Can high-energy electrons coming from Earth produce water on the Moon? The possibility that it is a major source other than the solar wind | sorae website portal to the universe

“Without atmosphere”moon“It is a very dry celestial body, but there is a small amount of”water」 (※1) Its existence is known, and its main origin is “Solar wind” is considered. However, the fact that water evaporation was not observed as it passed through the tail of the Earth’s magnetic field (Earth’s magnetic field), where the solar wind is blocked, has remained a major mystery regarding water on the Moon.

*1…The term “water” used in this paper includes not only H2O, an ordinary water molecule, but also hydroxyl groups (OH).

A research team led by Shuai Li of the University of Hawaii at Manoa conducted research on the lunar lander “Chandrayaan 1From monitoring dataWater is produced on the Moon’s surface even when the solar wind passes through the occluded geomagnetic tail.I discovered it. This is the result of observationHigh-energy electrons from Earth will be added to the main source of water on the MoonIt means there is a possibility.

■Water on the Moon is created by solar wind

The Moon, Earth’s only natural satellite, is a barren celestial body with a very dry surface, unlike Earth’s surface. However, as a result of many years of exploration, it was discovered that there is a small amount of solid water (ice) on the surface of the Moon. Water ice is a permanent shadow that forms inside craters near the poles. (※2) It is known to be abundant in the region, but it is also known to occur in other regions at lower latitudes. As evidence of this, very thin water vapor has been observed to rise in areas exposed to sunlight and escape into space. The fact that water exists even though the escape process is active means that water is still being produced on the Moon’s surface.

*2…A place where the sun’s rays do not shine almost permanently due to the angle of the moon’s rotation and revolution.

The solar wind has been strongly supported as the process by which water is produced on the surface of the Moon. The solar wind is a flow of charged particles (electrified particles) emitted from the surface of the Sun, and its main components areproton (hydrogen ion)He is. Because the solar wind has high energy, when it collides with the rocks that make up the surface of the moon,The hydroxyl (OH) groups in minerals are separatedIn order to do thisWater is produced when solar wind protons combine with hydroxyl groups.. It is estimated that such chemical reactions caused by high-energy charged particles occur not only on the Moon, but also on other celestial bodies that do not have an atmosphere.

However, there were some mysteries surrounding this generation process. The key isTerrestrial magnetism (Earth’s magnetosphere). Magnetism has the property of bending the path of the solar wind, trapping the solar wind within the magnetosphere of a celestial body. The Moon’s magnetic field is so weak that it can almost be ignored, but the presence of the Earth’s magnetic field, which is strong, cannot be ignored.

Because of the “push” between Earth’s magnetic field and the solar wind, Earth’s magnetic field extends to the far side of the Sun, like the tail of a comet. Because the Moon orbits close to the Earth, it periodically enters the extended geomagnetic tail. Then the process of producing water by the solar wind stops in the Earth’s magnetic field, where about 99% of the solar wind is blocked. If the water on the Moon is produced by the solar wind, the amount of water on the Moon should decrease over time, as no water is produced and evaporates during this period.

But in actual observation,The amount of water on the Moon remains almost unchanged even when it blocks the solar wind.I’m starting to understand that. Since the solar wind is blocked for about 27% of the lunar day, it has been a long-standing mystery as to who produces the water instead of the solar wind.

■Do Earth-derived electrons also form water on the Moon?

Li and his team conducted research to understand the process by which water is saved during periods when the solar wind is blocked. Previously, Li and his colleagues discovered that the near side of the Moon (the side facing Earth) contains an unexpectedly high amount of iron oxide, suggesting that the source of the oxygen that oxidizes the iron is on Earth. It is clearly the upper atmosphere. Based on the results of this study, it is very likely that material flowing from Earth to the Moon also plays a role in producing water during periods when the solar wind is blocked.

Lee and his colleagues analyzed observational data from the Chandrayaan 1 lunar probe and looked at changes in the amount of water on the Moon’s surface as the Moon moved in and out of the Earth’s magnetosphere. Chandrayaan-1 is equipped with a remote sensor called the “Lunar Surface Mineral Mapping Instrument”, which can detect water on the Moon’s surface with high sensitivity.

As a result of their analysis, Li and his colleagues discovered that there is a relationship between changes in the amount of water on the Moon and movement in and out of the geomagnetic tail. First, the amount of water on the Moon’s surface increased as the Moon entered and left Earth’s magnetosphere. This result is as expected, as near the geomagnetic boundary (the magnetosheath), the interaction between the magnetic field and the solar wind increases the amount of high-energy solar wind reaching the moon’s surface, leading to a temporary increase in the amount of water. produced.

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But surprisingly,The amount of water on the Moon’s surface barely changes even when we are inside the Earth’s magnetosphere.This has been confirmed. This suggests that a water production process similar to that which occurs in the solar wind also operates within the Earth’s magnetosphere.

Li and others were included in the plasma layer surrounding Earth.High energy electronsThey are similar to the protons of the solar windIt plays a role in the decomposition of minerals to produce waterI think so. Plasma sheets consist of charged particles trapped by the Earth’s magnetic field, and are rich in high-energy electrons. If this is true, the new role of terrestrial magnetism and plasma sheets has been demonstrated.

■Research is important for manned lunar exploration missions

This research may have discovered a new, previously unknown interaction between the Earth and the Moon, in which Earth-derived electrons are involved in the production of water on the Moon’s surface. Water on the Moon’s surface has the potential to become a valuable source of water for future human lunar exploration missions, and knowing the details of its production process is important when considering effective methods of water extraction.

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Li and his colleagues propose observations using a new lunar probe to confirm this hypothesis. By ascertaining the ups and downs of the Earth’s magnetic field and the amount of plasma and water in more detail, we will be able to understand the process of water production on the Moon’s surface in more detail.

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Written by Riri Aya