Water likely accumulated on the moon slowly over billions of years, rather than during one big event, according to a new study by an international team of scientists
The researchers, including Paul Hayne, a planetary scientist at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado Boulder, published their findings April 7 in the journal Nature Astronomy.
The study gets at a lunar mystery that has puzzled scientists for decades. Observations from NASA missions and other sources have provided tantalizing hints that water might be plentiful on the moon. It gathers as ice in the deep, dark craters around the moon's South Pole.
But how that ice got there, or why it seems to exist in some craters but not others, hasn't been clear.
The team's findings can't pin down the exact source, but they rule out a few possibilities—including water arriving on the moon at once on a humongous comet crashing into the lunar surface.
"It looks like the moon's oldest craters also have the most ice," said Hayne, an associate professor in the Department of Astrophysics and Planetary Sciences. "That implies the moon has been accumulating water more or less continuously for as much as 3 or 3.5 billion years."
Water on the moon would be a goldmine for astronauts, Hayne said. Future lunar explorers could mine ice for drinking water, or even to produce rocket fuel by splitting apart the hydrogen and oxygen atoms.
"Finding water beyond Earth in liquid and usable form is one of the most important challenges in astronomy," said Oded Aharonson, lead author of the study and a planetary scientist at the Weizmann Institute of Science in Israel, in a press release [link to WIS release].
Permanent lunar shadows
Hayne cited several possible sources for the moon's water: Volcanoes in the distant past may have transported water from deep inside the moon to its surface . Water may have also traveled to the moon on comets or asteroids, and it may have arrived via solar wind—a steady stream of charged particles that flows away from the sun and into the solar system.
"Through the solar wind, a constant stream of hydrogen bombards the moon, and some of that hydrogen can be converted to water on the lunar surface," Hayne said.
Regardless of where the water came from, scientists like Hayne are fairly certain that ice has built up in what are known as "cold traps"—craters on the lunar surface that exist in permanent shadow and haven't seen the sun for, in some cases, billions of years.
Observations from the Lyman Alpha Mapping Project (LAMP) instrument on the NASA's Lunar Reconnaissance Orbiter (LRO), which launched in 2009, found evidence of what might be ice in some of those craters.
"What's clear is that the ice has a patchy distribution," Hayne said. "It's not concentrated in the same quantities in every crater. And there was no great explanation for that."
Ice boxes
Hayne, Aharonson and co-author Norbert Schörghofer wanted to come up with an explanation—and to do that, they hit rewind on the moon's history. Aharaonson led the work as a visiting scholar at CU Boulder in 2025. The team used lunar surface temperature data from LRO's Diviner instrument and a series of computer simulations to estimate the evolution of craters on the lunar surface.
Hayne noted that the moon didn't always sit in the orientation we know today. Instead, its tilt relative to Earth has shifted over time. As a result, craters that are in shadow today may not always have been in shadow.
Drawing on their simulations, the researchers came up with a list of the moon's cold traps that have been darkest the longest.
The team also discovered something intriguing: The moon's oldest and darkest craters are also where LAMP had seen the greatest signs of ice.
The team's results may give astronauts hints about where to go looking for water. The moon's Haworth Crater, which sits near the South Pole, for example, has likely been in shadow for more than 3 billion years. It's a top candidate for storing a lot of ice, Hayne said.
The planetary scientist said that researchers need to collect more detailed observations of craters on the moon that may harbor ice. He's developing a new instrument to do just that called the Lunar Compact Infrared Imaging System (L-CIRiS), which NASA plans to deploy near the moon's South Pole in late 2027
"Ultimately, the question of the source of the moon's water will only be solved by sample analysis," he said. "We will need to go to the moon to analyze those samples there or find ways to bring them from the moon back to Earth."
