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Solar wind may have forged water on the Moon — exactly where future NASA missions will need it

The regolith itself contains a stockpile of hydrogen and water thanks to solar wind, and it's most abundant at the lunar poles, says a study of Chang'e-5 samples.

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The future landing site of Artemis III may be richly laden with hydrogen and water. That’s the conclusion of a recent study of regolith grains from China’s Chang’e-5 lander.

Space weather researcher Yuchen Xu, of the Chinese Academy of Sciences, and colleagues published their findings in a recent paper in the Proceedings of the National Academy of Sciences.

What’s New — Grains of lunar regolith (fine grains of rock and glass) from the Chang’e-5 landing site have a lot of hydrogen stuck to them, and nearly all of it got there thanks to solar wind. When solar wind, a constant barrage of charged particles from the Sun, blasts the surface of the Moon, it leaves hydrogen ions stuck in the outer layers of individual, tiny grains of regolith. Sometimes those hydrogen ions bind with oxygen atoms, forming hydroxide or water.

Scientists already found hydrogen stuck to the regolith grains brought back by the Apollo missions, but regolith at the Chang’e-5 landing site contains a lot more hydrogen than the Apollo samples. And Xu and colleagues say that could mean there’s even more hydrogen — and water — buried in the regolith near the lunar poles.

(The instruments Xu and colleagues used couldn’t distinguish hydrogen from water or hydroxide, so as a general rule, when we say “hydrogen” in this story, we mean “hydrogen, water, and other molecules with hydrogen in them.”)

All six Apollo missions landed close to the Moon’s equator — in the lunar tropics, if you’re willing to think of the Moon as having tropics. That meant that most of what we knew about the chemical makeup of regolith came from places where daytime temperatures were highest. And volatile chemicals like hydrogen, hydroxide, and water, embedded in the surfaces of hydrogen grains, tend to escape, or “out-gas,” when they’re heated.

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But in late 2020, Chang’e-5 landed 43 degrees north of the equator, which is the lunar equivalent of landing in Michigan. The highest daytime temperature here is about 350 Kelvin, compared to about 380 Kelvin at the Apollo sites. Xu and colleagues examined a few of regolith grains Chang’e-5 brought home, and those grains turned out to be richer in hydrogen than the Apollo samples. Xu and colleagues say that’s because solar wind deposits hydrogen everywhere on the Moon, but at higher latitudes, the cooler temperatures, leave more of the hydrogen stuck in the grains.

Digging Into The Details — Lunar geologists have known for a while that solar wind leaves hydrogen stuck to regolith. But they haven’t been sure how long the hydrogen would actually stay put, especially when the regolith was sheltered from the solar wind. If you heat regolith, volatile hydrogen molecules tend to break free and drift away, a process called “outgassing.” At the surface, there’s more or less a balance between hydrogen outgassing and solar wind adding more hydrogen. If a moonquake or meteor impact deposits a fresh layer of regolith over an old one, though, the old layer can still out-gas, but it’s shielded from the solar wind that could restock its stash of hydrogen.

Chang’e-5 scooped up regolith from a few centimeters beneath the surface, and Xu and colleagues found that the 17 grains they studied were still rich in hydrogen. That suggests that regolith actually holds onto hydrogen, water, and other compounds pretty reliably, at least at the fairly temperate latitudes where Chang’e-5 landed.

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“We didn’t know how well the solar wind-derived water was preserved in the topmost few centimeters of the surface, where the solar wind irradiation was shielded by only a single grains but the diffusion loss of water was going on,” study co-author Yangting Lin, also of the Chinese Academy of Sciences, tells Inverse. “Our analysis and reheating experiments demonstrate that the solar wind-derived water is stably preserved.”

And when Xu and colleagues heated a few of the grains in the lab, they had to turn the heat up to well above noontime temperatures for about 28 hours — just to get 20 percent of the hydrogen trapped in the grains to out-gas. It turns out that hydrogen embedded in lunar regolith is pretty stable.

The heating experiment also helped Xu and colleagues build models of how much hydrogen gets locked in regolith grains at different latitudes, with different daytime temperatures, on the Moon. And that’s how they concluded that polar regolith should contain about 9,500 parts per million of hydrogen, on average — including about 560 parts per million of water. That lines up well with data from the Moon Mineralogy Mapper, which suggested somewhere between 400 and 700 parts per million of water trapped in the regolith near the lunar poles, not counting the ice in deep, permanently shadowed polar craters.

Here’s The Background — China’s Chang’e-5 lander sent two kilograms of regolith and Moon rock back to Earth in late-2020. That included a few scoops from the top few centimeters of regolith, along with rock samples drilled from about two meters deep. The sample capsule, which landed in Mongolia in mid-December 2020, contained the first pieces of the Moon brought back to the Earth since 1976.

Chang’e-5 gave scientists a closer look not only at higher lunar latitudes (a.k.a., Moon Michigan) but also at a much younger part of the Moon’s crust. By studying the samples and confirming their age, researchers hope to find evidence of relatively recent (in geological terms) volcanic activity on the Moon. But so far, they’ve found much older signs of volcanic and meteoric cataclysms: beads of volcanic glass much older than the nearby basalt rock, and bits of rocky debris from far-flung meteor impact craters.

And recently, a closer look at exactly 17 of the regolith grains Chang’e-5 sent home is shedding light on where the Moon keeps its scattered bits of water ice — and how solar wind keeps it stocked.

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Why It Matters — NASA is already planning to harvest water — and possibly plain old hydrogen — from the lunar surface to support a base on the Moon or a supply station on the way to Mars. Astronauts will need water to drink and to grow plants on the Moon or Mars. And rockets use hydrogen and oxygen as fuel. In other words, the Moon and its hydrogen-laden regolith could become the spacefaring equivalent of a gas station, letting astronauts fuel up and grab a drink on their way to Mars.

Artemis III, the first Artemis mission to land on the Moon, will probably land somewhere near the Moon’s South Pole, which is exactly where Xu and colleagues’ findings — and remote sending data from spacecraft orbiting the Moon — suggest that there should be the most hydrogen and water embedded in the regolith. So that’s very good news for long-term lunar planning.

Meanwhile, Xu and colleagues’ heating experiment suggests that getting at that material could be as simple as heating the regolith and collecting the gas it releases. (Of course, when you’re talking about setting up an industrial process on the Moon, “simple” doesn’t necessarily mean “easy.”)

What’s Next — Upcoming missions, including Artemis III, will bring home more samples of the lunar surface for scientists around the world, like Xu and colleagues, to study. And Lin is looking forward to that.

“I would like to have lunar soil samples from high latitude, especially polar regions,” says Lin. “It can be expected in the near future lunar exploration. In addition, we would like to have ancient lunar soil samples (e.g. 3-4 billion years ago).”

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