Cities won’t survive on the moon as there isn’t enough water, scientists say

If, like Elon Musk , you have been dreaming of a future in which large populations of humans can live on the moon , you’re likely to be disappointed.

Even though there could be trillions of pounds of water trapped inside tiny glass beads spread across the lunar surface, scientists estimate that this won’t be enough to sustain a big city for very long, according to a study published Monday in the journal Frontiers in Space Technologies .

Musk announced in February that his aerospace company, SpaceX , had shifted its focus from developing a settlement on Mars to “building a self-growing city on the Moon,” because it would be quicker to achieve, potentially “in less than 10 years.”

That raised the question, “Well, how big a city could you support on the moon with that water supply? And the answer was very surprising,” lead study author Martin Elvis, a senior astrophysicist at the Harvard and Smithsonian Center for Astrophysics, told CNN.

Taking 1 billion tons of water as a “generous” estimate, Elvis and his colleague, astrophysicist Jonathan McDowell, who is now an honorary professor at the UK’s Durham University Space Research Centre (SPARC), calculated how long populations of various sizes could last if they were using water for drinking, hygiene, growing food, rocket fuel and even breathing.

They found that a population of 1 million people — about the number of people who live in Sweden’s capital, Stockholm — would finish the water supply in under two and a half years, according to the study.

However, if the water was efficiently recycled, such as at the 98% water recovery rate that the International Space Station has achieved, it would take a city of 1 million people 100 years to deplete it. And “that depends on people being very nice” about their water usage, Elvis said, including “always using rather dubious toilets and not doing laundry,” and this behavior is “unlikely to be sustainable, I would think.”

“For such a large investment, a lifetime of about a century seems to fall short of the sustainable, long-term settlement beyond Earth that some advocate,” the researchers noted.

Yang Guanyu/Xinhua/Getty Images

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They calculated that a small city of about 100,000 people — slightly fewer than the number of people living in Sandy Springs, Georgia — could survive for 1,000 years, which “seems long enough to justify being called sustainable.”

“If you have a small town of only 10,000 people… then you’ve got… 10,000 years’ worth of water, and that starts feeling like, okay, on a 10,000-year timescale, we can find another solution,” McDowell told CNN.

For example, there is “a lot of water in the solar system,” including water in the asteroid belt and on the moons of Saturn , he said. So, maybe in the future, those on the moon could import “deliveries of water in the quantities required to sustain a large population. It’s hard to see practically how to do that with current technology, but in the long term, maybe you can do that.”

In the short term, he suggested that instead of a city on the moon in 10 years, as Musk proposed, “I would say within 30 years we might have a town on the moon. That would be the way that I would translate what Elon said.”

“This is an important study. I think the authors have done a valuable service by injecting some realism into the discussion of longer-term plans for establishing large human settlements on the Moon,” astronomer Ian Crawford, a professor of planetary science and astrobiology at Birkbeck, University of London, who was not involved in the research, told CNN.

“If such settlements are to become established, I think the authors are correct in pointing out that water (or at least hydrogen to manufacture water from oxygen in lunar rocks) will need to be imported,” Crawford added over email.

A “Moon Village” of 1,000 inhabitants — about the amount of people who stay in Antarctica during the winter — could be “relatively carefree in our use of lunar water,” the researchers noted in the study. Crawford said in the short term, there is probably enough water at the lunar poles to support small-scale research outposts of this size, “and I think this will be the most likely use in the coming decades.”

However, he added that it is “also important to note that the water ice at the lunar poles is of great scientific interest, and we need to be able to study it properly before it is all dug up for water.”

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The ice on the moon has accumulated over billions of years since its formation and “therefore preserves a chemical and physical record of the environment in which our Earth has existed for all this time, and which led to the development of life on Earth,” Simeon Barber, a senior research fellow at the UK’s Open University who looks for water on the moon, told CNN over email.

“We should at the very least work out secrets this ice holds for our own evolution and that of our planet — and of other planets — before we consider extracting it for industrial use,” Barber, who was not involved in the study, added.

Agriculture puts the most demand on water resources, with the study using the World Bank’s estimate that feeding one person for one day requires two to five tons of water, or 2,000 to 5,000 liters. Ways of cutting water usage could include adopting a vegan diet, as the water needed to produce beef is six times that of soybeans, and chicken needs double, according to the study.

However, even if people optimize their usage of water, there is still “huge uncertainty in how much water there actually is on the moon,” McDowell said. “But also, how easy is it going to be to extract?”

McDowell, who said “water is going to be the gold of the solar system,” added that there is some suggestion that in the very cold, shadowed craters, the water content could be quite high , “which would make it relatively easy to extract. But on a lot of the area of the South Pole, there might be water in trace amounts among the rock and that, even if it adds up to a lot, it would be super hard to actually get out of the rock.”

Barber said the “extreme cold and extensive darkness” in those regions make them an “inhospitable place for robotic or human workers. The water is not in the form of pristine sparkly ice cubes that we could melt and drink; it is likely laced with a cocktail of poisonous chemicals that first must be removed.”

The eight biggest craters on the moon are estimated to contain about 34 million tons of water, according to the researchers. With so few big craters to explore, interested parties might “try to grab the biggest and best of them and exploit the water,” said Elvis, who urged those involved in the space race to work with — not against — each other.

The researchers “are surely correct that careful management of the limited reserves of lunar water will be required,” Crawford said. “In turn, this implies the need for an internationally agreed regulatory framework, to prevent a ‘goldrush’ mentality developing which could prematurely deplete a valuable lunar resource.”

如果你像埃隆·马斯克一样，梦想着未来人类可以在月球上大量居住，那么你很可能会失望。

科学家周一在《空间技术前沿》杂志上发表的一项研究显示，尽管月球表面散布的微小玻璃珠中可能蕴藏着数万亿磅的水，但这仍然不足以长期维持一座大城市的运转。

马斯克在 2 月份宣布，他的航天公司 SpaceX 已将其重点从在火星上建立定居点转移到“在月球上建造一座自我发展的城市”，因为这将更快实现，可能“在不到 10 年的时间内”就能实现。

“这就引出了一个问题，‘那么，用月球上的水资源，你能养活多大的城市？’答案非常令人惊讶，”哈佛大学和史密森天体物理中心的资深天体物理学家、该研究的主要作者马丁·埃尔维斯告诉 CNN。

埃尔维斯和他的同事，天体物理学家乔纳森·麦克道尔（现为英国杜伦大学空间研究中心 (SPARC) 的荣誉教授）以 10 亿吨水作为“慷慨”的估计，计算出如果不同规模的人口将水用于饮用、卫生、种植食物、火箭燃料甚至呼吸，他们能够维持多久。

研究发现，100万人口（大约相当于瑞典首都斯德哥尔摩的人口数量）在不到两年半的时间内就会用完这些水。

然而，如果水资源得到高效循环利用，例如达到国际空间站98%的水回收率，那么一个拥有100万人口的城市需要100年才能耗尽这些水资源。埃尔维斯说，这“取决于人们在用水方面非常讲究”，包括“总是使用不太卫生的厕所，而且不洗衣服”，而这种行为“我认为不太可能可持续”。

研究人员指出：“对于如此巨大的投资而言，大约一个世纪的寿命似乎不足以实现一些人所倡导的地球外可持续、长期定居。”

杨冠宇/新华社/盖蒂图片社

在太空竞赛中，中国推迟了史无前例的探月任务发射，该任务旨在寻找水资源。

他们计算得出，一个人口约 10 万的小城市（略少于佐治亚州桑迪斯普林斯的人口）可以生存 1000 年，这“似乎足以证明它是可持续的”。

麦克道尔告诉 CNN：“如果你有一个只有 1 万人口的小镇……那么你就有……相当于 1 万年的水量，这让人感觉，好吧，从 1 万年的时间尺度来看，我们可以找到另一种解决方案。”

例如，他指出，“太阳系中有很多水”，包括小行星带和土星卫星上的水。因此，或许在未来，月球上的人们可以进口“足够维持大量人口生存的水”。他补充道：“以目前的技术水平，很难实际操作，但从长远来看，或许可以做到。”

短期来看，他认为，与其像马斯克提议的那样在 10 年内在月球上建造一座城市，“我认为 30 年内我们或许能在月球上建一座城镇。这就是我对埃隆所说的话的理解。”

“这是一项重要的研究。我认为作者们为在月球上建立大型人类定居点的长期计划的讨论注入了一些现实主义，做出了宝贵的贡献，”伦敦大学伯克贝克学院行星科学和天体生物学教授、天文学家伊恩·克劳福德（他并未参与这项研究）告诉 CNN。

“如果这样的定居点要建立起来，我认为作者们指出水（或者至少是氢气，以便从月球岩石中的氧气中制造水）需要进口是正确的，”克劳福德在电子邮件中补充道。

研究人员在报告中指出，一个拥有1000名居民的“月球村”（大约相当于冬季南极洲的人口数量）可以“相对自由地使用月球水”。克劳福德表示，短期内，月球两极的水资源可能足以支持这种规模的小型科研站，“我认为这将是未来几十年最有可能的用途。”

但他补充说，“同样重要的是要注意，月球两极的水冰具有重要的科学价值，我们需要在将其全部挖掘出来获取水之前对其进行适当的研究。”

美国宇航局采取措施，避免在建造耗资300亿美元的月球基地过程中出现代价高昂的延误。

英国开放大学高级研究员西蒙·巴伯（Simeon Barber）在给美国有线电视新闻网（CNN）的电子邮件中表示，月球上的冰自其形成以来数十亿年间不断积累，“因此保存了地球一直以来所处环境的化学和物理记录，以及促成地球生命发展的因素”。巴伯致力于在月球上寻找水。

“至少在我们考虑将其提取用于工业用途之前，我们应该先弄清楚这种冰蕴藏着对我们自身、我们星球以及其他星球的进化而言的秘密，”巴伯补充道，他并未参与这项研究。

农业是用水量最大的行业。该研究援引世界银行的估算，指出养活一个人一天需要2到5吨水，即2000到5000升。研究还指出，减少用水量的方法之一是采用素食，因为生产牛肉所需的水量是大豆的六倍，而鸡肉的用水量是大豆的两倍。

然而，麦克道尔表示，即使人们优化用水，月球上究竟有多少水仍然存在“巨大的不确定性”。“而且，提取水究竟有多容易？”

麦克道尔说“水将是太阳系的黄金”，他还补充说，有迹象表明，在非常寒冷、阴影笼罩的陨石坑中，水的含量可能相当高，“这将使水的提取相对容易。但在南极的大部分地区，岩石中可能含有微量的水，即使总量很大，从岩石中提取出来也极其困难。”

巴伯表示，这些地区的“极度寒冷和漫长黑暗”使得它们“对机器人或人类工人来说都是不适宜居住的地方。那里的水不是我们可以融化饮用的纯净闪亮的冰块；它很可能掺杂着多种有毒化学物质，必须先将其去除。”

据研究人员估计，月球上最大的八个陨石坑蕴藏着约3400万吨水。埃尔维斯表示，由于可供探索的大型陨石坑数量有限，各方可能会“试图占据其中最大、最好的几个，并开采其中的水资源”。他敦促参与太空竞赛的各方携手合作，而不是相互竞争。

克劳福德说：“研究人员的观点无疑是正确的，即需要对有限的月球水资源进行谨慎管理。反过来，这也意味着需要建立一个国际认可的监管框架，以防止出现‘淘金热’心态，从而避免过早耗尽宝贵的月球资源。”