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Mercury

Closest planet to the Sun. Its mass is 0.056 that of Earth. On its sunward side the surface temperature reaches over 400°C/752°F, but on the ‘night’ side it falls to −170°C/−274°F.

Mean distance from the Sun

58 million km/36 million mi

Equatorial diameter

4,880 km/3,030 mi

Rotation period

59 Earth days

Year

88 Earth days

Atmosphere

Mercury's small mass and high daytime temperature mean that it is impossible for an atmosphere to be retained.

Surface

composed of silicate rock often in the form of lava flows. In 1974 the US space probe Mariner 10 showed that Mercury's surface is cratered by meteorite impacts.

Satellites

none

NASA's Mariner 10 probe, launched on 3 November 1973, arrived at Mercury on 29 March 1974, and provided the first close-up images of the planet. NASA launched a US$286 million mission to Mercury in 2004 to orbit the planet during 2008, 2009, and 2011. It will photograph the planet's surface, analyse its atmospheric composition, and map its magnetic field. The European Space Agency, in collaboration with Japan, plans to send probes to orbit Mercury, arriving in 2015 after a four-year journey.

Mercury's largest known feature is the Caloris Basin, 1,400 km/870 mi wide. There are also cliffs hundreds of miles long and up to 4 km/2.5 mi high, thought to have been formed by the cooling of the planet billions of years ago. Inside is an iron core three-quarters of the planet's diameter, which produces a magnetic field 1% the strength of Earth's.

Atlas rocket

Caloris Basin

terrestrial planet