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Satellite of Jupiter (the third-largest), discovered by Galileo Galilei on January 7, 1610. It has a diameter of 3630 km. The semi-major axis measuring 422,000 km, the orbital period of 1.77 days. Its density (∿3,6 g / cm3), which is the highest among those of all the satellites of the solar system, indicates that it is made almost entirely of rock material. I lack of impact craters completely, which shows that the satellite’s crust is geologically very young; also it was not found traces of water in any of its states. The satellite has an extremely rarefied atmosphere of sulfur dioxide. I was the only body in the solar system, other than Earth, which certainly has active volcanoes, the eruptions of which are similar to huge geysers, with jets that reach heights of hundreds of kilometers. The ejected material is not, as in terrestrial geysers, water (in liquid and steam), but sulfur dioxide that, in large part, condenses into small crystals that precipitate to the ground. The intense volcanic activity makes you think I will find mostly in liquid form; probably, at the bottom of a thin solid crust there is a ‘ocean’, formed mainly by sulfur and compounds of this element. The existence of active volcanism on a satellite relatively small as I was a surprising fact: in fact, the other bodies in the solar system that have similar size, is volcanically dormant for billions of years. I must therefore have an internal energy source different from those present in other planets and satellites. It is thought that his heating is produced by the combined effects of gravitational fields of Jupiter and neighboring satellites (Europa and Ganymede). The latter prevents that I rotate around Jupiter synchronously, that is, always giving him the same hemisphere. On the other hand the tides, induced gravitational attraction of Jupiter, force the satellite toward this state that, however, can not be reached: the result is the dissipation of mechanical energy into heat, which goes to heat the interior of the satellite . The power (∿100 million megawatts) that powers the volcanism of Io to be supplied by the energy of the motion of Jupiter’s rotation, which would slowly braked by interacting with its satellite. What you hear, it is the original sound of the satellite.