A new paper suggests a novel origin for the permafrost-locked water found on the lunar surface: what if it was captured from the upper parts of Earth’s atmosphere?
The Earth’s magnetosphere, which shields the planet from harmful rays blasted out by the Sun, isn’t spherical but rather cone shaped, thanks to solar wind, and for about five days each month around the time of the full moon, the moon passes through the outermost tip of that cone.
Published in Scientific Reports in March, the paper argues that as the moon passes through that cone, it scoops up ions of hydrogen and oxygen - the atomic ingredients in water - which are caught in the magnetic bands. If the moon had its own magnetosphere, it would interact with Earth’s as it passes through and possibly block the rogue ions, but it doesn’t.
"It is like the moon is in the shower – a shower of water ions coming back to Earth, falling on the moon's surface," geophysicist Gunther Kletetschka, an associate professor at the University of Alaska Fairbanks and one of the authors of the study, said in a UAF news release.
According to their estimates, it’s no small amount of water, either: about 3,000 cubic kilometers has escaped from the Earth to the moon via this method - that’s a little more than the volume of Africa’s Lake Victoria.
Of course, some water has also arrived on the moon in the same way it likely did on Earth: via impacts by asteroids and comets containing large amounts of it.
Lunar water was first discovered in 1976 when the Soviet Union’s Luna 24 probe delivered samples from the Sea of Crisis regolith. A two-meter columnar sample was taken, which revealed that the water amount increased with depth. However, Western scientists all but ignored this finding, leading to the US space agency NASA claiming it had made the discovery in 2020 using an infrared telescope mounted inside a Boeing 747 aircraft.
Finding a way to harvest and store lunar water will be vital to any permanent human mission on the lunar surface. Several space agencies have floated plans to build lunar bases in the coming decades and have included plans to explore lunar water in greater detail.
In addition, water is believed to exist on many other moons across the solar system, including Saturn’s moons Enceladus, Titan, and Mimas; Jupiter’s moon Europa; Uranus’ moon Miranda; and the dwarf planet Pluto.