A study found that up to 1 billion tons of water may exist at the Moon’s poles, but that level of resources would make it difficult to sustain a large lunar city over the long term. It said water shortages could be a key factor limiting the scale of human settlement on the Moon.

On Oct. 10, Japanese media outlet Gigazine reported that a research team led by Martin Elvis and Jonathan McDowell of the Smithsonian Astrophysical Observatory published the findings in the international journal Frontiers in Space Technologies.

The Moon’s poles contain permanently shadowed regions that sunlight does not reach. In an extremely cold environment, ice can be preserved for long periods, and has been seen as a resource for drinking water, food production and securing oxygen.

The team said, however, that the total amount of water is limited. If the Moon has 1 billion tons of water and each resident uses 500 tons a year for living and agriculture, a city of 1 million people would be sustainable for only about 2 years without water recycling.

The situation does not change much even if the International Space Station’s 98 percent water recycling rate is applied. With 2 percent of total annual use lost each year, water would be depleted in about 100 years if 1 million people live there. By contrast, a population of 100,000 could use it for about 1,000 years.

Moreover, 1 billion tons is an optimistic estimate. A previous study estimated about 34 million tons in eight major craters expected to be rich in water. The amount of water that can actually be mined has not yet been confirmed.

Power supply was assessed as relatively less constrained. It said that installing large-scale solar power facilities in areas near the lunar poles with abundant sunlight could support a population of 1 million people.

The researchers suggested improving water recycling efficiency, cutting consumption through vertical farming, transporting water from asteroids and exploring additional deposits as solutions.

They stressed that the Moon’s water should not be treated as an unlimited resource, and said systematic water management tailored to population size is needed for long-term settlement on the Moon.