Recently, the planetary science team of the Institute of Geology and Earth science of the Chinese Academy of Sciences published a paper in "Scientific Progress", and the results of the study showed that the Chang'e-5 probe found trace amounts of water in the lunar soil and lunar rock, which is a major discovery of human in situ exploration of the moon, and also let the controversy over whether there is water on the moon settled. So, where does the water on the moon come from? Is it available to humans?
Background map and water content of the Chang'e-5 sampling area
Whether the moon has water has been debated for years
The moon is the closest large celestial body to the earth, and in ancient times, human beings had a beautiful imagination of the moon, weaving a dreamlike mythological story. However, astronomers have long discovered that the moon is a "death star" without an atmosphere and full of craters. Nonetheless, in 1952 the discoverer of deuterium, American chemist Harold Yuri, proposed that there might be water or similar volatile material in the permanently shadowed areas of the moon.
Although humans did not have the ability to detect and verify this hypothesis at that time, the U.S. and Soviet moon exploration and moon landing competitions of the 1960s and 1970s provided the possibility for the moon to find water. In 1969, the Apollo 11 manned spacecraft successfully landed on the moon for the first time, but neither the lunar surface instruments nor the samples brought back by the astronauts found evidence of water on the moon. For a time, the moon became the consensus that it was a dry, waterless planet.
American astronauts display the national flag on the moon
During this period, the Soviet Union successfully returned to the exploration mission through three successful sampling and return missions of Luna 16, Luna 20 and Luna 24, and recovered about 300 grams of lunar rock samples. Soviet researchers analyzed lunar soil samples and found that there was trace amounts of water in the lunar soil.
In fact, the samples retrieved by the Apollo mission also had similar findings, but the United States and the Soviet Union detected that the water content in the lunar soil was very low, and more importantly, the possibility of contact with water vapor after the lunar soil returned to the ground could not be ruled out, which led to the uncertainty of declaring the existence of water on the moon.
Samples of lunar rocks retrieved during the Apollo manned lunar mission
With advances in remote sensing technology and the resurgence of lunar exploration in the 1990s, the lunar search for water entered a new phase. In 1994, American researchers probed the lunar poles through the clementine probe's radar and found evidence of suspected ice. In 1998, the U.S. Lunar Explorer detected that the lunar polar regions were rich in hydrogen through gamma ray and neutron spectrometers, providing circumstantial evidence for the existence of water.
Subsequently, countries stepped up their exploration efforts, and the U.S. Lunar Reconnaissance Orbiter used more sensitive seed probes and Lyman-α probes to detect water and hydroxyl signals in the lunar South Pole crater. The M3 probe aboard Chandrayaan-1 in India also found hydroxyl signals in the lunar polar region. In 2009, the United States used the LCROSS probe to hit the moon, carrying infrared and ultraviolet spectrometers that detected water and hydroxyl signals. However, the 3-micron infrared band cannot distinguish between water and hydroxyl groups and cannot be used as irrefutable evidence.
In fact, water on the Moon doesn't just exist in permanent shadows. In 2020, NASA announced that SOFIA had detected 6 mm of infrared radiation near Cavirus Crater in the south of the Moon during flight observations in August 2018. SOFIA, a Boeing 747 aircraft carrying a 2.5-meter telescope, and 6-millimeter infrared light, produced by the heat vibrations of water molecules, demonstrate the presence of water molecules on the lunar surface, not just hydroxyl groups.
The SOFIA platform discovered the presence of water molecules on the Moon
SOFIA found that the concentration of water near the Craters of Clavius is 100 to 400 ppm, which means that each ton of lunar soil contains 100 to 400 grams of water. However, the latest evidence of water on the moon comes from Chang'e 5.
Lunar Clavius crater
The United States found evidence of water on the moon, either from SOFIA, the Earth's stratosphere observation platform, or from the orbiter in the lunar orbit, and the Chang'e-5 soft landing on the lunar soil in situ detection, estimated that the lunar soil water content is about 120ppm, the lunar rock water content of about 180ppm, this is the first time that humans through in situ detection to prove that the moon has water, which is the real hammer of the moon has water evidence.
The source of the water remains controversial
The lunar surface can reach temperatures as high as 160 degrees Celsius during the day and as low as minus 180 degrees Celsius at night. In such a harsh environment, liquid water has long since evaporated. So, where exactly does the water on the moon come from?
First of all, we must know that there is no direct discovery of free liquid water or ice on the moon, and even no minerals whose main component is hydroxyl, whether it is remote sensing or in situ detection, the water on the moon is converted into water content through the infrared signal of water molecules and hydroxyl groups, and these "waters" are likely to be crystalline water and structural water in the lunar mineral lattice, or linger in other strange states.
Further, the water content of the lunar surface is negligible, and the water content of the lunar rock of 180 ppm is equivalent to only 180 grams of water in 1 ton of lunar rock, which is not to say that compared with most of the earth's soil, it is compared with the dry desert. The moisture content of sand in the desert is generally 2 kg to more than a dozen 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 proposed 3 different theories about the source of water on the moon:
The first hypothesis is that the water on the surface of the moon was blown by the sun. Not only does the solar wind blow away the Martian atmosphere, it also contains a lot of hydrogen, and when the solar wind blows to the lunar surface, the hydrogen in the solar wind and the oxygen in the lunar soil minerals combine 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 lunar surface hits a comet. Comets from the outer solar system contain large amounts of water ice, which enter the inner solar system and volatilize under the sun, but some comets hit the moon, bringing valuable water. Most of the water evaporated and escaped during the impact of the comet, but it may also be that some of the water seeped into the lunar soil and was preserved as water in the lunar soil.
A third guess is that the moon rocks themselves contain water. Solid planet minerals contain water, and in the case of the recent volcanic eruption in Tonga, the vast majority of the gases erupted by the volcano are water vapor. Researchers in the "Apollo" retrieved the lunar sample, found that some minerals contain very little water, can support this speculation, and the Chang'e-5 lander found that the water content of the lunar rock is much higher than the lunar soil, especially the lunar rock it detected, itself formed in the depths of the moon, is a meteorite impact sputtered out, which provides strong evidence for the endogenous theory of lunar water.
Basalt collected by astronauts from the moon
In summary, the water on the moon may be blown by the solar wind, or it may be magma itself when the moon formed, or it may be brought about by the impact of deep space objects such as comets. According to the detection and analysis of Chang'e 5, most of the water in the lunar soil is contributed by the solar wind, and most of the water in the lunar rock should be there before the magma solidifies.
The moon has to look at the polar regions to get water
The good news that the moon has water has been heard many times, but more seriously, the current moon has water, or only the result of spectral exploration of water molecules and even hydroxyl groups, at most it can only prove the existence of water molecules on the surface of the moon, to say that there is water vapor, water ice, and even liquid water and other genuine free water, still to be discovered.
Water is the source of life, and although scientific exploration has found that there is water on the moon, it cannot give birth to life on the moon in harsh environments. Even so, the presence of water makes the moon an attractive place. Many people and even some scientists have written papers that boldly imagine that if the water on the moon can be effectively mined 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.
U.S. commercial companies say they will use water from the moon to produce propellants and add propellants to deep space probes, manned spacecraft and geostationary satellites, making water on the moon a profitable resource.
Nevertheless, the results of the US SOFIA and Chang'e 5 exploration of the lunar surface to find water show that the water content on the lunar surface is too low, the difficulty and cost of directly obtaining water from the lunar soil are too high, and at least in the early stage of lunar exploration and development, it is of little economic value.
In addition, current remote sensing probes suggest that the permanent shadow areas of the lunar poles should be rich in water resources. Although the presence of water ice or volatile water-rich minerals in the lunar polar regions is only a reasonable guess based on remote sensing data, the water content of the permanent shadow area of the lunar base is much higher than that in the lower and middle latitudes of the lunar surface explored by SOFIA and Chang'e-5.
The blue area shows where water ice may be present at the moon's south pole
The presence of water in the lunar soil and lunar rock exposed to sunlight is a major scientific discovery, but in engineering, especially in the future construction of lunar bases for human beings, and even the preparation of propellants on the lunar surface, such a low water content obviously lacks practical value, so human beings have to use water on the moon and have to go deep into the permanent shadow area of the lunar north and south poles.
In the future, the mainland Chang'e 7 will carry a leap probe to detect in situ the shadow area of the lunar polar craters, and we expect it to bring more exciting good news.
Source/ China Aerospace News official WeChat public account
Text/Zhang Cedar
Editor/Yang Sishuang
Audit/Yang Jie
Executive Producer/Xu Chunmei
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