🌙 The Moon would be continuously supplied with water by... the Sun?
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Moreover, these eternally shadowed areas are not really so: the Moon's tilt has changed over time, plunging the craters alternately into light and shadow.
Lunar cold traps
These craters are depressions at the bottom of which the Sun supposedly "never" shines. In the absence of light, the temperature there remains below -200°C, allowing water ice to survive for billions of years without sublimating. These areas are called cold traps.
Their location evolves with the Moon's obliquity, i.e., the tilt of its rotation axis. Over time, this tilt oscillates between 18° and 28°, modifying the angle of solar illumination. A crater that is shadowed today was not necessarily so 3 billion years ago, and vice versa.
The researchers therefore calculated which craters had remained in shadow the longest. These are precisely those that contain the most ice. This correlation indicates that water accumulates progressively, somewhat like snow in a hollow, rather than arriving all at once.
Shackleton crater is located at the south pole of the Moon. NASA/Ernie Wright
The role of the solar wind in water formation
The solar wind is a continuous stream of charged particles, mainly protons (hydrogen nuclei), emitted by the Sun. When these protons hit the lunar surface, they can combine with oxygen present in soil minerals to form water molecules.
This newly formed water can then accumulate in cold traps where it is preserved as ice. However, this process requires oxygen to be available. Now, part of the oxygen could come from Earth's atmosphere, which slowly leaks into space, notably due to the Sun's effect, and reaches the Moon.
The solar wind is therefore a continuous source of hydrogen, while oxygen can come from Earth or from the lunar rocks themselves. This combination would explain the gradual arrival of water, without requiring a sudden input.