Photo: NASA
Scientists have found that craters near the south pole of the Moon that have spent the most time in shadow are likely to contain the highest amounts of water ice. These deposits were most likely formed gradually over billions of years rather than as the result of a single event, according to Space.com.
However, these so-called “permanently shadowed” craters are not always in darkness, as once believed. Changes in the Moon’s axial tilt relative to Earth and the Sun mean that illumination conditions have shifted over geological time. As a result, craters that were in deep shadow billions of years ago may no longer be permanently shadowed today — and vice versa.
In the 1960s, some planetary scientists suggested that polar craters, where sunlight strikes at extremely low angles, could be cold enough to trap water ice. But lunar soil samples returned during the Apollo program were found to be completely dry.
In 1994, radar data from the Clementine mission indicated possible water ice, later confirmed by the Lunar Prospector and the Lunar Reconnaissance Orbiter.
Water ice on the Moon could become a valuable resource for future astronauts, providing drinking water and also being split into hydrogen and oxygen for rocket fuel and breathable air. However, its exact origin remains uncertain.
Researchers including Paul Hayne, Oded Aharonson, and Norbert Schorghofer concluded that the ice did not originate from a single large event, such as a giant comet impact. Instead, it likely accumulated gradually over time.
Using temperature data and models of how lunar craters evolved as the Moon’s tilt changed, the team found that some craters that were in permanent shadow billions of years ago are not necessarily shadowed today. Ice in sunlit areas would sublimate and migrate into colder “cold traps.”
The researchers identified that the craters with the longest cumulative shadow history are the same ones where orbiting instruments, such as NASA’s LRO, have detected strong signatures of water ice.
One example is Haworth crater near the south pole, which has been in shadow for over three billion years and shows strong evidence of ice deposits.
The findings suggest that the Moon may have been accumulating water ice continuously for around 3 to 3.5 billion years.
