Water from Earth's atmosphere may have caused rain on the Moon

The moon may look like a giant ball of dry ice, but there's more water on it than you might expect. In a new study, scientists have shown that at least some of this water may have been spewed onto the Moon's surface from Earth's very atmosphere.

Over the past decade or so, water has been detected on the Moon by numerous spacecraft. Most of them are in the form of ice and are concentrated around the poles, while other regions of the Moon's surface may contain water-rich minerals. Finding and tracking these water bodies is an important task for future Moon missions.

But, how did water appear on the Moon?

The moon is not a giant dry rock.

The most widely accepted explanation is that most of them were created, or brought about, by the impacts of comets and asteroids colliding over billions of years. Some may have been deposited as hydrogen and oxygen ions from the solar wind. However, they can also be traced back to the Moon's own formation, when a giant planet crashed into early Earth about 4.4 billion years ago, and caused it to carry some water.

But now, researchers from the University of Alaska Fairbanks have proposed a new mechanism that has helped bring back some of the water on the Moon. And that's the fact that they fell as rain from the Earth's very atmosphere, for a few days each month.

Specifically, Earth's magnetic field creates a bubble called the magnetosphere, which protects us from cosmic rays. This bubble is quite rounded at the edge and slightly tapered to form a point at the back, like the tail of a comet. And for 5 days each month, the Moon will pass through this tail.

But some of Earth's magnetic field lines are fractured, with only one end remaining connected to the planet, and hydrogen and oxygen ions from the atmosphere can escape into space through these lines. However, as the Moon passes through the upper magnetosphere tail point, it causes some of these fault lines to reconnect, sending the discrete ions back to Earth. And part of this will hit the surface of the Moon.

'It's like the Moon is in a shower - a shower of water ions that come back to Earth, and they also fall to the surface of the Moon,' said lead researcher Gunther Kletetschka.

The researchers calculate that over billions of years, this process could have dumped about 3,500 cubic kilometers of water onto the Moon's poles. And that's just based on the lowest mass calculation, with only 1% of the ions escaping from the Earth reaching the Moon.

Of course, water on the Moon could very well come through many other mechanisms, and this new idea is just one of them.