Jupiter's satellites, first seen by the Voyager spacecraft in 1979, set the tone for exploration of the outer Solar System. Nothing matched our preconceptions.
Perhaps the most bizarre of Jupiter's large moons was the innermost, Io. Io, about the size of Earth's moon, has almost no impact craters, and to a planetary geologist, that means something is burying or destroying them. Io's surface is covered with red, yellow and white materials as well as dark volcanic vents. Even more surprisingly, nine vents were observed to be active during the Voyager encounters.
What can power so much volcanic activity on a moon as small as Io, which is the size of our own moon? On Earth, volcanoes are powered by steam, but Io's gravity is so feeble it long ago lost all its water vapor. We need something abundant in the universe, heavy enough for Io's weak gravity to hold it, easy to vaporize, and preferably capable of explaining the strange surface colors. Sulfur fills every requirement. Some geologists believe the sulfur coating is thin; others believe that almost all of Io's sulfur is on the surface in a layer kilometers thick.
Where does the energy come from? Our own moon is geologically dead, but Io is still very active. The energy comes from Jupiter. Jupiter's gravity long ago locked Io's rotation so that Io shows only one side to Jupiter, just like the moon does to the earth. But unlike earth, Jupiter has four large moons, and every time Io passes one of them, that satellite tries to twist Io around to face it. This tug of war pumps enough energy into Io to keep its interior hot, just like kneading a lump of modeling clay makes it warm. In the process, Jupiter's rotation slows down, but Jupiter is so huge the effect is too tiny to notice.
Created 21 December 2007, Last Update 25 May 2011
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