Scientists have long suspected that water exists on the moon, but exactly how it got there—and where it is most concentrated—has remained a mystery.
Now, a new study suggests that lunar water didn’t arrive all at once.
Instead, it likely built up slowly over billions of years, collecting in some of the coldest and darkest places on the moon.
The research, led by an international team of scientists including Paul Hayne from the University of Colorado Boulder, was published in Nature Astronomy.
It helps explain why water ice appears in some craters but not others, especially near the moon’s South Pole, where temperatures can drop so low that ice can survive for extremely long periods.
For years, data from NASA missions hinted that water ice might be hiding in deep craters that never see sunlight.
These areas, known as “cold traps,” are permanently shadowed regions where temperatures are so low that ice does not melt or evaporate. However, scientists didn’t fully understand how the water got there or why it seemed unevenly spread.
The new study suggests that the moon has been slowly collecting water for up to 3 to 3.5 billion years.
This finding challenges earlier ideas that a single large event—such as a massive comet impact—delivered most of the moon’s water in one go. Instead, the evidence points to a gradual process.
There are several possible sources of this water. Ancient volcanic activity may have released water from deep inside the moon.
Comets and asteroids could have brought water during impacts over time. Another surprising source is the solar wind—a stream of particles from the sun. When hydrogen from the solar wind hits the moon’s surface, it can combine with oxygen in the lunar soil to form water molecules.
No matter where the water came from, it appears to have slowly migrated into cold traps, where it became locked as ice. But not all cold traps are equal. To understand why some hold more ice than others, the researchers looked at the moon’s long history.
Using temperature data from NASA’s Lunar Reconnaissance Orbiter and advanced computer simulations, the team studied how the moon’s tilt has changed over billions of years. Unlike today, the moon’s orientation in the past was different, meaning that some craters now in permanent shadow may once have received sunlight.
The researchers identified which craters have stayed in darkness the longest—and these turned out to be the same places where the strongest signs of ice have been detected. In other words, the older and darker a crater is, the more likely it is to contain significant amounts of water ice.
One especially promising location is Haworth Crater near the moon’s South Pole, which may have been in shadow for more than 3 billion years. Scientists believe it could store large reserves of ice.
These findings are important for future space exploration. Water on the moon could be used for drinking, growing food, or even making rocket fuel. Researchers are now developing new instruments to study these icy regions more closely, and future missions may collect samples directly.
In the end, scientists say the only way to fully understand the moon’s water is to visit these sites and study them up close—or bring samples back to Earth.
Source: University of Colorado at Boulder.
