The extensive volcanic activity is unexpected for such a small object. We would not expect such a small object to have a molten interior. However, we have already seen that the gravitational effects of the other moons, enhances by orbital resonances, distort Io. As shown in Fig. 25.28, this distortion changes its orientation as the moon moves around in its orbit. The effect is to cause internal friction, heating the interior. This results in a very large heat flow through the surface, about 2 W/m2. For comparison, the average heat flow on Earth is 0.06 W/m2, although some of the

Fig 25.28.

Orbital resonances and tidal heating of Io.An orbital resonance with Europa keeps Io's orbit elliptical.This means that, even though Io rotates at a constant rate, equal to its orbital period, it doesn't go around the orbit at a constant rate. If we think of Io as an ellipsoid, with an inner and outer section, the effect of the torques caused by Jupiter is for the inner layer to be out of phase with the outer layer. The two layers move against each other and generate heat.

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