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Sidereal Messenger: Chinese probe detects water on the surface of the Moon


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For the first time, a surface probe has detected the presence of water on the Moon’s soil. The result was obtained by the Chinese spacecraft Chang’e-5, which landed there in December 2020, and has relevance for future plans for permanent occupation of the Moon. natural satellite, since water would be one of the main limiting factors.

The finding was reported by the teams of Lin Yangting and Lin Honglei of the Chinese Academy of Sciences and published in the latest issue of the journal Science Advances.

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In addition to landing an ascent module that brought back 1.7 kg of samples taken (in the first such mission since the Soviet Luna-24 in 1976), the Chang’e-5 mission was equipped with a mineralogical spectrometer. , a penetrating radar and a panoramic camera. And it was precisely the spectrometer that was responsible for the discovery.

This equipment works by analyzing the light coming from the ground and breaking it down, like a rainbow. This process makes it possible to search, in the midst of the colored pattern (the so-called “spectrum”), for signs indicative of the atoms and molecules that absorb and reflect light.

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It is not easy to detect water in the lunar soil. Its spectral signature is at a wavelength close to 3 micrometers (thousandths of a millimeter), in the infrared range (light invisible to the human eye), but the thermal radiation itself coming from the hot surface, illuminated by the Sun, “muffles” any signal with length greater than 2 micrometers. To find it, the researchers needed to apply a corrective model, which “compensated” for the thermal emission. And then the water (or hydroxyl, OH) signal appeared, at 2.85 micrometers.

The signal strength allows estimating the amount of water present. Which, as you can imagine, isn’t much (or it wouldn’t be that hard to spot). Analyzing the regolith (the sandy lunar soil), the Chinese researchers found an average of 120 ppm (parts per million) of water. That is, for every million molecules in a soil sample, 120 are water.

Despite the modest amount, it is a very important find. With the plans for a manned return to the Moon (nourished by Americans, Europeans and Japanese on the one hand, and Chinese and Russians on the other), the possibility of extracting and using water from the satellite itself proves to be a fundamental resource. (In addition to serving astronauts to drink, water can be broken down into oxygen for breathing and rocket propellant.)

The ground measurement also helps to corroborate a long series of recent studies that indicate, through images from telescopes and satellites, the presence of water even in illuminated regions of the Moon (where the Sun would theoretically make it sublimate and be lost. to space).

Apparently, the water “trapped” in the lunar regolith is implanted there by the interaction of the solar wind (composed largely of free hydrogen nuclei) with the oxygen in the ground, producing loose molecules that become trapped in the soil.

The spectrum from a rock near the Chang’e-5 lander, however, revealed an even higher amount of water, something like 180 parts per million. It is an indication, according to the researchers, that, in addition to interacting with the solar wind, rocks have water coming from an internal source on the Moon.

In addition to these small amounts of water mixed with the soil and rocks, future lunar explorers are keeping an eye on the bottoms of craters where sunlight never hits near the lunar poles. These, for now, have only been detected from orbit – but are thought to exist in far greater quantities, perhaps as ice that is easily accessible if these “wells of eternal darkness” can be explored.

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