Diameter: 4.88 x 106 m
Mass: 3.30 x 1023 kg
Orbit: 5.79 x 1010 m from the Sun
Composition: iron core, silicate mantle and crust
Distance from other planets: View matrix

Mercury is the closest planet to the Sun. It is also the smallest planet in our Solar System. Mercury is visible with the naked eye, and for that reason it was discovered very early in the history of astronomy. Its existence was first recorded by the ancient Greeks in the first millennium BC. Until the 4th century BC Greek astronomers believed it to be two bodies and so it had two names; Apollo was visible at sunrise and Hermes could only be seen at sunset. Mercury was given its current name by the Romans who named the planet after the Roman God Mercurius. He was the God of trade, and wore winged sandals to fly around swiftly, which earned him the nickname “the winged messenger”. Similar to Mercurius’ flying technique, Mercury has a very erratic orbit around the sun. Its orbital path is the most elliptical of all the planets. Mercury spins on its own axis 3 times in every 2 orbits around the sun, so its days are almost as long as its years!

The temperature on Mercury is also very erratic. Mercury has the most extreme temperature variations of all the planets with a range of 90 – 700 K. The average temperature is very hot, so the atoms in the atmosphere are easily blown into space by solar wind. The resultant atmosphere is very thin. Mercury has no satellites orbiting it. It also has a very small magnetic field, roughly equivalent to 1% of Earth’s magnetic field.

Mercury is the second most dense of the major bodies in our solar system (after Earth). It is composed of a very dense iron core with a diameter roughly equal to 3.6 x 106 m. Around the core is a molten silicate outer shell, which is 5 x 105 m thick. The surface has regions which are heavily cratered, and other regions which are smooth. The craters may have been caused by volcanic activity or deposition of matter after a collision. The largest known feature of Mercury’s surface is the Caloris basin, a large crater measuring 1.3 x 106 m across and caused by an early impact. Low resolution radar observations from Earth have shown evidence of water ice in craters near the North pole. Mercury has also been studied from much closer by two spacecraft. Mariner 10 made the first visit to Mercury in 1974-75. NASA’s Messenger probe carried out a flyby in 2008-09 and entered into orbit around Mercury in 2011.

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Jack is the content manager of and is a keen amateur journalist. After completing a degree in astronomy from Newcastle University, Jack went on to develop this website out of his passion for space.

  • Maryannu

    Knowledge of Mercury had been in existence since Sumerian times, 5,000 + years ago 🙂