Volcanoes of Io

Two: Super Volcanoes

Olympus Mons may be the largest known volcano in the solar system, but it has probably been extinct for millions of years. But there are active volcanoes in the solar system, volcanoes that dwarf anything Earth can offer. Strangely enough, these are not on any of the planets, but on a moon instead, and a relatively small one at that.

Io, one of the four large moons of Jupiter known as the Galilean Satellites after their discoverer, Galileo Galilei, is the most volcanically active body in the solar system. Unquestionably the weirdest moon in the solar system, Io orbits 262,000 miles (421,600 km) from Jupiter. This is closer than our moon is from Earth. Jupiter—which is more than 40 times wider than our moon—fills Io’s sky like an enormous striped balloon. Io is a large moon, about 2% bigger than our own. At first glance, Io resembles a cheese pizza, with its blotchy, swirling patterns of red, yellow, orange and white, or perhaps something that had been left sitting in the refrigerator too long. Unlike most of the other moons in the solar system, which are covered with craters, Io’s surface features hundreds of volcanic calderas, or large flat-floored depressions, many of which are violently active. In fact, there are more volcanoes erupting at any one time on Io than on Earth, making this moon the most volcanically active body in the solar system, with new eruptions breaking out all the time. Some of the most powerful eruptions can throw bright plumes of material up to 300 km (186 miles) above the surface. But these eruptions look nothing like those we see on our planet. Since Io has very little atmosphere the dust and gas cannot create the billowing clouds we see spewing from volcanoes on Earth. Instead, Io’s volcanoes look like vast garden sprinklers, the gas and dust curving in huge arcs, forming umbrella-shaped plumes over the vents.

Some of Io’s volcanoes spew molten rock, like volcanoes on Earth, while others erupt molten sulfur compounds. It is the sulfur that gives Io its extraordinary colors since sulfur changes color depending on its temperature. Io’s internal heat is created by tidal flexing caused by Jupiter—although since Io is very much closer to the big planet its internal tides are much more powerful than Europa’s. It is also affected by the gravity of nearby moons Europa and Callisto, as though caught in a tug of war between those moons and Jupiter. These forces cause the surface of Io to rise and fall by as much as 100 meters (300 feet) in the 43 hours it takes to make a single orbit of Jupiter! This movement creates a huge amount of friction within the crust of the moon which in turn generates vast amounts of heat.