Mercury

Mercury is the smallest planet in the Solar System and the one closest to the Sun. With almost no air to hold in heat, it's a world of extremes: scorching by day and frozen solid by night.

What kind of planet is it?

Mercury orbits the Sun at an average distance of about 0.39 AU, roughly two-fifths of the distance between Earth and the Sun. At about 4,879 km across, it's around 0.38 times Earth's diameter and the smallest planet in the Solar System. Its surface is packed with craters large and small, so at first glance it looks a lot like the Moon.

Inside, though, it's a very different story. Mercury is a rocky planet like Earth, but its iron-rich core is enormous, thought to take up about 85% of the planet's radius. That makes this little world remarkably dense: second only to Earth in the Solar System. Thanks to that big core, Mercury also has a magnetic field, although a weak one.

The surface features the vast Caloris Basin, about 1,500 km wide, along with cliff-like scarps that run for hundreds of kilometers. These cliffs are thought to have formed as the planet's interior cooled and contracted, wrinkling the surface as it pushed up. In other words, Mercury has been slowly shrinking over a very long time.

Mercury takes about 88 days to orbit the Sun but about 58.6 days to spin once on its axis. It has a unique rhythm: exactly three rotations for every two orbits. As a result, one full day from sunrise to sunrise (a solar day) lasts about 176 days. On Mercury, a day is twice as long as a year!

Atmosphere and weather

Mercury has almost nothing we'd call an atmosphere. There's only an extremely thin scattering of atoms such as sodium, potassium, oxygen, hydrogen and helium, supplied by the solar wind and knocked loose from the surface. It's so tenuous that scientists call it an exosphere instead of an atmosphere. So there's no wind, no clouds and no weather.

Without a blanket of air to trap heat, the temperature swings are staggering. The dayside facing the Sun reaches about 430°C (806°F), hot enough to melt lead, while the nightside can't hold on to its heat and drops to around -180°C (-292°F). The calculated average is about 167°C, but day and night are so extreme that it's hard to find anywhere actually close to that average.

Amazingly, this scorching planet has ice. Mercury's axis is barely tilted (about 0.03°), so the floors of some deep polar craters never see sunlight. Observations from the MESSENGER spacecraft confirmed that water ice is hiding in these permanently shadowed places.

Moons and rings

Mercury has no moons at all. It's thought that being so close to the Sun makes it hard for Mercury to hold on to small bodies against the Sun's powerful gravity. It has no rings either. Mercury and Venus are the only two planets in the Solar System without moons.

So no moon ever rises in Mercury's night sky. Instead, with no air, the sky is always black, and Earth and the Moon would appear side by side as two bright dots among the stars.

Exploration

Because Mercury sits so close to the Sun, it's hard to observe with telescopes from Earth and tricky to reach with spacecraft. A probe speeds up as it falls toward the Sun, so the biggest challenge in getting into orbit around Mercury is actually slowing down.

• Mariner 10 (USA, launched 1973): Flew past Mercury three times in 1974–1975 and took the first close-up images of its surface. Because the same side was sunlit on every pass, it could map a little less than half of the surface.
• MESSENGER (USA, launched 2004): After slowing down with several flybys of Earth, Venus and Mercury, it became the first spacecraft to orbit Mercury in 2011. Over more than four years it completed a map of the whole planet and found evidence of polar ice. In 2015, out of fuel, it ended its mission by crashing into the surface.
• BepiColombo (Europe/Japan, launched 2018): Built jointly by the European Space Agency and the Japan Aerospace Exploration Agency, it's made of two orbiters flying stacked together. It has been adjusting its speed with repeated flybys of Earth, Venus and Mercury, and is aiming to enter Mercury orbit in late 2026.

Fun facts

• Mercury is the fastest planet in the Solar System, racing around the Sun at an average of about 47 km per second.
• Jupiter's moon Ganymede and Saturn's moon Titan are both wider than Mercury, but Mercury is far more massive.
• Seen from Mercury's surface, the Sun looks about 2.5 to 3 times larger than it does from Earth.
• Because its orbit is a fairly stretched-out ellipse, some places on Mercury get a strange sunrise: the Sun rises, pauses, slips backward for a while, then rises again.
• Some calculations show that, averaged over a long time, the planet closest to Earth is Mercury, not Venus. Mercury always stays near the Sun, so it never gets very far from us.

See it in Planetica

Turn on Planetica's Solar System view and Mercury appears on the innermost orbit around the Sun. Planet positions are calculated from J2000 orbital elements for today's date, so Mercury will be at a different spot on its orbit each day you visit. Since it completes a lap in just 88 days, you'll notice it moving even if you check back only a few days apart.

Select Mercury to fly in for a closer look. Its size, surface gravity, day length, axial tilt, average temperature and orbital period are based on real values. With no moons or rings, Mercury circles the Sun alone.

Note that the surface patterns are created by Planetica's planet generator to match the planet type, so they differ from real maps of Mercury. Real features like the Caloris Basin aren't drawn as they actually are.

Other planets

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• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune