Ice at the moon’s poles likely formed over a long period, building up for at least 1.5 billion years rather than arriving in a single event, according to a new study published in Nature Astronomy.
The research was led by an international team of scientists, including planetary scientist Paul Hayne from the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.
A long-standing lunar mystery
For decades, data from NASA missions have suggested that water exists on the moon. Much of it appears as ice trapped in deep craters near the lunar South Pole.
These craters remain in permanent shadow. Sunlight never reaches them. Temperatures can drop to around minus 160 degrees Celsius (−256°F) , cold enough to preserve ice for billions of years.
Scientists have struggled to explain how this water formed and why it appears unevenly across different craters.
Evidence points to gradual accumulation
The new study supports a slow and steady process. Water likely arrived in small amounts over time. Possible sources include comet and asteroid impacts, ancient volcanic activity, and reactions driven by the solar wind.
Hydrogen from the solar wind constantly hits the lunar surface. Some of it reacts with oxygen in the soil to form water. The findings suggest that this process continued for at least 1.5 billion years and possibly as long as 3 to 3.5 billion years.
The study also rules out the idea that a single large comet delivered most of the moon’s water.
Older craters hold more ice
The researchers found that older craters tend to contain more ice than younger ones. This pattern points to long-term accumulation. Much of the ice is stored in “cold traps.” These are craters that have remained in darkness for extremely long periods.
Ice on the moon didn’t arrive in one big event — it built up slowly over at least 1.5 billion years, a new study finds.Scientists say ancient craters near the South Pole hold the most ice, making them key targets for future missions.#Moon #Space #NASA #Science pic.twitter.com/ePJebEQdAA
— Tom Marvolo Riddle (@tom_riddle2025) April 7, 2026
Observations from NASA’s Lunar Reconnaissance Orbiter, launched in 2009, support this. Its instruments detected signals consistent with ice in several shadowed regions. However, the ice is not evenly distributed. Some craters contain much more ice than others.
Moon’s changing tilt explains uneven distribution
To understand this pattern, the team examined the moon’s history. Oded Aharonson of the Weizmann Institute of Science led part of the work during a research visit in 2025.
Using temperature data and computer simulations, the team found that the moon’s tilt has shifted over time. Craters that are permanently shadowed today were not always in darkness.
The researchers identified which craters stayed dark the longest. These oldest and coldest regions match areas where the strongest signs of ice have been observed.
Key sites for future exploration
The findings could help guide future missions. Scientists can now focus on craters that are most likely to hold ice.
One leading candidate is Haworth Crater near the South Pole. It may have remained in shadow for more than 3 billion years, making it a strong location for large ice deposits.
NASA plans to explore these regions further under its Artemis program, which aims to return humans to the moon.
Water could support human missions
Lunar ice could become a valuable resource for astronauts. It can be used for drinking water or split into hydrogen and oxygen to produce fuel.
Hayne said more detailed observations are still needed. He is developing a new instrument, the Lunar Compact Infrared Imaging System (L-CIRiS), which NASA plans to deploy near the South Pole in 2027.
He added that the origin of the moon’s water will only be confirmed through direct sample analysis. Scientists will need to study material on the moon or bring it back to Earth.
Aharonson said finding usable water beyond Earth remains one of the most important challenges in astronomy.
