Where does the energy come from to heat up Jupiter's moon Io?
We know that Io’s volcanism is caused by tidal stresses as the other satellites tug on Io causing small deviations from a circular orbit, thus Jupiter’s tidal distortion of Io changes over the orbit and that generates heat in its interior. But what is losing energy that Io then gains? Is the orbit of Io shrinking, for example, or is Jupiter’s rotation slowing a tiny bit?
Chris Smith put this question to planetary geoscientist David Rothery from the Open University, with a little help from materials scientists Anna Ploszajski from University College London...
David - It’s quite true - you can’t get something for nothing. I suspect Io’s orbit is actually increasing; it’s getting further from Jupiter and therefore orbiting more slowly and Jupiter’s spin will be slowing down very slightly as well. So these tidal interactions are robbing energy from the combined system of Jupiter and it’s moons.
Chris - So as the thing goes round Jupiter it’s getting squeezed and stretched a bit and that’s causing friction in the material of the moon, and that’s where the heating comes from in the moon?
David - You can try this at home. Get a metal coat hanger, bend it to and fro a few times, just touch it to your lips, you’ll burn your lips. Not badly but you’ll feel the heat. It’s this internal friction because of the tidal stressing and it does the job. But the energy’s coming from Io’s orbit and Jupiter’s spin.
Chris - Anna…
Anna - So what you’re doing there when you’re bending the coat hanger is you’re manipulating the atoms inside the metal and the metal in this case, is behaving a bit like a plastic, so it’s very pliable. The reason is heats up is because the atoms as they slide over each other in their atomic planes are rubbing against each other. And yeah, you're right, that’s where friction comes from and so it gets released in heat.