth satellite within the radius of the earth 10. Alfin, a famous astronomer, believes that the moon was once an independent planet. When the moon was captured by the earth, its distance from the earth was about 26 times that of radius of the earth, and its angle with the earth plane was 149 degrees. If the moon enters the roche limit of the earth, the tides will produce a strong uneven gravity field, and the rocks on the surface of the moon will be broken and enter the orbital space where the moon runs, and most of the debris will return to the moon and hit it. There are a large number of moon-sea basins on the surface of the moon. 3.9 billion years ago, the event of digging people in the front of the moon may be an important evidence of the capture theory. Through careful calculation of the orbit of the Earth and the data of laser ranging, it is shown that the orbit of the Moon is farther and farther away from the Earth, and it retreats by about 3.8 cm every year. However, the capture theory can only explain some observed facts, which is not satisfactory. Therefore, people are constantly looking for new methods and putting forward new assumptions.
4 impact theory
Impact theory
Hypotheses about the origin of the moon, such as splitting theory, homology theory and floating theory, can only explain some observed facts and are not satisfactory. Therefore, scientists have been trying to find another way and put forward new hypotheses. Among them, the collision origin theory put forward in the mid-1980s has aroused great concern, which can explain more observational facts and is a reasonable hypothesis about the origin of the moon at present. The collision origin theory, also known as the "big collision split theory", holds that the earth was hit by a Mars-sized celestial body in the early days, and the impact debris (that is, a part of the silicate mantle of two celestial bodies) eventually formed the moon. According to the collision origin theory, there were a large number of nebulae in interplanetary space in the early stage of the formation of the solar system, which gradually increased after collision and accretion. It's about the existence of earth-moon system. A "protoearth" with a mass equivalent to 9/ 10 of the present mass of the earth and another celestial body "protomoon" with the size of Mars have been formed. In the course of their respective evolution, these two celestial bodies have formed a metal core dominated by iron and a mantle and shell composed of silicate. Because the two celestial bodies are not far apart, there is a chance of collision. The violent collision not only tilted the rotation of the original earth, but also tilted the original moon. The expanding gas "wrapped" dust and a small amount of mantle material and flew away from the original moon. The separated metal core slows down due to the obstruction of the expanding gas, and is absorbed by the "primitive earth" and becomes a part of the earth. The scattered gas and dust are distributed in the space outside roche limit in the form of a disk under the action of the earth's gravity. They first formed some small celestial bodies through accretion, and then accumulated and grew like snowballs. Finally, the present moon was formed. The theory of impact causes can reasonably explain the basic characteristics of earth-moon system. For example, the tilt and acceleration of the earth's rotation axis, the inconsistency between the moon's orbit and the equatorial plane of the earth, the moon is the only satellite of the sun with the mass ratio of 1/8 1, the moon is rich in refractory elements but lacks volatile elements and iron-affinity elements, the density of the moon is lower than that of the earth, and there is a large area of melting and magma ocean in the early stage of the moon's formation. Therefore, the theory of impact origin is a more reasonable and mature theory of the origin of the moon, which has been gradually supported by most scholars. [11] [8] Moon-orbiting spacecraft Smart- 1.
[12] In 2006, the lunar orbiter Smart- 1 of the European Space Agency completed the determination of the chemical composition of the lunar surface. The results show that the surface of the moon contains some chemical elements, including calcium and magnesium. It is always said that the moon was formed by the impact of a part of the earth. This discovery provides strong evidence for the "collision split theory" of the origin of the moon. In the late 1960s and early 1970s, American astronaut Apollo brought back lunar rock samples during his mission to the moon. Since then, planetary scientists have been surprised to find that these lunar rocks are very similar to those found in the deep mantle region of the earth. When more scientists carefully observe the moon rocks, the question of the origin of the moon becomes more suspenseful. Scientists have found that there is still a big difference between the moon and the rocks deep in the earth. Most importantly, the isotopes in the moon rocks are not consistent with those in the earth rocks. According to some scientists' speculation, in the early days of the earth's formation, a planet the size of Mars hit the earth, and the debris caused by it later gathered to form the moon. If so, the iron content of the moon will be lower than that of the earth. However, the content of light elements such as magnesium and aluminum is higher. Another theory about the origin of the moon holds that the earth and the moon produced two celestial bodies from a cloud of gas and dust at the same time. However, since the Apollo program, scientists have obtained a large number of shocking photos and 382 kilograms of lunar soil and rocks. The "collision split theory" of the origin of the moon gradually gained the upper hand.
5 nuclear explosion theory
At the beginning of 20 10, two scientists from South Africa and the Netherlands put forward new theories and explanations. They think that the moon was not caused by space impact or space explosion. It was separated from the earth by its own nuclear explosion. Rob de Meyer, a scientist from university of the western cape, and WIM van Vestlen, a scientist from the Free University of Amsterdam, the Netherlands, put forward this view based on a nuclear fission theory, which was described by scientists as early as19th century. According to this theory, both the earth and the moon come from the same rotating lava in the universe. Later, part of it separated to form the present moon. However, scientists at that time could not explain how the lava that formed the moon was separated from other reasons. Two scientists believe that the lava that formed the moon was separated in a nuclear explosion on earth. In their research paper "Another Hypothesis of the Origin of the Moon", two scientists explained that if the moon was separated from the Earth due to an external impact force, it should be composed of some materials that hit celestial bodies and the Earth. They said, "The model of the evolution of the solar system shows that the chemical composition of the earth and the chemical composition of the impacting celestial body cannot be the same." But according to the detected lunar specimens, the moon is almost the same as the earth in chemical composition. This discovery shows that the separation process of the moon does not involve the impact of celestial bodies. Scientists explained in their research papers that "the more similar the chemical composition of the moon is to the earth, the greater the possibility that the moon will be directly formed in the earth's material." Therefore, scientists believe that the energy that causes the moon to separate directly from the earth and enter the orbit is generated by the supercritical reactor at the boundary of the earth's mantle. The heat generated by this reactor is enough to evaporate and eject silicate and other substances on the earth. Clay Dilloway, a scientist in the American Science and Technology New Era magazine, also supports the views of the two scientists. According to their explanation, gravity concentrates a lot of heavy metals on the surface near the equatorial plane of the earth. Such as uranium and thorium. When these heavy metals accumulate enough and the concentration is large enough, an out-of-control nuclear chain reaction will occur, which is somewhat similar to some principles of nuclear power plants. " Dilloway explained, "In this way, a naturally formed earth nuclear reactor is pushed to a supercritical level, and then it will explode. After the moon was separated from the earth, it was pushed into orbit by a huge nuclear explosion. Of course, this theory is difficult to test. However, people do know that there are nuclear reactors on the earth, and its legacy is uranium mines mined today. " Meyer and Vestlen believe that in order to prove their theory, they need to rely on future lunar exploration missions to bring material samples back to the moon.
After 4.5 billion years of evolution, the moon has become a dead celestial body with almost exhausted internal energy and stagnant internal activities, with only extremely weak lunar seismic activity. The impact of small celestial bodies and huge temperature difference are the most important geological forces on the surface of the moon, causing mechanical fragmentation of rocks, thickening of the lunar soil layer and slow flattening of the terrain. Nowadays, the surface of the moon is a lifeless world, with no atmosphere, no water, no dryness, no silence and no life.