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There is no doubt that the Moon has water on its surface. The orbiters have detected ice deposits that linger in the perpetual shadows of the polar craters. And recent research shows that water also exists in the sunlit parts of the Moon.
Over the years, scientists have presented evidence that the Moon’s water comes from comets, asteroids, from inside the Moon, and even from the Sun.
But now new research points directly to Earth as the source of some of the Moon’s water.
The new study is entitled “The terrestrial wind as a possible exogenous source of hydration of the lunar surface.” The lead author is HZ Wang from Shandong University in China, and the article is published in The Astrophysical Journal Letters. Research suggests that particles from Earth can seed the Moon with water.
That the Moon has water is not surprising. Astronomers have detected water in all kinds of places in space, although most of it is ice. The prevailing theory for that water is that it reaches planets and moons as they form, perhaps delivered by asteroids or comets. But this article presents evidence that some of the water on the Moon’s surface came from Earth’s wind.
The solar wind is likely responsible for some of the Moon’s surface water. Lunar regolith contains silicates, and protons from the solar wind can deplete the oxygen in those silicates. That oxygen then easily combines with hydrogen to form water.
The problem with the Sun being the only source of water on the lunar surface is evaporation. Computer models predict that a large part, up to 50%, should evaporate from the high-latitude regions of the lunar surface during the full Moon. For three to five days in each cycle, the Moon is in the Earth’s magnetosphere, which means that the water should disappear from the surface, as the Earth’s magnetosphere prevents the solar wind from reaching the Moon and replenishing the water. from the surface during that period.
But that’s not what happens. In contrast, according to data from the Moon Mineralogy Mapper from India’s Chandrayaan-1 satellite, water does not disappear during full moons. The study authors say a “land wind” is replenishing it.
The solar wind and the terrestrial wind are different. The solar wind is primarily plasma made up of protons and electrons released from the Sun’s upper atmosphere. But the Earth’s wind is a flux of ions from the magnetosphere, measured by the THEMIS-ARTEMIS mission.
Japan’s Kaguya mission detected hydrogen ions from Eath’s exosphere embedded in the ground. It also detected high concentrations of oxygen isotopes coming from the Earth’s ozone layer and embedded in the lunar surface. This points to the idea of a “water bridge” from Earth to the Moon. This bridge is active during the days of the month when the Moon is inside the Earth’s magnetosphere and replenishes the water lost through evaporation.
While these findings go a long way in helping to explain the water on the lunar surface, they could also be of broader significance. If there is a bridge between the Earth and the Moon that creates water on the Moon, where else in the Solar System could this be happening?
The authors have proposed a mechanism for lunar water that coexists with the solar wind explanation. But it is not confirmed yet. Future studies may provide more evidence that a water bridge exists between the Earth and the Moon. China’s Chang’e 5 mission returned lunar samples to Earth in December. Those samples could contain evidence for the water bridge idea.
If it turns out to be correct, astronomers will immediately start to wonder (they probably have already) if a similar mechanism is at work elsewhere, perhaps right here in our own Solar System.
This study also points out how much more we have to learn about the interactions between planets, their moons, and their stars. The evolution of water in our and other solar systems may depend on these interactions. This research effort could also help us understand the potential habitability of distant exoplanets.
