The Moon may appear to be a massive, dry rock, but it has more water than you might expect. In a new study, scientists have shown that at least some of this water may have been delivered to the Moon’s surface from Earth’s own atmosphere.
For over a decade, water has been detected on the Moon by various spacecraft. Most of it exists in the form of ice and is concentrated around the poles, while other areas on the Moon’s surface may contain water-rich minerals. The search for and monitoring of these water-rich regions is a critical task for future lunar 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 it was generated or delivered by impacts from comets and asteroids colliding over billions of years. Some may have been deposited as hydrogen and oxygen ions from solar wind. However, they may also originate from the formation of the Moon itself, when a giant planet collided with early Earth about 4.4 billion years ago, bringing along some water.
Now, researchers from the University of Alaska Fairbanks have proposed a new mechanism that may have contributed to the Moon’s water supply. They suggest that it fell as rain from Earth’s own atmosphere for several days each month.
Specifically, Earth’s magnetic field creates a bubble called the magnetosphere, which protects us from cosmic rays. This bubble is fairly round at the edges and slightly elongated to form a point at the back, resembling a comet’s tail. For five days each month, the Moon passes through this tail.
However, some of Earth’s magnetic field lines are broken, with only one end still connected to the planet, allowing hydrogen and oxygen ions from the atmosphere to escape into space through these lines. Yet, when the Moon passes through the magnetosphere’s tail, it reconnects some of these broken lines, sending scattered ions back to Earth. Some of these ions will impact the Moon’s surface.
“It’s like the Moon is in a rain shower – a rain shower of water ions falling back from Earth, and they also fall onto the Moon’s surface,” said Gunther Kletetschka, the lead researcher.
The researchers estimate that over billions of years, this process could have deposited around 3,500 cubic kilometers of water at the Moon’s poles. And this is based on the lowest mass calculations, with only 1% of the ions escaping from Earth reaching the Moon.
Of course, water on the Moon could very well come from many other mechanisms, and this new idea is just one of them.
The study has been published in the journal Scientific Reports.
