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Scientists first saw patches of ice on Mercury 20 years ago, and that discovery raised a lot of questions: How could ice survive on one of the solar system’s hottest planets, and how did it get there in the first place?

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As the closest planet to the Sun, Mercury is hot. Like, unimaginably hot.

Temperatures on the surface can reach more than 400 degrees Celsius, and yet, back in 2012, NASA’s MESSENGER spacecraft spotted something incredible: masses of water ice on Mercury’s surface. Previously, radar telescopes on the ground had picked up patches of something bright and reflective in some of the planet’s craters, so this discovery confirmed scientists’ suspicions. The question was, though, how.

How could ice survive on one of the solar system’s hottest planets, and how did it get there in the first place? Based on recent research, the answer might be stranger than you’d think. Mercury’s ice may not exist in spite of the heat, but because of it.

Now, we’re talking about a place that’s way too hot for liquid water, let alone the frozen kind! But when you’re that close to a massive body like the Sun, it’s not just the temperature that matters—the gravity does too. On average, Mercury is about 60% closer to the Sun than the Earth is—close enough that the difference between the Sun's pull on the near side and the far side is pretty significant, especially when it passes closest to the Sun.

On top of that, Mercury is not a perfect sphere—it’s slightly stretched, and its stretched parts act like handles that the Sun tugs inward when it makes that closest approach. In the past, that repeated tug slowed down the planet’s rotation until it was locked to its period around the Sun. Today that same tug keeps Mercury locked in that pattern.

As a result, for every two years on Mercury, the planet experiences one day. And it lasts for nearly 176 Earth days. So, each day, each side of the planet spends over 4000 hours under the scorching Sun, and over 4000 hours in pitch darkness.

And as anyone who lives through Arctic winters will know, without the Sun’s heat for weeks on end, it can get seriously cold. That’s especially true on Mercury, which doesn’t have much of an atmosphere to trap heat, so the night side is directly exposed to the extreme cold of space. In fact, during the night, temperatures on the surface can plummet to minus 180 degrees.

Celsius—which is colder than it gets on Mars. And that’s where Mercury’s ice collects—in particular, in craters near the poles where shadows keep it dark and cold for even longer. So, that’s one mystery solved, but it doesn’t explain how the water got there in the first place.

See, when the Sun first ignited, about four-and-a-half billion years ago, its heat kept water molecules from forming unless they were about four times as far as where Earth orbits now. So scientists think that all of the water in the inner solar system must have been delivered later, by asteroids from farther out that collided with the planets after they formed. That’s the leading explanation for how the Earth got its oceans full of water, and until recently, scientists figured that was how Mercury got its water ice, too.

But it’s not the only explanation anymore. A 2020 study published in the Astrophysical Journal Letters suggests that Mercury may be creating that water… all by itself. According to their model, it all starts when energetic charged particles from the Sun strike the surface of the planet.

Mercury is especially exposed because of its lack of atmosphere—and while it has a weak magnetic field, lots of particles can still make it right through. Many of them are hydrogen ions, and when those hit the surface, they react with elements in the soil to create hydroxyl groups, which are molecules made of oxygen and hydrogen. On the day side of Mercury, the extreme heat gives the hydroxyl groups a lot of energy, so they begin moving around rapidly and reacting with each other.

Those reactions produce water vapor and hydrogen gas, which rise off the surface of the planet and drift around. Some of the molecules react further and get broken down, but some stick around long enough to drift over to the night side and land in craters. There, without the Sun’s heat, they freeze.

Some of these craters are deep enough that they never see the Sun, and over thousands of years, they’ve turned into significant patches of ice. This mechanism isn’t unique to Mercury—it happens on our Moon too, in tiny amounts. But on Mercury, the scorching temperatures on the day side and the constant barrage of solar particles make it a much bigger deal.

And according to the authors of the study, this method likely accounts for about 10% of Mercury’s ice. But while the remaining 90% likely did hitch a ride to Mercury on asteroids, those asteroids needed to get their water from somewhere, too. And according to the study’s lead author, they may have produced their water through the same process.

In fact, this research may even help us understand how water and ice got to other places in the solar system. Which isn’t a lesson most of us expected to learn from Mercury! Thanks for watching this episode of SciShow Space!

We’re still learning a lot about Mercury because, even though it’s pretty close as far as cosmic things go, it’s not that easy to get there! And if you want to find out more, you might like our episode on how we send probes to Mercury. {♫Outro♫}.