Water ice is no longer just a wild idea about the Moon. Evidence from lunar missions points to ice hiding in deep, permanently shadowed craters near the south pole. The bigger mystery is how that water got there in the first place, and why some craters seem to have kept much more of it than others.
A fascinating new study suggests the answer may not lie in one dramatic moment, such as a huge comet impact. Instead, the Moon’s ice may have built up slowly, over immense stretches of time, in places that have stayed cold and dark for billions of years.
Image Credit: Domenichini Giuliano via Shutterstock
Why the Moon’s South Pole Is So Important
Today’s plans for returning humans to the Moon are focused heavily on the lunar south pole. The reason is simple: this region may contain valuable stores of water ice.
The Moon is very different from Earth. Earth’s tilted axis gives us seasons, with the Sun appearing higher or lower in the sky throughout the year. The Moon has almost no tilt today. Near its poles, the Sun stays close to the horizon. Because of that, sunlight cannot reach the floors of some deep, steep craters.
These areas are called permanently shadowed regions. Some have not seen sunlight for extremely long periods. Without direct sunlight, temperatures can fall low enough for ice to survive. These especially cold places are known as cold traps.
NASA’s Lunar Reconnaissance Orbiter, launched in 2009, has helped scientists map and study these regions. Earlier observations suggested that some polar craters may contain ice. But the ice is not spread evenly. As planetary scientist Paul Hayne of the University of Colorado Boulder said in the Dutch release, “What is clear is that the ice is unevenly distributed.” Some craters appear richer in ice than others, and for a long time there was no good explanation for why.
The Oldest Shadows May Hold the Most Ice
The new study, published in Nature Astronomy, looked at whether the age of a shadowed region is linked to the amount of ice it appears to contain.
That question matters because the Moon has not always looked exactly as it does now. In the distant past, it had a greater axial tilt. Over billions of years, that tilt decreased. As the Moon gradually “straightened up,” more craters near the poles slipped into permanent shadow and cooled down.
Researchers from the Weizmann Institute of Science, the University of Colorado Boulder and the Planetary Science Institute studied when different craters became permanently shadowed and when they became cold enough to preserve ice. They combined temperature data, ultraviolet observations from the Lunar Reconnaissance Orbiter and computer simulations of how the Moon’s polar shadows changed over time.
Their conclusion was striking: the places that became cold traps earlier tend to show stronger signs of ice.
“We found that the earlier a region became shadowed, the larger the area that was able to accumulate ice,” says Prof. Oded Aharonson of the Weizmann Institute of Science. He added that the trend began at least 1.5 billion years ago and has continued even during the past 100 million years.
That pattern points away from the idea that all lunar water arrived in one sudden event. A single massive comet impact would not easily explain why the oldest cold traps seem to have collected the most ice. Instead, the evidence suggests that ice has been building up gradually from one or more continuing sources.
Hayne put it this way in the Dutch release: “It seems that the oldest craters of the Moon also contain the most ice.” According to him, that implies the Moon may have been collecting water more or less continuously for as long as 3 to 3.5 billion years.
Not Every Dark Crater Is a Good Ice Trap
Permanent shadow alone is not enough. For ice to remain on the lunar surface for hundreds of millions or billions of years, temperatures must stay extremely low, around minus 160 degrees Celsius.
Some shadowed craters are still too warm because nearby crater walls can radiate heat into them. That means a crater may be dark, but not cold enough to preserve ice over geological time.
This helps explain why the ice appears unevenly distributed. The study found that some places have been efficient cold traps for billions of years, while others became suitable only much later.
One important example is Shackleton Crater, close to the lunar south pole. It has long been considered a promising place to search for ice because it has been shadowed for about 3.5 billion years. But the researchers found that it became a true cold trap only around 500 million years ago.
Haworth Crater, also near the south pole, appears more promising. According to the study, it may have been in shadow and cold enough to collect ice for more than 3 billion years. That makes it a strong candidate for future missions seeking lunar water.
The source of the water is still not fully settled. It may have come from ancient lunar volcanoes, from asteroids and comets hitting the surface over time, or from solar wind. Solar wind carries hydrogen from the Sun, and some of that hydrogen may react with material on the lunar surface to form water.
The researchers also considered how water can be lost or buried. Their model describes the Moon a bit like a leaky bucket: water may be supplied and lost relatively quickly, while some of it survives only where cold traps can preserve it.
What Lunar Ice Could Teach Us
Lunar ice is valuable because it may be useful. Astronauts could potentially process it into drinking water or split it into hydrogen and oxygen for rocket fuel. But it is also scientifically valuable.
“The gold-standard proof of the existence of ice on the Moon would be a sample of it,” says Aharonson. Such a sample could help scientists compare lunar water with water on Earth and better understand where the Moon’s water came from.
Future spacecraft and rovers could target the oldest cold traps, including Haworth Crater, to collect better data or samples. The results could help mission planners decide where to land and where to search first.
For now, the main takeaway is that the Moon’s water story appears to be slow, uneven and shaped by ancient shadow. The ice did not simply appear everywhere at once. It collected where time, darkness and extreme cold gave it a place to last.
If you are interested in more details about the underlying research, be sure to check out the paper published in the peer-reviewed science journal: Nature Astronomy, listed below this article.
Sources, further reading and more interesting articles:
Why Jupiter's four largest moons are among the most interesting worlds of our solar system - (Universal-Sci)
How many of Earth’s moons crashed back into the planet? - (Universal-Sci)
Does our Moon Have an Atmosphere? - (Universal-Sci)
How many moons does our solar system have? - (Universal-Sci)
Who owns the moon? A space lawyer answers - (Universal-Sci)
Saturn’s moons may be younger than the dinosaurs – so could life really exist there? - (Universal-Sci)
Observational constraints on the history of lunar polar ice accumulation - (Nature Astronomy)
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