Recently, the planetary science team of the Institute of Geology and earth of the Chinese Academy of Sciences published a paper in scientific progress. The research results show that the chang'e-5 probe found trace water in lunar soil and lunar rocks, which is a major discovery of human in-situ lunar exploration and settles the dispute over whether there is water on the moon. So, where does the water on the moon come from? Can it be used by human beings? Background map and water content of chang'e-5 sampling area Whether the moon has water has been debated for many years The moon is a large celestial body closest to the earth. In ancient times, human beings had a beautiful imagination of the moon and woven dreamlike fairy tales. However, astronomers have long found that the moon is a "Death Star" with no atmosphere and craters. Nevertheless, in 1952, American chemist Harold Yuri, the discoverer of deuterium, proposed that there might be water or similar volatile substances in the permanent shadow of the moon. Although human beings were unable to detect and verify this speculation at that time, the US Soviet competition for lunar exploration and landing in the 1960s and 1970s made it possible to find water on the moon. In 1969, Apollo 11 manned spacecraft successfully landed on the moon for the first time, but neither lunar instruments nor samples brought back by astronauts found evidence of water on the moon. For a time, it became a consensus that the moon was a dry and waterless planet. American astronauts display national flag on the moon During this period, the Soviet Union recovered about 300 grams of lunar soil and lunar rock samples through three successful sampling and return exploration missions of "moon 16", "moon 20" and "moon 24". By analyzing lunar soil samples, Soviet researchers found that there was a small amount of water in lunar soil. In fact, there are similar findings in the samples retrieved by the Apollo mission, but the United States and the Soviet Union have detected that the content of water in the lunar soil is very low. More importantly, they can not rule out the possibility of contact with water vapor after the lunar soil returns to the ground, which leads to the uncertainty of announcing that there is water on the moon. Lunar rock samples retrieved from the Apollo manned lunar landing mission With the progress of remote sensing technology and the resurgence of lunar exploration in the 1990s, lunar water exploration has entered a new stage. In 1994, American researchers detected the lunar poles through the radar of the Clementine probe and found evidence of suspected ice. In 1998, the US lunar explorer detected hydrogen rich elements in the lunar polar region through gamma ray and neutron spectrometer, which provided circumstantial evidence for the existence of water. Subsequently, countries increased their exploration efforts, and the US Lunar Reconnaissance Orbiter used more sensitive seed detectors and Lehmann- α The probe detected water and hydroxyl signals in the lunar Antarctic crater. The M3 probe on India's moon ship 1 also found hydroxyl signals in the polar region of the moon. In 2009, the United States used the LCROSS probe to hit the moon, and its infrared and ultraviolet spectrometers found water and hydroxyl signals. However, the 3 micron infrared band can not distinguish between water and hydroxyl, which can not be used as irrefutable evidence. In fact, water on the moon does not exist in permanent shadows. In 2020, NASA announced that during its flight observation in August 2018, Sofia found 6mm infrared radiation near the kravius crater in the south of the moon. Sofia is a Boeing 747 aircraft with a 2.5-meter telescope, and the 6mm infrared light is produced by the heating and vibration of water molecules. This discovery proves that there are water molecules on the lunar surface, not just hydroxyl groups. Sofia platform found water molecules on the moon Sofia found that the water concentration near kravius crater is 100 ~ 400ppm, that is, 100 ~ 400g water per ton of monthly soil. However, the latest evidence of water on the moon comes from chang'e-5. Moon kravius crater The United States found evidence that the moon has water, either from Sofia, the earth's stratospheric observation platform, or the Orbiter on the moon's orbit. However, the soft landing of chang'e-5 carried out in-situ detection of the lunar soil and estimated that the water content of the lunar soil is about 120ppm and that of the lunar rock is about 180ppm. This is the first time that human beings have proved that the moon has water through in-situ detection, It can be regarded as evidence of the existence of water on the moon. The source of water remains controversial The maximum temperature of the moon during the day can reach 160 degrees Celsius, and the minimum temperature at night can drop to minus 180 degrees Celsius. In such a harsh environment, liquid water has long evaporated. So where does the water on the moon come from? First of all, we should know that there is no direct discovery of free liquid water or ice on the moon, or even minerals whose main component is hydroxyl. Whether it is remote sensing or in-situ detection, the water on the moon is the water content converted from the infrared signals of water molecules and hydroxyl. These "water" is likely to be crystal water and structural water in the crystal lattice of lunar minerals, Or in some other weird way. Furthermore, the water content of the lunar surface is very small. The water content of 180ppm of lunar rock is equivalent to only 180g of water in a ton of lunar rock. This water is pitifully less than that of most earth soils and dry deserts. The water content of sand in the desert is generally 2 kg to more than 10 kg per ton, while the water in the lunar soil is less than 1 / 10 of the water content of the earth's desert. At present, researchers have put forward three different views on the source of water on the moon: The first hypothesis is that the water on the moon is blown by the solar wind. The solar wind will not only blow away the Martian atmosphere, but also contain a lot of hydrogen. When the solar wind blows to the lunar surface, the hydrogen in the solar wind combines with the oxygen in the lunar soil minerals to form hydroxyl and even water molecules, which are preserved in the lunar soil. Researchers believe that most of the water in the lunar soil is blown by the solar wind. The second hypothesis is that the water on the moon is hit by comets. Comets from the outer solar system contain a lot of water ice. They enter the inner solar system and volatilize in the sun, but some comets hit the moon and bring precious water. Most of the water evaporated and disappeared when the comet hit, but some of it may infiltrate into the lunar soil and remain as water in the lunar soil. The third guess is that the lunar rocks themselves contain water. It is natural that the minerals of solid stars contain water. Take the recent Tonga volcanic eruption as an example, the vast majority of the gas emitted by the volcano is water vapor. In the lunar samples retrieved by Apollo, researchers found that some minerals contain very little water, which can support this speculation. However, the chang'e-5 lander found that the water content of lunar rock is much higher than that of lunar soil. In particular, the lunar rock it detected was formed in the depths of the moon and sputtered by meteorite impact, which provides strong evidence for the endogenous theory of lunar water. Basalt collected by astronauts from the moon To sum up, the water on the moon may be blown by the solar wind, the magma itself when the moon was formed, or the impact of deep space objects such as comets. According to the detection and analysis results of chang'e-5, most of the water in the lunar soil is contributed by the solar wind, while most of the water in the lunar rock should have existed before the magma solidified. The moon still depends on the polar regions The good news that the moon has water has been heard many times, but to be honest, at present, the moon has water, which is only the result of spectral detection of water molecules and even hydroxyl groups. At most, it can only prove that there are water molecules on the moon's surface. To say that there are real free water such as water vapor, water ice and even liquid water, it remains to be found. Water is the source of life. Although scientific exploration has found that there is water on the moon, life cannot be born on the moon under the harsh environment. Even so, the existence of water makes the moon an attractive place. Many people and even some researchers are boldly imagining that if the water on the moon can be effectively exploited and utilized, it will help astronauts maintain their activities on the moon, and even produce hydrogen and oxygen propellants to serve the plan of manned landing on Mars. American commercial companies said they would use the water on the moon to make propellant and add propellant to deep space probes, manned spacecraft and geostationary satellites, so as to make the water on the moon a profitable resource. Nevertheless, the results of lunar surface exploration by Sofia and chang'e-5 show that the water content on the lunar surface is too low, the difficulty and cost of directly preparing water from lunar soil are too high, and they lack economic value at least in the early stage of lunar exploration and development. In addition, the current remote sensing detection shows that there should be abundant water resources in the permanent shadow area of the moon's poles. Although there is water ice or minerals rich in volatile water in the lunar polar region, which is only a reasonable guess based on remote sensing data, the water content in the permanent shadow area of the lunar base is much higher than that in the low and middle latitudes of the moon detected by Sofia and Chang'e 5. The blue area shows where water ice may exist at the south pole of the moon The existence of water in lunar soil and rocks exposed to the sun is certainly a major scientific discovery, but in engineering, especially in the future construction of lunar bases and even the preparation of propellants on the lunar surface, such a low water content obviously lacks practical value. Therefore, if humans want to use the water on the moon, they have to go deep into the permanent shadow area of the north and south poles of the moon. In the future, China's "chang'e-7" will carry a leap probe to detect the shadow area of the lunar polar crater in situ. We expect it to bring more explosive good news. (Xinhua News Agency)