The first evidence was found that the Earth replenishes the water supply of the Moon

Before the astronauts landed on the moon, the surface of our space satellite was considered dry as a desert due to extreme temperatures and the harshness of the space environment. But since then, everything has changed. Research has found lunar water in the form of ice in the shadow of polar craters, bound in volcanic rocks, and oxidized iron deposits in lunar soil. Despite these findings, there is no reliable confirmation of the origin of water on the lunar surface.

The moon in the shadow of the Earth's magnetosphere in the artist's view (gray indicates oxygen ions, blue - hydrogen ions, solar wind is yellow). Image source: E. Masongsong, UCLA EPSS, NASA GSFC SVS

The prevailing theory for the origin of water on the Moon is that positively charged hydrogen ions in the solar wind bombard the Moon's surface and react spontaneously to form water (as hydroxyl OH– and molecular H₂O). However, a new international study published in the Astrophysical Journal Letters suggests that solar wind may not be the only source of water-forming ions.

Researchers have shown that particles from Earth can also replenish the Moon's water supply. In addition, this means that similar processes can occur with other planets and their satellites. In other words, from the surface of Mars to the moons of Jupiter and the rings of Saturn, comets, asteroids, and Pluto, and in the clouds far beyond our solar system, water could appear not only and not so much in the early period of the formation of the solar system, but gradually get to the surface of cosmic bodies - in dynamics.

But back to the moon. Although the solar wind is a likely source of water on the moon's surface, computer models predict that up to half of it should evaporate and disappear in the polar regions during about three full moon days, when the moon passes through the Earth's magnetosphere and is closed from the solar wind. A new analysis of maps of the lunar surface with data on the distribution of hydroxyl showed that, contrary to expectations, surface water on the Moon does not disappear during this period of screening of the solar wind by the Earth's magnetosphere. It follows that the water supply on the Moon is replenished by something other than hydrogen ions in the solar wind.

A close study of 10- year-old images from the Chandrayaan-1 satellite and the study of data on the ionic situation in the orbits of the Earth and the Moon by the Japanese satellite Kaguya, carried out anew by many specialists, led to the conclusion that lunar water can be replenished by streams of magnetospheric ions, also known as the "earth wind". Thus, looking at the data in a new light disproves the shielding hypothesis and instead suggests that the Earth's magnetosphere itself creates a "water bridge" that could replenish the Moon's water supply.

In light of these discoveries, future studies of the solar wind and planetary winds may reveal more mysteries about the evolution of water in our solar system and the potential effects of solar and magnetospheric activity on other moons and planetary bodies. Expanding this research will require new satellites equipped with complex spectrometers for mapping hydroxyl groups and water, as well as particle sensors in orbit and on the lunar surface, to fully confirm this mechanism. These tools can help predict the best regions for future exploration, mining, and possible settlement on the Moon.