We Knew There Was Water on the Moon, But Not Why Some Craters Were Empty. Now We Have the Answer

Water on the Moon: A Valuable Resource with Hidden Challenges

For decades, scientists have suspected the presence of water on the Moon, but it wasn’t until recent missions that its existence was confirmed—primarily in the form of ice. This discovery was a major milestone for future lunar exploration and potential colonization, as water is essential not only for human survival but also for producing oxygen and rocket fuel through processes like electrolysis. However, accessing this resource is far from straightforward. Water is not evenly distributed, and while ice has been detected in certain polar craters, many others with seemingly ideal conditions remain completely dry. This inconsistency puzzled researchers for years—until a new study provided a clear and detailed explanation.

Permanently Shadowed Regions: Cold Traps for Ice

Lunar ice has been primarily found in what are known as permanently shadowed regions (PSRs), deep inside craters near the Moon’s south pole. Due to the Moon’s low axial tilt, some of these craters never receive direct sunlight, allowing temperatures to remain below -200 °C. Under such extreme conditions, any water molecules—delivered by comet impacts, solar wind interactions, or surface chemistry—can become trapped and preserved indefinitely. These cold traps are therefore prime candidates for hosting ice deposits. Yet, surprisingly, not all shadowed craters contain water, raising a critical scientific question: why are some of them empty?

The Mystery of the Dry Craters

The absence of ice in certain craters that meet the necessary thermal conditions has long been a mystery. If temperatures are cold enough to preserve water, why isn’t it present everywhere? This is where a recent study led by scientists at the University of Colorado Boulder provides groundbreaking insight. By combining surface temperature data from NASA’s Lunar Reconnaissance Orbiter (LRO) Diviner instrument with advanced computer simulations of the Moon’s long-term rotational evolution, researchers discovered that the Moon’s orientation has shifted significantly over billions of years. This phenomenon, known as true polar wander, means the Moon’s rotational axis—and thus its poles—have not remained fixed in place throughout its history.

The Key Lies in a Crater’s Thermal History

A region that is a permanent cold trap today may not have been one in the distant past. Due to shifts in the Moon’s axis—driven by internal mass redistributions such as the formation of large volcanic basins—some areas now in permanent shadow may have been exposed to sunlight long ago. This means that while they are currently cold enough to retain ice, they never had the chance to accumulate it, as they were too warm in earlier epochs. In other words, the presence of water depends not only on current conditions but also on a crater’s long-term thermal history. Craters that have remained in shadow for billions of years are far more likely to harbor ice.

Implications for Future Lunar Exploration

This discovery has significant implications for upcoming lunar missions, especially those aiming for human settlement. Instead of focusing solely on current geography or surface temperature, mission planners and scientists must now consider the age and long-term thermal stability of potential landing sites. The study suggests that older craters at the lunar south pole are more likely to contain ice, making them top priorities for exploration. This deeper understanding will help optimize site selection and increase the chances of accessing one of the Moon’s most valuable resources: water.