The Moon.

It’s a dry, dusty landscape. Devoid of life, and certainly devoid of water.

Or is it? Certainly that’s how people picture the Moon, and it was the Apollo astronauts’ experience. It was so dry and dusty that it caused serious problems with their equipment.

But in the decades since the Apollo missions, astronomers have learned that there’s much more water on the Moon than we ever imagined. And that water, in the form of ice, could hold the secret to colonizing the Moon, and make it a perfect stop-off point for a mission to Mars. The first time scientists seriously suggested we might find water on the Moon was in 1961, when researchers at Caltech published a paper on the topic.

They said we might find water ice in craters near the Moon’s poles. There, in that shadowy, cold environment, it would be possible for ice to survive for millions of years. But it wasn’t until 2009 that we were able to prove them right.

Astronomers knew that asteroid and comet impacts would’ve brought some water to the Moon. And there’s more water forming on the Moon all the time, when the hydrogen in solar wind interacts with oxygen in the rock and dust. Bringing water to the Moon is one thing, but keeping it there is another.

Originally, few people thought there would be any water on the Moon. For one thing, the temperature swings wildly between day and night, reaching an average high of 107 degrees Celsius, above the boiling point of water. For another, even water vapor doesn’t survive long on the Moon.

That’s because the sunlight decomposes it into hydrogen and oxygen, and then that lightweight hydrogen escapes the Moon’s weak gravity. In fact, when Apollo astronauts brought back Moon rocks with traces of water, people were so sure the Moon was dry, that they just assumed the samples had been contaminated. Even ten years ago, while astronomers suspected there might be water, they hadn’t been able to prove it.

So how did we finally discover it? Well, the short answer is we threw stuff at the Moon to see what would happen. The long answer is that it took decades of unmanned missions and careful measurements.

The first really solid evidence for water on the Moon came in 1994 with NASA’s Clementine probe. By bouncing radio waves into shadowy parts of the Moon, we were able to discover areas of the Moon that reflected like an icy surface, instead of a rocky one. The results weren’t enough to say that there was definitely water ice down there, but it was a tantalizing hint that there might be.

Next came Lunar Prospector in 1998. This NASA probe measured the contents of the Moon’s regolith. Scientists used the data to calculate how much hydrogen was in the regolith, which was important, since water — aka H2O — has hydrogen in it.

Lunar Prospector found that there was a lot more hydrogen near the poles, suggesting we might find water there. But there was a wrinkle: the extra hydrogen they’d found near the poles was bound up in what’s known as a hydroxyl group, which consists of a hydrogen and oxygen atom. These hydroxyl groups are found in water, but they can also be bound to minerals.

So they might not have been signs of actual water the way we imagine it. At the end of its mission, Lunar Prospector tried something else to solve the water mystery. It pointed itself towards the surface of the Moon and crash landed.

The goal was to try to throw some water into the atmosphere where it could be detected. It was a great idea, but it didn’t work out the way astronomers had hoped. They didn’t find anything new.

But then, in 2008, things really kicked off. First, a study came out that analyzed lunar samples from the Apollo missions. They showed that those samples really did contain water, hidden away inside ancient beads of lava, proving that the Moon once had plenty of the stuff.

Next, the Indian spacecraft Chandrayaan-1 did a similar trick to Lunar Prospector, and crash-landed in a crater to see what would be thrown up. And this time, astronomers found clear signs of hydroxyl groups in the lunar soil. Over at NASA, they rushed to launch a satellite that could settle the question once and for all.

They called it the Lunar Crater Observation and Sensing Satellite. It reached the Moon in late 2009, and produced similar plumes of debris by crashing the rocket that carried it there into a crater. Then, it flew through the debris plume and tried to detect water.

The data showed pure, crystalline water-ice. Since there are no visible slabs of ice on the surface near the impact site, this probably came from small chunks mixed in with the lunar regolith. When researchers analyzed the data, they estimated that in just that one 20-meter crater, there were 100 kg of ice.

This is a big deal for future space missions. For one thing, humans obviously need water to, you know… stay alive. And bringing it with us is expensive, because water’s heavy!

For another, you can use water to create rocket fuel. If we went to the Moon today, we’d have to bring any fuel we needed for the return journey with us. That’s a lot more weight to blast up out of Earth’s atmosphere.

Being able to create rocket fuel on the Moon would make space travel, including missions to Mars and beyond, much more practical. And the more we know about the nature and amount of water on the Moon, the closer we get to finally colonizing our closest neighbor. Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible.

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