Io is an odd object. For starters, its name doesn’t have any consonants – as far as I know, it’s the only major solar system object with only vowels! However, what it does have a lot of are volcanoes.
And oh my, does it have volcanoes.
In fact, Io is the most geologically-active object in the solar system. By some estimates, it resurfaces itself in as little as a millenium. (By contrast, here on Earth, we have some surface rocks that are billions of years old – and the Earth is itself an unusually active planet.) In fact, Io is so volcanic that its geography has changed visibly even in the last few years .
But why is Io so active? One possible factor is composition.
Most gas giant moons are balls of ice, with a varying amount of rocky material in their cores. The Jovian moons are somewhat more rock-heavy while the Saturnian, Uranian and (presumably) Neptunian moons are believed to be mostly ice. This is because the gas giants orbit beyond the so-called ‘snow line’ – the maximum distance from the Sun at which water ice could have been evaporated within the debris disc, during the solar system’s formation. On the inner planets, the ice would have been boiled away, with surface water later arriving either due to outgassing from the planets’ interiors or by deposition from comets. The gas giants were far enough away from the young Sun that their water reserves were never ‘steamed off’ – so their moons accreted a lot of ice.
Io, however, is a ball of rock.
Io’s average density is about 3.5 grams per cubic centimetre, roughly the same as our Moon. It’s also roughly the same as the density of the Earth’s surface rocks (typically something like 3 g/cm^3 – obviously this varies with the type of rock!). This tells us that there’s hardly any ice at all inside Io . In fact, Io’s dramatic yellowish surface colouration is caused mainly by sulfur deposits. Io is rocky because the young Jupiter was hot, glowing with residual heat from its formation – hot enough that it had its own, miniature snow line. Io orbits inside that line, the other Galilean moons orbit outside it. The heat from Jupiter boiled all the light volatiles off, so Io accreted from the heavier, rocky elements.
However, being a ball of rock isn’t the whole picture. Having a supply of rock for lava is obviously nice, but Io is small enough that it should have frozen solid by now. Our Moon is a similar size and mass and is also entirely rock . Our Moon, however, is pretty much solid all the way through, aside from a small partly-molten region around its inner core . Io, by contrast, appears to have a deep mantle with only a relatively-thin solid crust. Something is keeping Io hot – but what?
The answer comes from its giant neighbour.
After the Sun, Jupiter is the next-biggest object in the solar system. It weighs in at a shade under 318 Earth masses. And it has a gravitational field to match. Despite being hundreds of millions of miles away, even at opposition, it’s Jupiter’s gravity that is the main driver of the 100,000-year cycle of ellipticity changes in the Earth’s orbit. This directly influences our climate in the form of the Milankovitch cycles, which have been implicated as a factor in the triggering-process for ice ages.
If that’s the effect it has on us, consider the effect it has on poor little Io! Io orbits Jupiter at an average separation of 421,700 kilometres. By contrast, our Moon orbits at an average of 384,400 kilometres, so Io is almost as close to Jupiter as the Moon is to us. However, Jupiter is more than three hundred times heavier than the Earth. Just as the Moon raises tides in our seas, Jupiter raises tides inside Io’s mantle. And they’re enormous. In fact, they’re so big that the unfortunate moon is actually stretched somewhat along the Jupiter-oriented line – measured that way, its diameter is 3,660.0 Km, but measured another way it’s only 3,630.6. That’s a difference of nearly 30 Km – or nearly three and a half times higher than Mount Everest.
Essentially, Jupiter is stretching Io on a tidal rack.
But there’s worse. Io isn’t Jupiter’s only moon. It’s not even Jupiter’s biggest . Relatively nearby are the other three ‘Galilean’ moons. (These four bodies – Io, Europa, Ganymede and Callisto – were all observed together by Galileo back in 1610, hence the name. The revelation of their existence also caused a theological crisis within the Catholic Church.) The other Galileans are all epically-sized bodies themselves – Ganymede is bigger than Mercury. If they were on any other orbit, there would be no question at all about them meriting planetary status.
Anyway, the other Galilean moons tug on Io with their own not-inconsiderable gravitational fields. This affects its orbit in much the same manner as Jupiter influences the Earth’s – but on a rather-greater scale and rather-faster time period. As a result, Io’s interior is pulled one way then the other in an eternal tug-of-war between Jupiter and the Galileans. The result is frictional heating, which has served to keep Io’s interior hot and molten for billions of years. In addition, the constant tidal stressing keeps the crust thin and weak.
And all of this paves the way for a state of continual volcanic convulsion.
Io is a remarkable body with a lot of interesting behaviour. It may, however, be a long way from unique. Exoplanet research is starting to uncover a population of planetary bodies in close orbits to their stars, and this has recently been extended to rocky bodies such as Kepler-10b. These objects much presumably be subject to similar tidal stresses as Io, and may show similar behaviour. Io may eventually become a benchmark for comparison to these objects, and this highlights the importance of planetary science for the wider world of astronomy.
One example is the Acasta Gneiss, with an estimated age of about 4 billion years.
 See, for instance, this page from the New Horizons mission.
 By contrast, Ganymede’s density is 1.94 g/cm^3, rather more icy, and Saturn’s moon Dione has a density of 1.46 g/cm^3, showing that there’s not that much else there except ice.
 Note, though, that the formation mechanism for our satellite was entirely different. Our Moon formed from debris thrown off the early Earth after an enormous collision around 30-50 million years after the formation of the solar system.
 For our Moon, we have direct evidence for its internal structure – seismological readings taken both by the Apollo astronauts and by automated probes.
 In fact there are at least 60 moons, and a ring system – Jupiter is an epically-sized planet with an epically-sized satellite system!