Harvard-Smithsonian Center for Astrophysics scientists studying water ice believed to be stored in the Moon’s ‘pits of eternal darkness’ have concluded that there likely isn’t enough of this critical resource to support long-term future human colonies, meaning visions of large-scale cities on Earth’s largest natural satellite may be more science fiction than science fact.
Conversely, the researchers calculated that smaller settlements with fewer than 10,000 human colonists could have enough accessible water to survive for centuries. They also suggested that other technological advances in water harvesting or advances in scanning techniques that explore below the top few meters of surface regolith may reveal significant amounts of hidden lunar water.
Water Ice Stored in Moon’s Pits of Eternal Darkness
The Moon’s surface is covered with visible evidence of asteroid collisions. Along with its textured appearance, some of these massive impact craters contain areas that are permanently shaded from direct sunlight.
The Harvard-Smithsonian research team behind the new study said some of the earliest “pits of eternal darkness” could be over one million years old. More critically, they can maintain a temperature of 110K, or just above absolute zero. At this low temperature, frozen water ice trapped in the sunless craters won’t lose more than a millimeter of thickness to space every billion years.
When announcing the new study, the researchers noted that “a succession of Moon-orbiting missions” has mapped out possible locations of stored water ice since 2013. The results suggested that the Moon’s pits of eternal darkness may contain as much as a billion tons, with the largest concentrated around the Moon’s northern and southern poles.
Since those missions, scientists and industrialists have begun to speculate about the viability of long-term human lunar habitation. Unlike astronauts trapped in capsules or space stations, lunar colonists could theoretically possess enough on-site water to support farming, full-service bathroom facilities, and production of hydrogen-based fuels.
Still, researchers wondered if one billion tons of stored water ice is enough to support the types of cities that billionaire industrialists and science fiction authors have proposed.
On-Site Solar Power Could Support One-Million-Person Lunar City
To start, the team modeled how long large human settlements ranging from 100,000 to one million people could survive on the water stored in lunar pits of eternal darkness. This included water-recycling rates up to 98%, as achieved on the International Space Station (ISS).
After running the numbers, the team found mixed results.
First, even colonies up to one million humans wouldn’t run out of power. The team said that number is based on solar power generation and excludes nuclear reactors.
Instead, the Harvard-Smithsonian team proposed constructing one-kilometer-tall solar energy towers near the rims of the craters. Their models suggest that an array of towers covered in photovoltaic arrays could generate three gigawatts of usable electricity.
The team noted that the materials needed to build solar panels, including the primary ingredient, silicon, are plentiful on the Moon, meaning future lunar colonists could manufacture new solar power-generating towers on-site. Such lunar colonists could theoretically maintain enormous AI data centers near these towers capable of servicing paying customers back on Earth.
“That would be the start of a true lunar economy,” the research team explained.
Even Generous Water Ice Estimates Limit Large Lunar Colonies
When the researchers crunched the water numbers, long-term lunar habitability became significantly more dubious. In fact, most estimates had future lunar colonists exhausting the most generous one-billion-ton projected supply of water ice within “a few years.”
When researchers modeled an ISS-level water recycling efficiency level using those same water estimates, a colony of one million humans would run out of water “in just over a century.”
“And today’s best estimates of the amount of water on the Moon are about 30 times less than a billion tons, shortening the time to water exhaustion by the same factor,” they add.
For example, if more conservative ‘pits of eternal darkness’ water ice estimates are closer to reality, the researchers said that “even a small city” likely wouldn’t make it past a decade before they had tapped the Moon dry.
Untapped Sources Could Improve Long-Term Habitation Prospects
Fortunately, the news was not all bad. When the team assumed a smaller “Moon village” of over one thousand people, up to a larger settlement of 10,000, the researchers found that the water ice stored in the Moon’s pits of eternal darkness “would instead be sustainable for several centuries or more.”
The team also suggested alternate solutions for supporting a larger, long-term lunar settlement. These included potential improvements in water recycling, improved farming efficiency, or mining nearby asteroids for water.
The team also noted that additional pockets of stored lunar water ice may be awaiting discovery, suggesting that finding such lunar water may be easier than achieving sufficient farming and recycling efficiency improvements.
“Current water surveying techniques do not reach below the surface more than a few meters, while the rubble-like rocks of the surface regolith typically extend tens of meters down and may contain water in the cold traps,” they explained.
The study “No Cities on the Moon: a Billion Tons of Water is not enough for Sustainability” was published in Frontiers in Space Technology.
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.