Io, Jupiter’s most interesting moon.It is not the Earth but Jupiter’s innermost moon, Io, which is the most volcanically active body in our Solar System. Between 1995 and 2003, the space probe Galileo was able to detect around 120 volcanoes that shoot gas and dust up to a height of 400 kilometres. On average, every one hundred years the material emitted forms an approximately one centimetre thick layer all over Io, so that the surface is continually changing. Lava flows up to 300 kilometres long and with temperatures of 1500 degrees Celsius, containing sulphurous material and melted silicates, run over the surface. Various sulphur compounds are responsible for the reddish-yellow colouration.
Where does the energy required for volcanism come from? On its orbit, Io is subject to the influences of the gravitational fields of Jupiter and the neighbouring moons Europa and Ganymede. Together with Io they orbit the planet in resonance – during one Ganymede orbit, Europa orbits Jupiter twice and Io three times. Thus, the three moons are regularly positioned in a line and the two outer moons exert a combined force on Io, their innermost partner. At the same time, Jupiter pulls much more strongly in the opposite direction and this leads to a strong tidal force. One the one hand, this causes an approximately one hundred metre-high bulge in the crust to move across Io’s surface like a flood wave. On the other hand, the interior of the moon is kneaded like dough. In the process, the material heats up so much that it melts and becomes lava, powering explosive volcanoes.