Haemus Montes rises 10 kilometres above the surrounding plains.
IO - MOON OF JUPITER
Io is a small world, only slightly larger than our Moon. Claims of active volcanism on Venus are controversial, and Triton's geysers are just feeble smears, but Io is most definitely alive. It is without doubt one of the most geologically violent places we know. Of the solid Solar System bodies Io is also the most colourful. Its spectacular yellow, red, brown and black splodges earned Io the nickname "the Pizza Planet".
Orbit
Io orbits Jupiter at a distance of 421,600 million kilometres, with an orbital eccentricity of 0.0041. Io completes one orbit of Jupiter every 1.769 days, and this period is the same as Io's rotational period. Io exhibits synchronous rotation
Io is also trapped in a resonant situation with Europa. Europa's orbital period is exactly twice that of Io's. The consequence of this resonance is that Io suffers additional gravitational forces every time Io travels past Europa, leading to distortion of Io's shape and severe internal heating. The orbital resonance leads to Io radiating at a rate of 80 million million Watts.
More about Io's interior
Io.
Hubble Space Telescope view of Io, and its shadow, passing over Jupiter's turbulent clouds.	Europa, left, and Io, right, as seen by Voyager 1.	Io's shadow travels over Jupiter's clouds at 17 kilometres per second.
Physical properties
Io is only slightly larger than our own Moon. With a diameter of 3642.6 kilometres, it is only 168.4 kilometres wider than the Moon. Io is also more massive than the Moon. Whereas the Earth-Moon system could almost be considered a double planet, Io however is dwarfed by the huge size of Jupiter.
Io has a density of 3530 kg m-3, giving it the highest density of the Galilean satellites. The density of the Galilean satellites decreases with distance from Jupiter, supporting the theory that Io, Europa, Ganymede, and Callisto formed with Jupiter. Io having the highest density is therefore to be expected.
Io's interior structure.
Interior
Although Io's density is similar to our Moon's, the behaviour of the interior is dramatically different. Our Moon is geologically dead, but Io, suffering from tidal heating, has a great deal of internal heat that drives surface processes.
Magnetic field
A weak magnetic field may have been detected that originates from Io. As the Galileo spacecraft approached Io, on-board instruments started to detect large changes in the magnetic field environment. However, it is not known whether this is a detection of a magnetic field from Io, or a detection of the interaction between Io and Jupiter's powerful magnetic field.More about Io and Jupiter's magnetism
Surface
Io has one of the most dynamic and active surfaces in the entire Solar System. The strong internal heating resulting from the orbital resonance between Io and Europa manifests itself as constant volcanic activity on the surface.
The surface of Io is also the youngest surface in the Solar System - the volcanic activity wipes clean the historical record. Indeed, there are no impact craters on Io. The activity on the surface, the hotspots, the eruption plumes, can even be monitored from as far away as Earth (778 million kilometres).
Volcanic activity on Io can be seen from Earth, using the Hubble Space Telescope.	Voyager view of Io's dynamic surface, and the large ring around the volcano Pele.	Galileo image of Io surrounded by sodium. The red spot on Io is Pele volcano and the bright flare is Prometheus' plume.
More about Io's surface
Atmosphere
The atmosphere may be a localised atmosphere, that is, it only exists above active volcanic vents, and may not surround the entire moon. The Galileo spacecraft has imaged Io, through different filters, allowing us to see a sodium cloud surrounding the moon.
Such constant surface activity is bound to generate an atmosphere, even if it is only a tenuous one and rapidly leaks into space. Material from Io's volcanoes may be responsible for colouring Jupiter's tiny moon Amalthea, and sulphur has been found within Jupiter's magnetosphere.
Discovery and exploration
Since the discovery of Io in 1610, very little was learnt of Io for over 300 years. Io was nothing more than a point of light. The greatest discoveries had to wait for the Voyager mission, which revealed for the first time that Io was the most dynamic moon of all.