Discovered: Known since 3,000 years BC
Diameter: 4,900 km
Temperature on surface: -173 degrees Celsius - 425 degrees Celsius
Surface gravity: 3.7 m/s2
Distance from the Sun: 46 - 69 million km
Time taken to rotate on axis: 58 Earth days
Time taken to orbit Sun: 88 Earth days
Mercury is the closest planet to the Sun and the smallest in our Solar System. Named after the Roman messenger god, the planet is not much larger than the Earth's Moon and superficially resembles it with a surface pock-marked by enormous craters.
These craters were caused by meteorites smashing into the planet's surface in the early stages of the Solar System's evolution, some four billion years ago. Although it's only a third of the size of Earth, Mercury is almost as dense. Scientists put this down to a massive iron core, which is also responsible for Mercury's magnetic field.
Mercury is a mysterious planet
The planet holds a number of mysteries, which are only just being unraveled by space scientists:
• Its surface rock appears to have very little iron despite the presumed iron-rich core.
• There is evidence that Mercury is the hottest planet in the Solar System might even have patches of ice in permanently shadowed polar craters.
• Mercury has no real atmosphere; only a thin exosphere of gases such as oxygen, sodium, hydrogen and helium released from underlying rocks.
VIDEO: Mercury’s Lost Atmosphere.
• Mercury’s magnetic field was discovered, by the Messenger mission, to be offset to the north of the planet’s centre. This offset is very large relative to the planet’s size.
• A final mystery is the irregularly shaped hollows which have been seen in images taken of Mercury. No-one is certain what has caused them.
Mercury is difficult to study because of its closeness to the Sun.
It was first investigated by Mariner 10 which flew past three times between 1973 and 1975. NASA’s Messenger mission carried out three fly-bys of the planet in 2008 and 2009 before starting a year-long orbital mission in March 2011.
VIDEO: Messenger arrives at Mercury
UK company, Astrium Limited and a team at the University of Leicester are developing a spectrometer to study Mercury’s surface composition as part of the BepiColumbo mission, a collaboration between ESA and JAXA.