There is no doubt that the moon has water on its surface. Orbiters have discovered ice deposits that persist in the eternal shadows of the polar craters. And recent research shows that water is also present in sunlit parts of the moon.
Over the years, scientists have come up with evidence that the moon’s water comes from comets, asteroids, from inside the moon, and even from the sun.
But now new research is pointing the finger directly at the earth as the source of some of the moon’s water.
The new study is entitled “Earth Wind as a Possible Exogenous Source for the Hydration of the Moon’s Surface”. The main author is HZ Wang from Shandong University in China. The paper was 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 discovered water in all sorts of places in space, even though most of it is ice. The prevailing theory for this water is that planets and moons matter as they form, possibly supplied by asteroids or comets. However, this paper provides evidence that some of the water on the lunar surface comes from the earth’s wind.
It is likely that the solar wind is responsible for some of the moon’s surface water. The lunar regolite contains silicates, and protons in the solar wind can reduce the oxygen from these silicates. This oxygen then easily combines with hydrogen to form water.
The problem with the sun being the only source of lunar surface water is evaporation. Computer models predict that a large part of it – up to 50% – should evaporate from regions of high latitude of the lunar surface during the full moon. Each cycle, the moon is in the earth’s magnetosphere for three to five days, which means that water should disappear from the surface as the earth’s magnetosphere prevents the solar wind from reaching the moon and replenishes surface water during that time.
This picture from the study shows the moon in the earth’s magnetosphere. The moon is protected from the solar wind 3 to 5 days a month and instead exposed to the earth wind. Photo credit: Wang et al., 2021.
But that doesn’t happen. Instead, according to the Indian Chandrayaan-1 satellite Moon Mineralogy Mapper, the water does not disappear during the full moons. The study’s authors say that an “earth wind” replenishes it.
The solar wind and the earth wind are different. The solar wind is primarily plasma, made up of protons and electrons released from the sun’s upper atmosphere. However, the earth wind is a flow of ions from the magnetosphere, measured with the THEMIS-ARTEMIS mission.
This figure from the study shows a north (B) overview and a south (C) overview of the polar regions of the moon. It shows data from the Chandrayaan-1 satellite’s Moon Mineralogy Mapper indicating the OH / H2O abundance. The red and black bars on the outside of each picture show the moon being exposed to solar and earth winds. The little red square is the Goldschmidt crater, which due to its composition has an abnormal amount of OH / H2O. Photo credit: Wang et al., 2021.
Japan’s Kaguya mission discovered hydrogen ions from Eath’s exosphere embedded in the ground. It has also detected high concentrations of isotopes of oxygen, derived from the earth’s ozone layer and embedded in the surface of the moon. This points to the idea of a “water bridge” from the earth to the moon. This bridge is active on the days of the month when the moon is in the earth’s magnetosphere and replenishes the water lost through evaporation.
While these results go a long way in explaining lunar surface water, they could also have greater 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 it happen?
This image from the study shows satellite data for an entire month, including the full moon. During the time the moon is out of the solar wind and in the earth’s magnetosphere, the surface water persisted. A more detailed explanation of this figure can be found in the study. Photo credit: Wang et al., 2021.
The authors have proposed a mechanism for moon water that coexists with the explanation of solar wind. But it’s not yet confirmed. Future studies could provide further evidence that there is a water bridge between the earth and the moon. China’s Chang’e 5 mission brought lunar samples back to Earth as early as December. These samples could contain evidence of the water bridge idea.
If it turns out to be correct, astronomers will immediately (probably already) wonder if a similar mechanism is working elsewhere, perhaps right here in our own solar system.
This study also shows how much more we need to learn about the interactions between planets, their moons, and their stars. The development of water in our and other solar systems can depend on these interactions. This research effort could also help us understand the potential habitability of distant exoplanets.
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