Moon Cities Face a Water Problem, New Study Finds

Visions of sprawling cities on the moon have long been a staple of science fiction, and some of the world's best-known tech executives say they are close to reality. A new analysis suggests the lunar surface may not be able to support anything close to that scale for long.

In February, SpaceX CEO Elon Musk wrote on social media that the company had shifted its focus from settling Mars to building a "self-growing city on the moon." He said this could potentially be achieved in under 10 years, compared with more than 20 for Mars. Blue Origin CEO Jeff Bezos, meanwhile, has claimed that "millions of people" will be living in space by 2045.

Those claims caught the attention of Martin Elvis, a senior astrophysicist at Harvard University's Smithsonian Astrophysical Observatory. Together with Jonathan McDowell, an honorary professor at the Durham University Space Research Centre in England, he decided to test them against the numbers. Their paper appears in the journal Frontiers in Space Technologies and focuses on two essentials: power and water.

Power turned out to be the easier problem. The researchers found that solar and nuclear sources would be enough to sustain one million people on the lunar surface. Water is where the picture changes.

Water does exist on the moon, and most of it is believed to sit in permanently shadowed regions at the poles, where sunlight never reaches. Assuming roughly one billion tonnes, which is in line with current estimates, the pair found that a population of one million could last only about 100 years. That figure also depends on recycling 98 per cent of the water, the rate achieved on the International Space Station.

Elvis said the total sounds large but is modest in context. He compared it to an aquarium the size of Central Park filled until the Statue of Liberty is just underwater. "It's a small lake," he said, "and a small lake is not going to supply a big city for very long."

A smaller settlement fares better. A town of 10,000 people would be sustainable for centuries under the same assumptions. "If you want to cover the face of the moon with cities, like in the best science fiction, then you have to find a way to get the water there, because it's not there right now," McDowell said.

The assumptions themselves may be generous. Elvis said a billion tonnes is probably "20 or 30 times too big," and that only one-third to one-half of the water present would likely be recoverable. His best guess is that the amount of water that can be counted on is about one per cent of the original figure, which would shrink 100 years to one.

There is also the question of what form the water takes. Gordon Osinski, a professor of planetary geology at Western University in London, Ont., said satellite measurements carry uncertainties. He believes the water is there but wants it confirmed on the ground. It could resemble Arctic permafrost, he said, with sub-millimetre ice crystals mixed into soil that are hard to extract.

McDowell said the findings should not be read as discouraging. "The truth is never negative," he said. "What's negative is to actually go and try and do something that isn't going to work."