If, like Elon Musk, you dream of a future in which large numbers of people can live on the moon, you will likely be disappointed.
Although there may be trillions of pounds of water trapped inside tiny glass beads scattered on the surface of the moon, scientists estimate that this amount will not be enough to sustain life in a large city for a long time, according to a study published in the journal Frontiers in Space Technologies, on Monday.
Musk announced in February that his space company, SpaceX, had changed its focus from developing a settlement on Mars to “building a self-growing city on the moon,” because achieving this would be faster, and could happen “in less than 10 years,” he said.
This raised another question: "How big a city could this amount of water support on the Moon? The answer was very surprising," Martin Elvis, lead author of the study and an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, told CNN.
Taking 1 billion tons of water as a "generous" estimate, Elvis and fellow astrophysicist Jonathan McDowell found that 1 million people, roughly the population of Stockholm, would consume that entire amount in less than two-and-a-half years if it were used for drinking, hygiene, growing food, rocket fuel and breathing, according to the study.
But efficient water recycling can extend this period significantly. At a recovery rate of 98%, as on the International Space Station, a city of 1 million people could be sufficient for water for about 100 years. However, this would require extreme rationing of their use, which Elvis finds likely unsustainable.
The researchers concluded that one century of water supply does not appear to be enough to establish a sustainable, long-term settlement off Earth, despite the huge investments required to achieve this.
The researchers believe that a small city of about 100,000 people could last for 1,000 years, a period that “seems long enough to be described as sustainable,” according to the study.
McDowell told CNN: "If the town's population is only 10,000 people, the water will be enough for about 10,000 years, and then it starts to make sense, as we will have enough time to find another solution."
For example, there is "a significant amount of water in the solar system," including the asteroid belt and on Saturn's moons, McDowell explained.
He added: "Therefore, in the future, lunar residents may be able to import water in quantities necessary to support large populations. It is difficult to imagine how this would be done using current technology, but it may become possible in the long term."
In the short term, McDowell believes that building a town on the moon may take about 30 years, not 10 years, as Elon Musk suggested.
"Village on the Moon"
Astronomer Ian Crawford, professor of planetary science and astrobiology at Birkbeck University in London, who was not involved in the study, told CNN: “This is an important study, and I think the researchers have provided a valuable service by bringing a measure of realism to the debate about long-term plans for establishing large human settlements on the moon.”
He added that establishing such settlements would require importing water, or the hydrogen needed to make them from oxygen found in moon rocks.
In the study, the researchers indicated that a village of about 1,000 people on the moon, roughly the number who stay in Antarctica during the winter, might be able to use lunar water “without major restrictions.”
The amount of water at the moon's poles may be sufficient in the short term to support small research sites of this size, Crawford said, adding that he believes "this will be the most likely use over the coming decades."
But he stressed the importance of properly studying the water ice at the moon's poles before extracting it completely and using it as a water source, given its scientific importance.
Ice has accumulated on the moon's surface since its formation billions of years ago, so it "preserves a chemical and physical record of the environment in which Earth existed throughout this period, which gave rise to life on our planet," Simon Barber, a lead researcher at Britain's Open University who studies water on the moon, told CNN.
Barber, who was not involved in the study, added: “We should, at the very least, uncover the secrets this ice holds about our evolution and the evolution of our planet and other planets, before thinking about extracting it for industrial use.”
“A scramble for resources”?
Agriculture consumes the largest share of water, as feeding one person daily may require between 2,000 and 5,000 liters, according to World Bank estimates adopted by the study. Adopting a completely plant-based diet may help reduce consumption, as beef production requires six times the amount of water required to produce soybeans, while chicken requires twice the amount, according to the study.
But even with rationalization, it's still unclear how much water actually exists on the moon and how easily it can be extracted, McDowell said.
He explained that water may be abundant in very cold and shaded craters, which makes extracting it relatively easy, but it may also be present in small quantities among rocks in large areas of Antarctica, which may make extracting it extremely difficult even if its total quantity reaches large levels.
“The extreme cold and prolonged darkness” make these areas a “harsh environment for robots or humans,” Barber said. He added that the water is not in the form of pure ice cubes that can be melted and drunk, but rather it is likely mixed with toxic chemicals that must be removed first.
Researchers estimate that the eight largest craters on the Moon contain about 34 million tons of water. With a limited number of large craters to explore, those interested may seek to “take the biggest and best ones and exploit the water in them,” said Elvis, who called on countries participating in the race to explore the moon to cooperate rather than compete with each other.
Researchers are "certainly right that careful management of the lunar water's limited reserves will be necessary," Crawford said, adding that this would require an "internationally agreed upon regulatory framework" to avoid a resource run and premature depletion of a valuable lunar resource.
