Approximate mean distance from Sun: 58 million kilometers (36 million miles)
Orbital period (year): 88 Earth days
Solar day (from one noon to the next): 176 Earth days
Atmosphere: Extremely thin atmosphere called an "exosphere," contains oxygen, sodium, helium, potassium, hydrogen and some other elements.
Mercury is the smallest planet in the Solar System.
Now that Pluto is a dwarf planet, Mercury is the baby of the planets. A hollow container the size of Mars – the second smallest planet – would have room for nearly three Mercurys.
Mercury probably used to be bigger than it is now.
Mercury could be described as a Moon-sized metal ball with a rocky crust, because its metal core is so big compared with the outer layers. This suggests that Mercury was once larger, and that a collision might have caused it to lose an outer layer.
Mercury is so close to the Sun that it's very hard to observe.
We used to know less about Mercury than many other planets. Astronomers must observe Mercury at twilight when it's low in the sky, and the features aren't clear then. Half the planet had never been seen until 2008 when NASA's Messenger probe took pictures. Interestingly, 13 times a century we can see the disk of Mercury cross in front of the Sun. This is called a transit and the next one will be on May 9, 2016.
Astronomers used to think that Mercury did not turn on its axis.
Since Mercury is close to the Sun, we would expect it to keep the same side facing the Sun, as the Moon does while it orbits Earth. However in 1965, astronomers studied the planet with radar, and discovered that it rotates three times every two years.
Mercury has a very elliptical orbit. An ellipse is a squashed circle.
The average distance between Mercury and the Sun is 36 million miles. Yet it gets as far away as 43,380,000 miles (aphelion) and as near as 28,580,000 miles (perihelion). If you could stand on Mercury at perihelion, the Sun would look three times as big as we see it from Earth.
Mercury is the fastest-moving Solar System planet.
Mercury's average speed is 107,000 km per hour (67,000 miles per hour). It's so close to the Sun that it has to move fast to keep from being pulled in by the Sun's gravity. If you could go that fast, you could get all the way around the Earth at the equator in less than 25 seconds. No wonder Mercury has the name of the swift Roman messenger god.
A day on Mercury is longer than a year on Mercury.
A year on Mercury is very short, only 88 Earth days long. On Earth, a day – from one noon to the next noon – is 24 hours. On Mercury it's 176 Earth days. That means that a day on Mercury is two years long.
Mercury has enormous temperature changes during a day.
The day side can heat up to 430 °C (800 °F). That's hot enough to melt the metals tin, lead and zinc. On the nigh side, the temperature can drop to -179 °C (-290 °F). This is as cold as some of the moons of Saturn, even though they are more than twelve times as far away from the Sun as Mercury is. By the way, the lowest temperature ever recorded at the surface of the Earth was a comparatively mild -89 °C (-129 °F).
There is ice on Mercury.
NASA's Messenger probe detected ice at the poles. There are places on Mercury that never get direct sunlight so it stays very cold.
One of the biggest impact craters in the Solar System is on Mercury.
Mercury is cratered like the Moon. And something pretty big hit Mercury in its early days to make a hole the size of the Caloris Basin. This impact crater is about 1550 km (960 miles) in diameter. That's more than the distance from New York City to Chicago or from London to Rome.