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Diameter4880 km, 0.38 x Earth's
Mass0.06 ME
Mean Density5.43 g/cm3
Mean Distance from Sun58 x 106km = 0.387AU
Eccentricity of Orbit0.206
Inclination of Equator to Orbit
Inclination of Orbit to Ecliptic
TemperatureDay: 430°C
Night: -183°C
Length of Day58.65 days
Length of Year88 days
No. of MoonsNone
Surface PressureNone
Gravity0.376 x Earth's
Escape Velocity4.25 km/s

The cratered surface of Mercury

Mercury is the closest planet to the Sun and the second smallest in our Solar System. It has only ever been visited by one spacecraft Mariner 10 so relatively little is known about this planet. Mariner 10 mapped about 45% of the surface, showing that is bears a close resemblance to the surface of the Moon. It is rocky and covered in craters caused by meteors hitting the surface. The largest of these craters is 1300km in diameter and is called the Caloris basin.

Mercury has the most elliptical orbit of all the planets except for Pluto and has the most extreme temperature range in the Solar System, reaching a maximum of 430 C at noon, and a minimum of 183 C at night. It is in synchronous lock with the Sun, rotating about its own axis three times for every two orbits of the Sun.

Mercury has no permanent atmosphere as it was stripped from the planet early in its life by the solar wind. This does, however, contribute a small amount of hydrogen and helium as a temporary atmosphere.

The planet has a very high density, and is therefore believed to be around 70% iron, concentrated in the core. This is surrounded by rock, which makes up the remaining 30% of Mercury's mass.

Mercury is the only terrestrial planet other than Earth to have a magnetic field. It is very weak, but is still significant with a strength of 3.5 x 10-7 Tesla. This was unexpected as the much larger planets Mars and Venus are incapable of supporting such a field.

Planets Introduction



The Moon










Extra-Solar Planets

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Authors: Carolyn Brinkworth and Claire Thomas

Last updated: July 2001