No cities on the moon — there isn't enough water, scientists say (op-ed)
Even a small city would run dry after just a decade or so.
by Martin Elvis · Space
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The discovery of water at the poles of the moon has inspired speculation about whether we can build there – cities, industrial neighborhoods, places where people work and live. But is there enough water, and can we use it efficiently enough? In their new article in Frontiers in Space Technologies, Dr. Martin Elvis and Dr. Jonathan McDowell do the math. Even at the most generous estimates of the available water and recycling efficiency, a city of one million people would only last 100 years before all the water runs out. In this guest editorial, which Frontiers in Space Technologies provided, Elvis breaks down what we can and can't do with the technology available.
The discovery of water in dark craters at the moon's poles has sparked a "moon rush" to establish bases there, and then villages, cities and heavy industry. How sustainable is this rush?
Soon after the moon was formed, asteroids bombarded it, creating its pockmarked topography. Near the moon's poles, there are dozens of crater floors that have not seen direct sunlight in about four billion years. Many of these cryogenically cold "pits of eternal darkness" are below a temperature of 110 Kelvin (minus 262 degrees Fahrenheit), forming "cold traps" in which water ice won't sublimate away by more than a millimeter in a billion years, even in the vacuum at the moon's surface. And there is water to freeze: some of those asteroids brought it along with them. Since 2013 we have known that that water is still there, possibly as much as a billion tons of it, thanks to a succession of moon-orbiting missions that have mapped out likely locations in increasing detail.
The presence of water at the lunar poles changes everything about human visits to the moon. Suddenly it is practical to envisage a "moon village" where people could survive without importing every last bit from Earth. They could grow their own food and have showers and functional toilets, unlike the astronauts on the International Space Station (ISS). This possibility has sparked ambitions from the likes of Jeff Bezos to move heavy industry to the moon and Elon Musk's aims to have "self-growing cities" there. These are enticing visions, but are they attainable?
The good news
We realized that we could make a first feasibility estimate simply by looking at how much power and water a large population would use on the moon, and how long a city of 100,000 or one million people could survive there. How sustainable is a city on the moon?
The good news is that power, even without nuclear reactors, is not a problem. That's because the rims of the craters containing the pits of eternal darkness are in almost permanent sunlight. (That's why they are sometimes, slightly incorrectly, referred to as the "peaks of eternal light.") Up on those highly illuminated rims it would be possible to generate three gigawatts of electricity using kilometer-tall towers covered with photovoltaic arrays. It should be possible to manufacture solar panels on the moon. There's certainly plenty of silicon. This could even make AI data centers on the moon possible, by placing them near these sunny peaks. That would be the start of a true lunar economy.
The bad news
The bad news is that, even assuming a generous one billion tons of water to start with, without recycling the answer to our question of sustainability is that the city would last only a few years. Even in the best-case scenario, with 98% efficient recycling of water — the same as achieved on the ISS — a one-million-person city will run out of water in just over a century. That is a problem for sustainable moon settlement. 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. This means that even a small city would run dry after just a decade or so. A moon village with a thousand people, or even a town with 10,000 people, would instead be sustainable for several centuries or more.
There are potential solutions. We could improve the efficiency of water recycling by a factor of five or more; lower water usage from new techniques such as vertical farming; import water, probably from accessible asteroids; or, simply, find more water. Of these it may be this last hope that is most promising. 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. If the ambitious plans of the space billionaires are to be realized, then finding more water will be the first step.