|Diameter||4880 km, 0.38 x Earth's|
|Average Density||5.43 g/cm3|
|Average Distance from Sun||58 x 106km = 0.387 AU|
|Eccentricity of Orbit||0.206|
|Inclination of Equator to Orbit||0°|
|Inclination of Orbit to Ecliptic||7°|
|Length of Day||58.65 days|
|Length of Year||88 days|
|No. of Moons||None|
|Gravity||0.376 x Earth's|
|Escape Velocity||4.25 km/s|
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 not very much is known about this planet. Mariner 10 mapped about 45% of the surface, showing that it looks very similar 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, it can reach a maximum of 430 C at noon, but drop to a minimum of –183 C at night. It rotates about its own axis three times for every two orbits of the Sun.
Mercury has no permanent atmosphere as this was stripped from the planet early in its life by a stream of particles, from the Sun, called the solar wind. This does, however, provide 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 do not have such a field.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001