The origin of the moon
The history of the evolution of the moon is interesting not only in itself, but also as part of the general problem of the origin of the Earth and other planets of the solar system. Recently, we have learned a lot about the physical and chemical characteristics of the moon. These data were obtained not only from the Earth, but also with the help of spacecraft. For example, the automatic stations Surveyor-5, -6, and -7, which made a soft landing on the moon in 1967 and 1968, made it possible for the first time to determine its chemical composition. Samples of lunar rocks delivered by American astronauts under the Apollo program (1969–1972) and Soviet automatic devices of the Luna series (1970–1976) made it possible to measure their chemical and physical characteristics in detail and determine the age of the moon .
The data obtained allow us to learn a lot about the history of the moon, but the question of its origin is still controversial. There are several theories of the appearance of the moon.
The theory of Earth’s “capture” by the Moon by the Earth is popular among scientists, although at first glance it seems unlikely, since upon capture, the Moon would have to lose a large energy equal to Gm1m2 / 2c, where m1 and m2 are the masses of the Earth and the Moon, G is the gravitational constant, c – semimajor axis of the orbit (average distance between the Earth and the Moon). Cadogan (1983) advocated this assumption. However, Gold (1975) disputes the capture hypothesis on the grounds that such a process is completely implausible, although theoretically possible. Taylor (1987) considers: “The hypotheses according to which the Earth captured the already formed Moon are no longer taken seriously. Firstly, they face serious dynamic problems, and secondly, they do not explain the exotic geochemistry of the Moon.”
It is hard to imagine how the speed of the moon could decrease so much that the “capture” of its Earth was possible. But even if scientists were able to discover this mechanism, the main question – how the moon was formed before the capture – would remain open.
The theory of the “double planet” is based on the fact that the Moon was formed from a cloud of small solid particles circulating around the Earth at the last stage of its formation. Scientists suggest that these particles differed in chemical composition from the Earth and contained more water or less heavy elements such as nickel and iron. But if this were so, then the Earth – Moon system would have to have a larger specific angular momentum than this follows from the relation between mass and angular momentum for planets. It is estimated that the moon could form from such particles in a very short time – in about 80 Earth years.
They recently tried to revive the theory of separation of the Moon with the idea that the moment of inertia of the Earth decreased when its substance was divided into a metal core and silicate mantle; rotation speed increased from this, which caused a part of the substance to break away as an independent body. But all the same, this requires a high initial speed of rotation of the Earth, so that the giant energy of rotation then passes into the heat of the earth’s interior, and most of the moment would be carried away from the Earth-Moon system, for example, by ejection of a significant mass. The problems associated with the conservation of energy and angular momentum make the theory of separation of the moon from the earth unlikely. Recent chemical data, especially regarding iron and rare earth elements, have shown that the composition of the lunar surface is significantly different from the earth’s surface. Therefore, the theory of separation is not being considered seriously.
According to another theory, at first several small moons were captured, and later a modern moon was formed from them. Only after this did tidal effects begin to play a noticeable role, so small satellites could be near the Earth for a long time. The destructive capture, as a result of which the Moon was literally “torn apart” when it passed near the Earth, could explain its loss of the original iron. On the other hand, a capture during a collision could explain the relatively late bombardment of the moon. In this case, an excess of energy was consumed in collisions with small moons, and the bombardment occurred when the moon, moving away from the Earth, met with the remaining satellites.
According to available data, it can be assumed that the Earth was formed with a rotation period of about 10 hours, which gave it a large specific angular momentum. One Moon (or several moons) was captured by the Earth; this Moon (or moons), turning around the Earth, attached other bodies to itself, and some threw it from near-Earth orbit into the near-solar one. In this case, the Moon turned in a forward direction in an orbit with the main semiaxis of about 40 Earth radii, which did not lie in the plane of the equator of the Earth. The rapid removal of the Moon from the Earth was to begin only in the recent geological past, when the oceans and the continental shelf became powerful enough to increase tidal friction.