Mercury's Atmosphere — Almost No Air at All

Earth's atmosphere is what makes our planet livable. It gives us air to breathe, scatters sunlight to make the sky blue, and works as a giant blanket that keeps the surface from getting too hot during the day or too cold at night.

Mercury has nothing like that.

Mercury's "atmosphere" is so thin that scientists don't even call it an atmosphere. They call it an exosphere, and it's basically a vacuum with a few stray atoms drifting around in it. Mercury's air is so thin that down at the surface, it's about a quadrillion times less dense than the air you're breathing right now.

Does Mercury Have an Atmosphere?

Technically, yes — but only in the loosest sense of the word. Mercury has a layer of gas around it, but it's so thin it doesn't behave like an atmosphere at all. It can't block radiation. It can't trap heat. It can't scatter sunlight. It can't carry sound. It's so thin that the few atoms in it don't even bump into each other very often.

For comparison:

• Earth's atmosphere at sea level has about 10 quintillion atoms per cubic centimeter.
• Venus's atmosphere has about 92 times that.
• Mars's atmosphere has about 1% of Earth's.
• Mercury's exosphere has fewer than 1 trillion atoms per cubic centimeter — about ten million times thinner than Earth's air.

What Is an Exosphere?

An exosphere is the very topmost layer of any atmosphere — the part where atoms are so spread out that they don't really collide with each other anymore. Earth has an exosphere too, but it sits hundreds of miles above the surface, above the thick part of our atmosphere where weather happens.

Mercury skips the thick part entirely. Right at the surface, where on Earth you'd find air pressure and clouds and weather, Mercury already starts in exosphere mode. There's nothing else.

What Is Mercury's Exosphere Made Of?

Mercury's exosphere is a strange mix of light gases. The main ingredients are:

• Oxygen — not the breathable kind we inhale on Earth, but stray oxygen atoms.
• Sodium
• Hydrogen
• Helium
• Potassium
• Calcium

These atoms don't last long. They get knocked off the surface by sunlight, by tiny meteoroid impacts, and by the solar wind (a stream of charged particles flowing out of the Sun). Some atoms drift back down to the surface, while others escape into space and are gone forever. Mercury's exosphere is constantly being made and lost at the same time.

Why Mercury Can't Hold an Atmosphere

Three things make it almost impossible for Mercury to keep an atmosphere:

1. Mercury is small. Its gravity is only 38% of Earth's. Gas molecules need a fair amount of gravity to stay near a planet, and Mercury's just isn't strong enough to hold them.
2. Mercury is hot. Daytime temperatures of 800°F give gas molecules tons of energy. Energetic gas molecules zip around faster, and faster molecules escape gravity more easily.
3. Mercury is close to the Sun. The solar wind is much stronger near the Sun. It blasts past Mercury and strips atoms off the planet's surface and away from any atmosphere it might have.

Add those three things together, and you get a planet that simply cannot hold onto a real atmosphere. Whatever gas leaks out from inside Mercury, or gets delivered by comets, gets carried off into space within a few thousand years.

Why the Sky Stays Black at Noon

On Earth, the sky is blue during the day because our atmosphere scatters sunlight. Blue light scatters more than red light, which is why we see blue overhead.

Mercury's exosphere is far too thin to scatter sunlight that way. So even at high noon, the sky on Mercury stays black, just like at night. The Sun blazes in the middle of it, and you can see stars all around if your eyes can adjust to the brightness.

It's a strange image: a brutally hot planet, with the Sun three times its normal size and seven times brighter than it looks from Earth, sitting in a black sky full of stars at midday.

Mercury's Magnetic Field (and Magnetic Tornadoes)

One of the biggest surprises Mariner 10 sent back from its 1974 flyby was that Mercury has a magnetic field. That was unexpected. Scientists thought small planets cooled too fast to keep a liquid outer core, which is what generates a magnetic field. Mercury was supposed to be a frozen lump of metal.

It turns out Mercury's magnetic field is real, but tiny — only about 1% as strong as Earth's. The bubble of space carved out around the planet by that magnetic field is called a magnetosphere, and Mercury's magnetosphere is the smallest of any planet that has one. Even so, that little field does some genuinely amazing things.

When the solar wind — the stream of charged particles flowing out from the Sun — hits Mercury, it can push so hard against the planet's magnetic field that it punches through. Where it does, scientists have seen huge twisting funnels of plasma spiraling down to the surface. They call them magnetic tornadoes, and NASA's MESSENGER spacecraft was the first to spot them in 2008.

These tornadoes carry the solar wind directly to Mercury's surface. Each one is hundreds of miles wide and lasts only a few minutes, but they're a major reason why so many atoms get blasted off Mercury's surface in the first place.

Mercury Has a Tail Like a Comet

Here's one of the strangest things about Mercury's exosphere. Because the solar wind is constantly sweeping past Mercury and pushing atoms off the planet, those atoms get carried away in a long, thin stream behind it — pointing away from the Sun, like a comet's tail.

Mercury's tail is made mostly of sodium atoms, and it can stretch for millions of miles behind the planet. From Earth, with the right kind of telescope, scientists can actually photograph Mercury's tail. It's the closest thing to a comet a planet can be.

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