Science Puzzle
Why Is Io Covered in Volcanoes?
Jupiter's moon Io is about the size of our Moon, yet it's the most volcanic world in the solar system. It has more than 400 volcanoes, and some blast plumes of gas and dust hundreds of kilometres into space. Our Moon, by contrast, has been cold and quiet for billions of years; small worlds usually lose their inner heat quickly.
Where does Io get the heat for all those volcanoes?
The Answer
From being squeezed. Jupiter's enormous gravity stretches Io, and because Io's orbit is slightly oval, the stretching keeps changing. Flexing back and forth like that heats Io's insides enough to melt rock.
Jupiter pulls harder on the near side of Io than on the far side, stretching it into a slight egg shape. If Io's orbit were a perfect circle, the stretch would stay the same and nothing much would happen. But Io's orbit is kept slightly oval by regular tugs from two of Jupiter's other moons: in the time Ganymede goes round once, Europa goes round exactly twice and Io exactly four times, so the tugs always line up in the same places.
As Io moves nearer to Jupiter and further away again, the stretch grows and shrinks. Its solid surface rises and falls by as much as 100 metres on every 42-hour orbit. All that flexing creates heat inside, just as bending a paperclip back and forth makes it hot, and the heat melts rock that bursts out as volcanoes.
In 1979 three scientists published a paper predicting that Io would be volcanic because of this squeezing. Days later, the Voyager 1 spacecraft flew past and photographed an eruption. The same squeezing helps keep a liquid ocean under the ice of Europa.
The principle: Tidal heating. A moon on an oval orbit round a giant planet is squeezed and stretched over and over by changing tides, and the flexing heats its interior enough to melt rock.