What secrets are hidden on the back of the moon that humans will never see?

All along, we often hear all kinds of rumors about the back of the moon. Many science fiction novels claim that the back of the moon is the base of aliens, and alien spacecraft can be identified from the images on the back of the moon. Even the missing bombers of the US military appeared on the back of the moon at the end of World War II, and the missing planes and ships in Bermuda were hijacked to the back of the moon by aliens. They claim that Apollo astronauts saw amazing objects on the back of the moon, which may be the remnants of aliens on the moon, and so on. There are also rumors that the interior of the moon is hollow and there is a huge city inhabited by aliens. These rumors based on misinformation are constantly being interpreted and even published in regular publications, and it is difficult to distinguish between true and false.

The root of these legends is that when you observe the moon from the earth, you can't see most of the back of the moon. Therefore, before there was a lunar probe, the back of the moon was always a mysterious unknown world. Most of the legends about the back of the moon now come from before the spacecraft explored the moon. However, even after the spacecraft flew to the back of the moon dozens of times and got a lot of image data, science fiction fans can still find clues from the slightly blurred enlarged images to provide evidence for these rumors. Why does the moon appear front and back?

The back of the moon is mysterious because we can't directly observe the back of the moon from the earth. But why does the moon have a front and a back?

Let's talk about the movement of the moon first. The moon can be regarded as a rigid sphere, and its movement can be divided into two parts: the movement of the center of mass (revolution) and the rotation around the center of mass (rotation).

When studying the motion of the center of mass of the moon, we usually regard the moon and the earth as particles with all the mass concentrated at the center of mass, and only the attractive forces can form the earth-moon system. From the inertial coordinate system, the common center of mass of the earth-moon two-body system rotates around the sun, and the moon and the earth rotate around their common center of mass respectively. According to the theory of two-body motion, the moon moves in a plane elliptical orbit around the center of mass of the earth, and the geometric relationship between the moon equator and the earth and its orbit is shown in figure 1, in which the included angle between the moon equator and the moon orbit plane is 6 degrees 4 1 min. The average included angle between the lunar orbit and the ecliptic is 5 degrees and 9 minutes, ranging from 4 degrees and 57 minutes to 5 degrees 19 minutes.

Geometric relationship between the rotation axis and orbit of the moon around the earth

Because the earth rotates from west to east, celestial bodies all rise in the east and set in the west. The moon is no exception. What we see every day is that the moon revolves around the earth from west to east among constellations, which reflects the rotation of the earth. The time for the moon to orbit the earth once is called "sidereal moon", which lasts for 27 days, 7 hours and 43 minutes, 1 1.47 seconds. It is called "Stellar Moon" because it is calibrated by stars, that is, the time interval when the moon starts near the star and returns to the same position near the star. The average speed of the moon in its orbit around the earth is 1.02 km/s, and it moves eastward 13 degrees every day. Therefore, the moon rises at different times every day, sometimes during the day and sometimes at night. The rising time is delayed by 50 minutes every day on average compared with the previous day, and the original rising time is restored after one month.

The hemisphere of the moon facing away from the earth is far away from the earth, and the gravity of the earth is less than that of the hemisphere facing away from the earth, that is, the force on the belly and back of the moon is different. If the moon's rotation period is not equal to the period of revolution around the earth, then the earth's gravity on the same part of the moon will change with time, which will lead to friction between different strata of the moon and gradually slow down the rotation speed, and finally make the moon's rotation period just equal to its period of revolution around the earth, that is, the moon will be attracted by the earth's gravity tidal locking. After tidal locking, when you look at the moon from the earth, you can only see one side of the moon forever.

Due to the libration of the moon, a little bit of lateral back sometimes appears in the edge area of the moon disk. Generally speaking, 59% of the entire lunar surface can be observed from the earth.

The back of the moon is not mysterious. Since ground observation can't see the back of the moon, how can we see the back of the moon? What exactly is the back of the moon like? What is the difference between the back and the front?

So far, no spacecraft has landed on the back of the moon, but scientists are not short of detection data on the back of the moon. 1959, the Lunar 2 probe launched by the Soviet Union captured the image of the back of the moon for the first time when it flew over the moon. In the 1960s and 1970s, American and Soviet lunar probes obtained images of the back of the moon many times. Apollo 8 and Apollo astronauts later flew to the back and saw the desolate scene on the back of the moon with their own eyes.

Since the 20th century, lunar probes from the United States, China, Indian, Japanese and European countries have explored the moon in detail with higher resolution. The Lunar Reconnaissance Orbiter (LRO) launched on June 19, 2009 has obtained a full moon image with a resolution better than 1 m, which can clearly identify the lunar rover and the lunar module. What Apollo left on the surface of the moon, including the high-definition images on the back of the moon. Except for the lander and lunar rover that landed on the moon, most lunar probes are in polar orbit, that is, from the North Pole to the South Pole and back to the North Pole. So each flight is half on the back of the moon and half on the front.

During 12 years since 2003, 1 1 lunar probes, including Chang 'e-1, Chang 'e-2 and Chang 'e-3, flew to the moon successively, and obtained a large number of detection data on the back of the moon, including detailed information such as topography, material composition and surface environment.

Regrettably, all these missions to the moon did not find an alien base on the back of the moon, nor did they find any traces of artificial buildings or human activities. The back of the moon looks like a desolate continent that has been preserved for 4 billion years.

Generally speaking, the surface of the moon can be divided into two geographical units: the moon sea and the moon land.

Moon sea refers to a large area of black spots on the moon. In fact, it is some vast plains without a drop of water. The reflection of solar light in these areas is weak, which looks darker than the surrounding areas, much like the oceans on the earth, accounting for about 17% of the monthly surface area. There are 22 moon seas on the moon, of which 19 is distributed on the front of the moon, including rain sea, sea of tranquility, sea of clouds, Leng Hai and storm sea, accounting for about half of the surface area of the whole hemisphere, especially in the northern hemisphere. On the back of the moon, there are only the East China Sea, the Moscow Sea and the Zhihai Sea for three months.

The Moon Sea is a great plain formed after the formation of the lunar crust, where internal magma gushed out of the lunar surface and filled with low-lying basins. Most of the Moon Sea Plain is surrounded by mountains, which is similar to a basin on the earth and has the characteristics of circular closure. The most typical example is the Jade Sea surrounded by Apennines, Caucasus, Alps, Jura and Carpathians. The topography of the Moon Sea is much lower than that of the Moon. For example, Jinghai and Chenghai are about 1.700 meters lower than the average monthly horizon (the average radius is equal to the equipotential surface obtained by laser altimetry), and the wet sea is 5200 meters lower. The lowest is the southeast of Yuhai, and its deepest part is more than 6000 meters lower than the average monthly horizon. There are also some protruding mountains in the Moon Sea, including arc ridges, obliquely distributed ridges and irregular veined ridges.

In addition to the moon, the lighter part of the moon is similar to the "land" on the earth, called the moon land. According to the big collision theory of the formation of the moon, 4.6 billion years ago, a Mars-sized celestial body hit the primitive earth, and the impact projectile formed the moon in the earth orbit. After the formation of the moon, there was a global melting of magma. During the cooling process, molten magma crystallizes and differentiates. Dense matter sinks into the lunar mantle, while lighter matter floats on the lunar surface, and forms an ancient lunar shell after cooling and solidification. The Moon Sea was formed after the formation of magma ejected from cracks in the moon's crust. Therefore, the moon is older than the moon and the sea.

The lunar land is mainly composed of plagioclase with optical density, which has high reflectivity and looks brighter. The terrain of the lunar land is generally about 2~3 kilometers higher than the lunar horizon, so it is also called the lunar highland. Overall, the land area of the moon accounts for about 83% of the surface area of the moon. The land on the front of the moon is roughly equal to the sea, but the land on the back of the moon occupies an absolute dominant position.

There are many craters on the whole surface of the moon, whether it is the moon or the earth, which records the traces left by small celestial bodies hitting the moon for more than 4 billion years. The older the surface of the moon, the greater the distribution density of impact craters. The density of impact craters on the lunar land is higher and the terrain is more rugged. Therefore, the selection of Chang 'e-4 landing zone will be an important problem to be solved urgently.