Is there water on the moon? Chang'e 5 goes out into the wild and adds another "proof"

Is there water on the moon? How much water is contained in 1 ton of lunar soil?

The latest research from Chang'e-5 probe data shows that there is about 120 grams of "water" in 1 ton of lunar soil. The relevant research results were published online in Science Advances on January 8.

Background image and water content of the Chang'e 5 sampling area. Photo provided by the research team

Based on the data detected by the "Lunar Mineralogy Spectrometer" carried by Chang'e-5, researchers from the Institute of Geology and Geophysics of the Chinese Academy of Sciences and other units obtained for the first time the water content under in-situ conditions on the lunar surface.

It needs to be explained that the "water" studied by scientists is not the "water" we drink. "The 'water' detected by the spectrometer refers to the water molecules or hydroxyl groups in minerals, which can only be converted into the water we drink under certain conditions," said Lin Honglei, the first author of the paper and an associate researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences.

Half a century of debate

Is there water on the moon? This question is not only of interest to the public, but scientists also want to know the answer.

As early as 1952, American chemist Harold Urey boldly speculated that volatile substances like water might exist in the depressions on the moon that are never reached by the sun.

From 1969 to 1972, the US Apollo mission collected a large number of samples from the moon and returned them to Earth, finally giving people the opportunity to directly measure whether there is water on the moon. Unfortunately, the lunar soil is very dry, and the instruments left by astronauts on the lunar surface to detect the atmosphere cannot detect water. This seems to make "the moon is dry" a fact.

However, even though Soviet scientists measured trace amounts of water in samples collected from the Luna 24 mission in 1978, the results were not taken seriously.

Until the launch of the Clementine mission in 1994, research on lunar water remained stagnant.

In 2009, a different discovery was made. The lunar mineral mapping spectrometer on India's Chandrayaan-1 found that water is ubiquitous on the moon, and the water content increases with latitude.

"This detection result made scientists extremely excited. This may be the first time that many people realized that there is water on the moon." Lin Honglei said that the "water" here refers to water molecules or hydroxyl groups.

Since then, the Cassini probe to Saturn, the Deep Impact probe to comets, and the Lunar Observation and Sensing Satellite (LCROSS) have all used spectrometers to confirm the existence of water on the moon.

In short, after more than half a century of debate and exploration, various "hard evidence" have made people believe that there is water on the moon, but there has still been no detection of water in situ on the lunar surface!

The Chang'e-5 probe carried a lunar mineral spectrometer, which acquired the spectrum of the lunar surface during the sampling process. Lin Honglei said that these data gave us the first opportunity to detect water signals on the lunar surface at close range and with high resolution.

Unlike liquid water in the general sense, the "water" detected by the spectrometer on the lunar surface is hidden in the rocks. The water molecules represent "bound water" that can be released with a little heating, while the hydroxyl groups represent "structured water" that requires a higher temperature to precipitate.

Water in lunar soil

Most of it is contributed by the solar wind.

The Chang'e-5 spectrometer conducted spectral observations of an area approximately 2 meters square in the sampling area. In addition to the lunar soil, the observation objects also included a piece of rock that was not brought back.

Data analysis results show that the water content in the Chang'e 5 sampling area is below 120 ppm, while the water content in rocks is about 180 ppm. "This is equivalent to about 120 grams of water in 1 ton of lunar soil and about 180 grams of water in 1 ton of rock," Lin Honglei explained.

So, where does this water come from?

Combined with sample analysis, most of the water in the lunar soil is contributed by the solar wind.

Lin Yangting, one of the corresponding authors of the paper and a researcher at the Institute of Geology and Earth, said that there is a lot of hydrogen in the solar wind, which hits the lunar surface and combines with oxygen in the lunar soil to form hydroxyl groups or water molecules.

Compared with the 120ppm water content in the lunar soil, there is still an extra 60ppm of water in the rocks. Where does the extra water come from? Researchers speculate that the rocks come from an area older than the local basalt at the Chang'e 5 landing site, and the extra water may represent water from the moon's interior. "The low water content in the lunar soil may be due to the drier lunar mantle in the Chang'e 5 landing area or a large amount of degassing, which is consistent with the long-term volcanic eruptions in the Ocean of Storms," ​​said Lin Honglei.

Not long ago, the research team of the Institute of Geology and Geophysics of the Chinese Academy of Sciences published three papers in Nature. One of the papers reported the results of the detection of water in the moon's interior based on nano-ion probe analysis technology, and determined that the lunar mantle source area in the Chang'e 5 landing area was very "dry". It is speculated that one of the reasons may be the result of strong degassing caused by long-term volcanic eruptions in the Ocean of Storms.

Lin Honglei introduced that Chang'e-5 is the only mission that can both return samples and obtain in situ spectra of the lunar surface. The samples can be used to analyze in detail the distribution and existence form of water in the lunar soil particles, and the source can be traced using isotopes. The in situ spectra can be connected with orbital remote sensing to study the global distribution and temporal variation characteristics of lunar surface water.

The distribution of lunar surface water may be highly correlated with latitude. Chang'e-5 has the highest latitude among the samples returned so far, which is of great significance for studying the distribution and source of lunar surface water. Lin Yangting said that Chang'e-6 and Chang'e-7 will continue to detect the content and distribution of lunar surface water in situ and on orbital scales in the future. The results of this research will also provide support for the realization of the scientific goals of Chang'e-6 and Chang'e-7.

Related paper information:

https:doi.org/10.1126/sciadv.abl9174

Comprehensive sources: Science and Technology Daily, China Science Daily, etc.