True colour image of Mercury taken by MESSENGER spacecraft.    Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Named after the ancient Roman messenger to the gods, Mercury is the closest planet to the Sun. Due to this proximity, it is only visible from Earth just before the Sun rises in the morning or just after it sets in the evening. It is a terrestrial planet, being composed mostly of metals and silicates (rocks), and has the highest metal-to-rock ratio of all the planets.

Mercury has the unusual feature of spinning three times on its axis for every two orbits of the Sun – a trait known as spin-orbit resonance. This means that each solar day (the time taken between two meridian transits of the Sun – i.e.: from “high noon” one day to the next) on Mercury lasts two full Mercury years.

Quick Stats:

  • Classification:  Terrestrial Planet
  • Radius:  2,439.7 km   (0.38 x Earth)
  • Mass3.3022×1023 kg   (0.055x Earth)
  • Sidereal Rotation Period: 58.6 days
  • Orbital Period: 87.97 days
  • Orbital Distance: 0.39 AU (57,909,050 km)
  • Inclination: 7.0°
  • Eccentricity: 0.21
  • Axial Tilt: 2.04′
  • Density: 5.427 g/cm3
  • Surface Gravity: 0.38 g
  • Temperature-193°C to 427°C
  • Moons: 0


  • Mercury is the smallest planet in the solar system. Even the moons Ganymede and Titan are larger.
  • Even though Mercury is the closest planet to the Sun, it is not the hottest. That distinction goes to the planet Venus, whose primarily carbon dioxide atmosphere causes a potent greenhouse effect.
  • Mercury is to small and hot to retain a significant atmosphere, however the MESSENGER spacecraft has detected a very tenuous exosphere, which is constantly being lost to space and replenished by a variety of sources.
  • Due to the lack of atmosphere, the difference in temperature between the day and night sides of Mercury is more than 600°C.
  • Despite its searing daytime temperatures of over 400°C, water ice has been detected in the permanently-shadowed craters at Mercury's poles.
  • Mercury has the largest eccentricity of any planet, and its orbit ranges between 46 million and 70 million kilometers from the Sun.
  • Due to its eccentricity, Mercury's noon temperatures can vary by as much as 150°C between aphelion and perihelion (farthest and closest points to the Sun).
  • Mercury does possess a magnetic field, though it is only about 1.1% as strong as the Earth's.
  • Due to Mercury's spin-orbit resonance, and the fact that a planet's orbital speed varies during its year, an observer on the surface might occasionally see the Sun rise, then reverse direction and set, before rising again, all in a single Mercurian day!
  • The reasons for Mercury's high metal content are unknown, but the leading idea is that the planet experienced a massive impact in the early history of the solar system, with the blast stripping the planet of much of its silicate mantle and crust.
  • The largest known impact crater on Mercury is Caloris Basin, which is 1,550 kilometers across, and bordered by a mountain range around two kilometers high!
  • So far, only two spacecraft have entered orbit around Mercury: Mariner 10 and MESSENGER.

Terrestrial Planets to Scale

Terrestrial planets to scale.  Credit: NASA

Credit: NASA

The Planet Mercury

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