Tides and Volcanoes on Io
Tidal Forces Cause Volcanic activity on Jupiter's Moon Io
The same effects that cause ocean tides also cause Io to be the most volcanically active world in the solar system.
Tides
Imagine spreading your blanket out on a wide expanse of beach, lazily dozing off, and awakening to the waves wetting your feet. What caused this incoming tide? With his law of gravity, Isaac Newton explained how the Moon causes tides. Because the gravitational force decreases with distance the side of the Earth facing the Moon gets a greater gravitational tug from the Moon than the opposite side. The tidal force is the difference in gravitational force between two sides of an object, as opposed to the force itself. It pulls water upward on the portion of Earth directly under the Moon and tugs the water on the opposite side less. Hence Earth's oceans collect into two tidal bulges directly underneath and opposite the moon, causing high tides. We have two high tides and two low tides a day. The times for tides change as the moon orbits Earth.
In addition to sloshing the oceans around, tidal forces also bulge the rock comprising Earth itself. Just as repeatedly bending a wire will warm the metal, the constant bulging and squeezing of the rocks contributes to heating Earth's interior. The friction also slows Earth's rotational rate, very slowly. We always see the same face of the Moon because Earth induced tides on the Moon have slowed its rotation period to the point that it equals its orbital period around Earth. It is tidally locked. Earth's rotation rate will continue to slow until Earth is also eventually tidally locked to the Moon.
Volcanic Activity
Frictional tidal heating makes a contribution but is not the major heat source for Earth's interior. Earth and other planets formed from gradual collisions and accretion of material left over from the Sun's formation. These collisions combined with radioactive materials in Earth's composition generated a lot of heat. Hence Earth's hot interior provides the energy needed to drive volcanic activity and keep Earth volcanically active. Smaller planets and moons cool off faster just as the smaller amount of soup in a spoon will cool faster than the larger soup bowl. With cooler interiors these smaller planets and moons don't have enough interior energy to drive significant volcanic activity. The Moon and Mercury have had little if any volcanic activity in their past. Mars, slightly larger, had active volcanoes in the distant past, but died out as Mars' interior cooled.
Volcanoes on Io
Io is the closest, to Jupiter, of Jupiter's four major moons. Roughly the size of our moon, Io should not have significant volcanic activity. Yet it does. When the first voyager mission zipped past Jupiter in 1979, the shots of Io revealed seven active volcanic eruptions. Rather than being volcanically dead like the Moon, Io is more volcanically active than Earth and is the only other world in the solar system with confirmed active volcanoes. (The jury is still out for Venus.) Why? As the closest to Jupiter, it has the greatest tidal effects from Jupiter. Io's interior tidal heating provides enough heat energy to drive volcanic activity on Io despite its relatively small size. So Io has volcanoes while the more distant of Jupiter's moons, Ganymede and Callisto, are volcanically dead. Europa, the second closest of Jupiter's major moons, has enough interior tidal heating to melt the subsurface ice into interior oceans, but not to drive volcanoes. Ultimately Io's volcanoes have the same cause as your wet feet on the beach.
It seems retrospectively obvious that Io should have an interior heated by Jupiter induced tides. Just days before the Voyager reached Jupiter, S.J.Peale, P. Cassen, and R. Reynolds published a paper predicting this tidal heating effect and possible volcanoes on Io, but otherwise volcanoes on Io were not widely expected prior to the voyager mission. In principle we should be able to apply scientific laws and theory to accurately predict what will happen, but we often get it wrong. Nature has a way of constantly outsmarting us and coming up with effects that nobody thought of. That is why we need to constantly observe nature, do experiments, and explore our universe.
