Water Ice on Mercury

Mercury is the closest planet to the Sun. Its day side hits 800°F. Its surface is covered in dust and bare rock, with almost no atmosphere. So nobody, for a very long time, thought Mercury could possibly have water.

But it does. Tucked away in deep craters at Mercury's north and south poles, there are billions of tons of frozen water. It has been sitting there in the dark for billions of years. NASA's MESSENGER spacecraft confirmed it in 2012, and BepiColombo photographed those polar craters again in January 2025.

This is one of the strangest discoveries in modern space science: the planet closest to the Sun is also one of the few places where water can survive on the surface for billions of years.

Yes, There Is Water on Mercury

Mercury's water is not in liquid form, and it's not in oceans, lakes, or rivers. It's all ice. And it sits only in very specific places: the floors of deep craters near Mercury's two poles. Anywhere else on the planet, water would either boil off in the heat or get blasted into space by sunlight.

Scientists first started suspecting Mercury had polar ice in 1991, when radio telescopes on Earth (specifically the Arecibo radio telescope and the Goldstone Deep Space Communications Complex) bounced radar off Mercury and got back the kind of bright signal you'd expect from ice. But nobody could be sure until a spacecraft went to look.

That confirmation came in 2012, from the MESSENGER orbiter. MESSENGER's neutron spectrometer detected hydrogen-rich material at Mercury's poles, exactly where the radar had suggested. The amount of hydrogen matched what you'd expect for water ice covered by a thin layer of dark, organic-rich material.

How Can the Closest Planet to the Sun Have Ice?

This is the part that surprises everyone. The trick is in two facts about Mercury that don't seem related at first.

Fact 1: Mercury barely tilts on its axis. Mercury's axial tilt is only about 2 degrees. Earth tilts 23.5 degrees, which is why we get seasons — different parts of Earth lean toward the Sun at different times of year. Mercury barely leans at all. So at Mercury's poles, the Sun is always low on the horizon, never directly overhead.

Fact 2: Mercury has very deep craters at its poles. Mercury has been hit by countless meteoroids over billions of years, and many of those impacts left deep, steep-walled craters near the poles. With the Sun always low on the horizon at the poles, the rims of those craters cast shadows that reach all the way across the crater floor. The shadows never go away.

Put those two facts together and you get something amazing: permanently shadowed craters. The Sun has never shone on the bottom of those craters. Sunlight has literally never reached the floor. Not last week, not last year, not at any point since Mercury was a freshly-formed planet.

Cold Traps: Where Ice Can Live Forever

With sunlight permanently blocked, the floors of those polar craters are some of the coldest places in the entire inner solar system. The temperature down there stays below -280°F (-170°C) for every minute of every Mercury year, forever.

Scientists call these spots cold traps. Anything cold that ends up on a cold-trap crater floor stays cold — and stays put. There's no sunlight to evaporate it, no atmosphere to blow it away, and no rain or wind to wear it down.

So when water vapor drifts into one of those craters from anywhere else on Mercury (or arrives from a comet), it freezes solid the moment it lands. And then it just stays there, in the dark, for as long as Mercury exists.

Some of the best-known crater cold traps on Mercury are named after artists, writers, and musicians:

• Prokofiev — named for the Russian composer Sergei Prokofiev. The largest known polar ice deposit on Mercury sits inside this crater.
• Kandinsky — named for the Russian painter Wassily Kandinsky.
• Tolkien — named for the author of The Lord of the Rings. Probably contains water ice on its floor.
• Gordimer — named for the South African writer Nadine Gordimer. BepiColombo photographed it in January 2025.

How Much Ice Is on Mercury?

The most-cited estimate, based on MESSENGER's measurements, is that Mercury holds somewhere between 100 billion and 1 trillion tons of water ice at its poles. That's enough water to fill Lake Erie roughly two-and-a-half times over, all sitting on a planet that's hot enough to melt lead during the day.

The ice isn't a single solid sheet. It's spread across many different polar craters, some bigger and some smaller. Some of the deposits are pure water ice, while others appear to be mixed with darker, carbon-rich material that may have come from comets or the breakdown of organic compounds. The dark layer also helps insulate the ice underneath.

Where Did Mercury's Ice Come From?

This is one of the questions BepiColombo will help answer when it arrives in 2026. Right now, scientists have two main theories, and the truth is probably some mix of both:

Comets. Comets are giant icy snowballs that travel through the solar system, and they sometimes crash into planets. Every time a comet hit Mercury, it would have vaporized into a temporary cloud of water vapor. Most of that vapor would have boiled off into space — but some would have wandered into the cold traps at the poles and frozen there.

Outgassing from inside Mercury. Like Earth, Mercury has water locked away inside its rocks. Volcanic activity (or even meteoroid impacts) can release that water as vapor. Once again, most of it escapes into space, but some makes its way to the poles and freezes.

Scientists have been adding up roughly how much ice each source could have contributed over Mercury's 4.5-billion-year history. Both processes can probably explain the amount of ice we see, so the question is which one did most of the work. BepiColombo is going to help figure that out.

Could Humans Live on Mercury?

The discovery of polar ice has changed how scientists think about Mercury. If humans ever try to set up a base on Mercury, those polar ice deposits would be the place to do it. Water can be:

• Drunk (after melting and filtering)
• Split into hydrogen and oxygen — oxygen for breathing, hydrogen as rocket fuel
• Used to grow food in greenhouses

And the polar craters offer something else: stable temperatures. Anywhere else on Mercury swings 1,100°F between day and night. But inside a permanently shadowed crater, the temperature stays cold and steady all the time. That's actually easier for spacecraft and habitats to handle than the wild swings of the regular Mercury surface.

That said, even with the polar ice, humans living on Mercury would face huge challenges:

• No air to breathe outside the habitat.
• Intense solar radiation, especially during the long Mercury day.
• Tiny meteoroids constantly hitting the surface.
• The nearest help (Earth) would be 3 to 22 minutes away by radio.
• The journey from Earth to Mercury takes years (and a lot of fuel).

So while Mercury isn't impossible for humans, it would be one of the hardest places to live in the solar system. More on Mercury's wild conditions here.

Is There Life on Mercury?

No life has ever been found on Mercury, and scientists don't think there's any reason to expect it.

For life as we know it to exist, you usually need three things: liquid water, an energy source, and protection from radiation. Mercury's polar ice gives you the water (frozen, but at least present). The Sun's heat is plenty of energy. But Mercury's lack of atmosphere and weak magnetic field mean radiation is a huge problem.

Even if microbes somehow got delivered to Mercury (on a meteorite, say), they would likely die within a short time. The polar craters are interesting, though: deep, dark, cold, and stable. They're a tiny bit like the deep-sea hydrothermal vents on Earth where extreme microbes live. But until a spacecraft lands at one of Mercury's polar craters and tests for biological molecules, this remains a question without a real answer.

For now, Mercury looks like a dead world — a fascinating one, full of surprises like its iron core, its shrinking surface, and its hidden polar ice, but lifeless.

Frequently Asked Questions