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2025-01-18 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >
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Shulou(Shulou.com)11/24 Report--
Original title: "[cosmic gossip] where did the moon come from? The mysterious far side of the moon (text version) "
Where did the moon come from? Mercury, Venus and Mars, which are also planets in the inner solar system, why they either have no moons, or have only very small ones. Even the largest Phobos, whose average diameter is only about 20 kilometers, is too small to form a regular sphere and looks like a big potato.
Phobos and Phobos, as Earth's satellites, the moon is much larger, with a diameter of about 3500 kilometers, which is equivalent to 1 big 4 of the Earth.
Moon vs Mercury for such a big guy, its origin seems complicated and confusing. In the last century, there were mainly several guesses about the origin of the moon in the mainstream scientific community.
One is that the moon was once a part of the earth, and then the earth rotated so fast that it broke itself up. The moon was formed by a part of the earth's material thrown out at that time. This is called "separatist theory".
The other is that the moon is not a satellite of the earth, but it has become a satellite only because it is captured by the earth's gravity. This is called "capture theory".
There is also a saying that the moon was born with the earth and was formed directly from the primordial accretion disk of the solar system, just as the sun and the planets were born in the same molecular cloud. This is called accretion theory (homology theory).
Accretion theory (homology theory) compared with the above three hypotheses, the scientific community today is more likely to agree with the formation of the earth-moon system or the "Giant impact hypothesis, GIH".
The hypothesis that the primitive Earth was hit by Theia holds that a Mars-sized object called "Theia" collided with the Earth one day and a large amount of material was thrown out and moved around the Earth. Over time, these fragments slowly gathered and eventually formed the moon as it is today.
The big collision theory, but after analyzing the isotopes of lunar rock samples brought back by Apollo, scientists found that lunar and earth rocks are strikingly similar in terms of isotopes, which means that the moon and the earth have the same composition. But this brings a problem: because the element content of the celestial body itself will be affected by the solar wind, if the Theia were alien, it would have been very different from the position of the sun and the earth in the first place. so it shouldn't be so similar to the earth in terms of isotopes. This has also become one of the main problems encountered in the big collision hypothesis.
In an article published last month (October 2022) in the Astrophysical Journal KuaiBao, researchers gave different details of the "collision between heaven and earth" between Theia and Earth. They have simulated hundreds of collisions by computer, and this one pays more attention to the details of the collision than previous ones.
The researchers found that after the big collision, the material thrown out may not have become planetary rings, but formed two relatively independent parts, one closer to Earth and the other relatively far away. Because it exceeds the Roche limit of the earth, the distant one will not be torn apart by the earth's gravity, and over time it will form the later moon, while the relatively recent one will be torn apart by the earth's gravity and gradually fall back to the earth. Since most of the material that formed the moon came from the earth, this may explain the isotope identity in the big collision hypothesis.
Two large chunks of matter were produced after the impact of Theia, and the farther one formed the details of the moon about the big collision, and we can also find clues from another place, that is, the far side of the moon.
We know that because the moon is locked by the tide, it always has only one face facing the earth. The side facing the earth is called the front, and the back to us is called the back.
Before man launched the lunar probe, we always thought that the front and back of the moon should be about the same. But when they first saw the photos sent back by the probe, astronomers were surprised to find that the far side of the moon was so different. Unlike large basins on the front, the far side of the moon is a rugged plateau.
Through the probe's gravitational measurements on the front (left) and back (right) of the moon, astronomers analyzed the mass distribution inside the moon: the average thickness of the lunar crust on the front is about 30 kilometers, while that on the back is 50 kilometers.
In addition, through the analysis of lunar meteorites, it is also found that there are great differences in chemical composition between the front and the back.
There are signs that the moon has become a "double-dealing" must be closely related to its original formation process. These differences are also where the big collision hypothesis needs to be further explained and improved.
In fact, there are already some upgraded theories based on the big collision hypothesis.
For example, there is a theory called "front heating theory (Nearside heat theory)". It is said that after the big collision, the moon and the earth are in a state of melting, especially the earth, which is like a super big stove, burning the "little coal ash" of the nearby moon red. However, the far side of the moon cools faster because it is facing the earth, thus forming a thicker lunar crust.
The side of the moon facing the earth cools faster, and you might say: is the amount of heat on the earth too insignificant for the distance between the earth and the moon?
The actual distance between the earth and the moon is indeed, in terms of today's distance between the earth and the moon, it is really impossible for the earth to transfer so much heat to the moon. But current research suggests that the moon is moving away from the earth at a rate of 3.83 centimeters a year. In other words, at the beginning of the formation of the earth-moon system, the two were once very close, even only one or two times the diameter of the earth. In such a close situation, the front of the moon can receive enough heat, and the volcanic activity on the surface is more intense, which is why we see a large area of lava plain on the front today.
But there are still some problems with this hypothesis. for example, if the two had been so close, the moon would not have moved away from the earth slowly. Instead, it would have gradually approached the earth and eventually hit it.
Another updated version of the big collision hypothesis does provide a "Second impact" hypothesis.
It is believed that after the first big collision in Theia, another small celestial body crashed head-on into the moon, which is still in a molten state. Then the spattered material moves to the back and precipitates, forming the plateau topography. The front that bears the impact, due to the melting of the surface, leads to differences in chemical composition.
But there are also problems with this conjecture. For example, if the impact had occurred in the early stage of melting, the impact basin on the front of the moon would have been flattened long ago and could not have been preserved until now.
Similar to the secondary impact hypothesis, there is another hypothesis that when a big collision occurs, there are not only one satellite, but two satellites formed by the debris around the earth. Because of the difference in the speed of the two satellites, the smaller one rear-ended the other big one. So similar to the second impact hypothesis, fragments from the small satellite splashed down on the other side of the big satellite after the collision, and the two later merged to form the final moon.
Whatever the hypothesis, there is still a lack of practical evidence for the differences between the opposite side of the moon and the origin. Now, however, with the development of national lunar exploration programs, it will bring new evidence for the study of lunar evolution.
For example, two years ago, China's Chang'e-5 brought back 1.73 kilograms of lunar samples from the moon. Based on the sample, scientists published three papers in the journal Nature, pushing back the duration of lunar volcanic activity by 2 billion years, refreshing man's understanding of the evolution of the moon. But why the interior of the tiny moon cools so slowly has become a new problem for academia.
But it wasn't long before the problem was solved by Chinese scientists. After analyzing lunar basalt samples, the researchers found that there were more calcium and titanium than expected. Because the basalt is formed by the cooling of the magma on the moon's surface, this means that the melting point of the mantle is greatly reduced because the magma contains more calcium and titanium as it goes deep into the mantle.
Not only in China, NASA is now planning to re-study the lunar samples brought back by Apollo that have been sealed for 50 years. It is believed that there will be more and more similar studies on the evolution of the moon in the future.
However, at present, all we have obtained are only samples from the front of the moon. In order to thoroughly solve the mystery of the origin of the moon, it is necessary to analyze the samples on the other side of the moon. In this regard, China's Chang'e-4 has become the first probe to land on the far side of the moon, and the subsequent Chang'e-6 is scheduled to complete the task of sampling and returning to the far side of the moon. It is believed that by then, we will have a better understanding of the truth about the origin of the moon.
This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo
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