Io is the first of Jupiter's Galilean satellites. It is linked in laplacien resonance with Europa and Ganymede. As it makes four revolutions around Jupiter, Europa and Ganymede make two and one respectively. It is not an ice satellite because it is metallic and rocky in composition. Io is one of the rare active bodies in our solar system. Sulphurous plumes have been observed several times. The surface of this satellites is different to all others in the solar system. It is very colourful and have many stains of varying sizes, shapes and colours. (Image 1)

Distance to Jupiter 421 800 km Period of revolution 1.77 earth days Diameter 3636,2 km Mass (Earth=1) 0.0149491 Density (water =1) 3.534 Composition Silicates, iron and sulphur Albedo 0.60 Temperature on the surface -143 °C

Volcanism : Io was photographed for the first time by the Voyager 1 probe in 1979. Because the satellite is so small, we expected to discover a totally cold and inactive world, Like the Moon. One scientist only, Peale, had predicted the contrary. The shots showed volcanic activity thanks to the plumes of erupting sulphur dioxide (Image 3) rising to 260 km high, appearing on the edges of the satellite, and the presence of calderas. While studying the system, the Galileo probe took new snap shots of Io.

We can see lava flows of melted silicates (Image 6), like on Earth, but far hotter. The nature of the volcanism was identified: the irruptive plumes do not come directly form the calderas but from areas close to it covered in frozen sulphur dioxide. When it comes in contact with the lava flows, the sulphur dioxide vaporizes to create plumes. The most active volcanoes are in the equatorial zone with a lifetime that can be limited. The sulphur is responsible for the diversity of colours on the Io.

The high temperatures of the lava indicates that it must not come from deep within the satellites otherwise it would have had time to cool. It comes from hot spots on the surface. Io radiates 40 more internal heat than Earth does. The high temperatures of these regions can be explained by the phenomenon of dissipation of tides. Io is deformed and so the internal friction makes some of the material melt. This participates in the volcanic activity. The vast expanses between the volcanoes stay frozen.

More recently, the probe New Horizons destined for Pluto passed close to Jupiter and its satellites. New images of Io showed particularly impressive plumes of up to 330 km high. The small shiny spots at the base of the plume is the hot lava.(Image 4)

The surface of the satellites is continuously modified by this activity and so is not much more than 10 million years old. For example, here is represented the evolution of the Pele volcano (Image 2) and the lava flows (Image 6).

The highest points on the landscape are about 10 km high. On image 5, the mountains are lit up by The setting Sun and the shadows allowed for the altitudes of the central mountain to be estimated to about 4 km high. These mountains seem to collapse in more malleable areas of the surface. Land slides produced piling up of debris creating the mountain ranges.

The determination of the internal structure was done by measuring the gravitational field around Io (the way in which mass is distributed inside a body modifies the gravitational field around the body and therefore changing the trajectory of probes flying overhead) and measurements of the magnetic field.

In the case of Io, the gravitational field shows that the satellite has a metallic core of iron and sulphur surrounded by a silicates shell with added sulphur which spans up to 30 km of the surface. Above this is the lithosphere, also comprised of silicates and sulphur. The core should be roughly 35 to 60 percent of the total radius (image 7). The volcanic activity on the surface leads to the suggestion that the outer surface could be partly in fusion and animated by convection currents (like Earth's atmosphere).

A magnetic field was measured around Io. It is either Io's own magnetic field, that is one created by the effects of a dynamo in its core, or an induced magnetic field, that is one that is a modification of Jupiter's due to magnetic materials inside Io.