The story behind the double-sided moon

  The moon is facing the side of our earth, bright and dark, and very beautiful, which has inspired countless literati; but the back of the moon has a twisted face, which is very ugly. What causes such a gap?
  The moon is the closest celestial body to the earth. On a clear night, when a bright moon is high in the sky, the ancients can even distinguish the general structure of the moon surface with the naked eye—the bright place is the land of the moon, and the dark place is the moon. sea. Today, with the help of spacecraft and space probes, we can already draw a complete map of the moon, detect the material composition of the moon, and even simulate the internal structure of the moon.
  However, the moon still has unsolved mysteries, such as its origin and its evolution process. In addition, because the tides are locked, the moon is always shown on the front. It was not until the Soviet Union Luna 3 spacecraft returned in 1959 that we saw the back of the moon for the first time, but it looked a little strange and was very different from the front. Scientists want to figure out why the moon has two completely different sides? Or, can the differences between the two sides tell us some stories about the early moon?
  ”Face” of the moon
  in front of the moon, on the sea is the main structure of the landscape, covering an area of up to 31.2%, which consists of iron-rich basalt (volcanic eruptions of lava cools down) components. Both the lunar sea and the lunar land (mainly composed of plagioclase) have smooth surfaces, and they set off each other to form a pattern of light and dark. This makes the front of the moon extremely beautiful, and the moon sea has also become a source of inspiration for the “Moon Man” in Western culture and the “Chang’e and Yutu” in Eastern culture.
  The back of the moon is not so good-looking, there is a lack of basalt, the area of ​​the moon sea is only 2.5%. Here, the protagonist instead of the moon and sea are the dense craters and craters on the moon and land. The largest crater in the solar system, the Aitken Basin in Antarctica and its surrounding craters occupy a large part of the landform on the back of the moon. The crater has a diameter of about 2500 kilometers and a depth of 8 kilometers. On the whole, the back of the moon is very rough and not very beautiful.
  Scientists believe that for a celestial body the size of the moon, the influence of many astronomical processes on it should be balanced on both sides, and there should be no difference between two sides. For example, the impact of foreign celestial bodies, in the sky of the moon, the earth only occludes a small part of the moon, unable to provide protection to the moon, so the craters and basins on both sides of the moon should be evenly distributed. In addition, these basins were formed when asteroids or other rocks fell into the moon. These impacts would deform the lunar crust. Volcanic magma would gush out from cracks on the surface of the basin and silt down to low-lying areas to cool down and form the moon. sea. If so inferred, the Aikento Basin in Antarctica should also be covered by basalt, but this is not the case. It can be seen that the “double-sidedness” of the moon is not just as simple as creating a difference in topography.
  In order to obtain more information about the back of the moon, at the beginning of the 21st century, NASA restarted its long-interrupted lunar exploration mission, focusing on the difference between the back of the moon and the front. The Lunar Reconnaissance Orbiter found that the two sides of the moon are not only different in surface morphology, but also in the material composition and thickness of the lunar crust on both sides. On the front side of the moon, the lunar shell is very thin, only 20-30 kilometers, but the Krip rock here is very rich and contains a lot of radioactive elements that can generate heat (such as thorium). It seems that the lava inside the moon is all deposited on the front side when it cools. ; On the back of the moon, although the lunar crust is very thick, up to 60 kilometers on average, the material type is relatively single, mainly plagioclase.
  Why is the moon so asymmetric? Scientists have used their imaginations. They either put forward the hypothesis of celestial collisions, or went to the moon to find answers. All the speculations told of a violent history of early moon experience.
  Two moons, two collisions
  For the origin of the moon, the most mainstream hypothesis in the scientific community is the collision theory proposed in 1975. This theory holds that in the early days of the solar system, the planet “Theia” of the same magnitude as Mars hit the earth, and a large amount of debris was thrown into the orbit around the earth. These materials gather, cool down quickly, and form the moon. When the moon cools, the heavy matter sinks and the light matter rises to the surface. This process is called differentiation. Finally, the iron core is formed, and from the inside to the outside are the liquid outer core, the lunar mantle and the lunar crust. Although the moon mantle has begun to crystallize, the radioactive elements that can generate heat still keep it highly plastic. In the following millions of years, new asteroid impacts created a huge lunar basin, cracking the moon mantle, and then magma surged and lava began to cover the lunar surface.
  The impact theory explains why the overall composition of the moon is close to the earth’s mantle, why the moon is rich in high-melting materials, the angular momentum of the earth-moon system, and the mass of the earth and the moon, but it cannot explain the difference between the two sides of the moon. Therefore, scientists proposed a new hypothesis that the deep lunar shell on the back of the moon came from the second, smaller moon. In other words, when “Theia” hit the earth, the debris of the debris formed two moons, one large and one small. After tens of millions of years, another collision occurred—the two moons were in close contact and cooperation. Two for one.
  Eric Espan of the University of California used a computer to simulate the process at that time. 4.5 billion years ago, “Theia” hit the earth, debris splashed, and gathered into many small satellites (including two moons). The large moon absorbed or expelled other satellites, leaving only a small moon with a diameter of 1,000 kilometers. It is stuck at the so-called “Lagrangian point” between the earth and the big moon (the point at which a small object can be stabilized under the action of the gravity of two large objects). In this way, the orbits of the earth and the two moons are stabilized, and each other is in peace. However, the tidal force of the earth will continuously push the two moons outward. Gradually, this triangular balance is broken, and the gravity from the sun begins to work. The small moon began to deviate from the Lagrangian point, approached the larger moon, and eventually crashed into the larger moon.
  This process takes place very slowly. The relative speed between the two is 2.5 kilometers per second. This speed is very fast in daily life, but it is very slow if compared with the impact speed of an asteroid. This low-velocity impact does not form a crater, but it can crush the small moon into pieces and thicken the lunar crust on the back.
  The lunar probe found that the lunar crust of the moon is mostly covered with a layer of porous and highly crushed rock and particles. Scientists call it a coarse weathered layer. Its thickness on the lunar land is between 10 and 20 meters. There are 3 to 5 meters in the moon sea. This rough weathering layer is most likely to be produced when a small moon collides with a large moon and then breaks into pieces, so Eric Espan believes that the hypothesis of “two moons” is consistent with the observation results of the lunar reconnaissance orbiter.
  The power of high-temperature
  Eric Ai Sipang theory is very impressive, it is actually the moon impact theory of the origin of a modified version. But not all scientists agree with the idea of ​​”two moons”. Some scientists believe that the positive extreme high temperature is the main reason why the moon formed its appearance today.
  Stein Higulardson of the University of Pennsylvania pointed out that the collision of “Theia” with the earth is very violent, which will inevitably heat our earth and the moon, but the moon is much smaller than the earth, and it cools The speed will be faster. Because the distance between the earth and the moon was very close at that time (only 1/20 to 1/10 of today’s distance), and the earth was very hot, with a temperature exceeding 2500°C, this caused a very large temperature difference between the front and back of the moon-the front of the moon was exposed to the earth The radiation, extremely high temperature, and the backside has been slowly cooling.
  This temperature difference plays a very important role in the formation of the lunar crust. The content of aluminum and calcium in the lunar shell is high, and these components are difficult to evaporate. When high-temperature gaseous substances begin to condense, aluminum and calcium will be the first to precipitate. There is no doubt that this process is more likely to occur in the lower-temperature back atmosphere. Over the next tens of millions of years, these components combined with the silicon in the lunar mantle to form plagioclase, which then accumulated into a lunar crust. Therefore, there will be more plagioclase on the back of the moon, and the moon shell will naturally be thicker.
  Temperature not only affects the formation of the lunar crust, but also the size of craters on the lunar surface. Scientists analyzed the data brought back by the lunar probe and found that the number of impact craters on the front and back of the moon is actually the same, but the craters on the front are relatively large in size. There are 8 basins with a diameter of more than 320 kilometers on the front of the moon, and only one on the back. The temperature of the lunar crust will affect the impact effect. If the temperature of the front side of the moon is a few hundred degrees higher than the temperature of the back side, the size of the basin formed on the front side by an impact of the same intensity will be twice that of the back side.
  theory of the origin of the “great decoration” brought by the Aitken Basin is not the starting point of all scientists. Peter Schultz of Brown University in the United States set his sights on the Aitken Basin in the South Pole, hoping to find the answer there. Schultz believes that the Aitken Basin is one of the largest impact craters in the solar system, and that the energy of such a large-scale basin impact must be enough to give the moon a comprehensive “big decoration”, so the moon’s “face change” “It should be closely related to the birth of the Aitken Basin.
  He pointed out that the early moon should be very uniform, and its two sides are very similar to the back of the moon today. 4.3 billion years ago, a huge asteroid crashed into the south pole of the moon from the side. The shock wave generated by the impact swept across the entire moon, transmitting energy along the inside of the moon to the front of the moon, destroying the lunar shell there. On the front side, all bursts and vibrations will cause magma to seep out and flow to the lunar surface. Even if they did not break out of the ground, they would come very close to the surface of the moon, and subsequent collisions with other celestial bodies would still cause magma to flow out and form the lunar sea, transforming the front face into today’s appearance.
  Regarding the double-sided moon, although scientists have different opinions, they have their own reasons. It is difficult to prove who is right and who is wrong. Perhaps the real situation will be the synthesis of these hypothetical models. In order to clarify this problem, we need more field verification. In the future, the new space program will bring back more samples from the back of the moon to help us solve the mystery of the double-sided moon