Io

Io The third-largest satellite of Jupiter, diameter 3643 km, and the innermost of the four Galilean satellites; also known as Jupiter I. It orbits in 1.769 days at a distance of 422 000 km, keeping the same face turned towards Jupiter. Its geometric albedo is 0.6 and its opposition magnitude 5.0. The two Voyager spacecraft in 1979 revealed that Io has explosively erupting volcanoes which eject plumes of material over 300 km high, some of it landing up to 500 km from the eruption site. The sources of the eruptions are volcanic calderas or fissures, of which there are over 300. Huge lava flows radiate from many of the volcanoes, and the whole surface is yellowish in colour due to deposits of sulphur or sulphur oxides. There are extensive plains and mountainous regions on Io but no impact craters, indicating that its surface is geologically very young. Io's density, 3.53g/cm³, suggests that it has an iron–sulphur core about 1500 km across and a silicate mantle. Io's volcanic activity is the result of heat released by tidal forces, which deform the satellite as it moves closer and further away from Jupiter in its orbit.