Io is a most unusual moon of the planet Jupiter, with a bright, blotchy, yellow-and-orange appearance. What lies behind its pretty face?
What We Know About Io
Io is the only heavenly body known to have active high-temperature volcanism. (Signs of geologically young volcanism have been seen on the other inner planets, if you count a billion years as "young," but nothing truly active yet. And Enceladus, a moon of Saturn, has active icy geysers.) It's so active, in fact, that not a single impact crater has been found on it. Every bit of Io's surface is covered with fresh volcanic ash. Io is not just volcanic, it's supervolcanic.
Io is made of rocky material, like Earth and unlike most of the moons of the outer planets. Magnetic evidence suggests that it has a substantial layer of melt beneath its surface, effectively a magma ocean, that maintains its vigorous eruptions. Because the little planet has lost nearly all of its water, sulfur is the major volatile element in Io's magmatism, which accounts for its otherworldly colors of yellow and orange and brown.
Io is so volcanic because it sits in a gravitational wringer. The side closest to Jupiter is pulled harder than its other, outer side. So Io feels a force stretching it. Io's own gravity tends to UNstretch it, relaxing it back into a perfect sphere. Its 42-hour orbit is not a true circle, so as Io moves closer to and farther from Jupiter, stretching and relaxing, it gets quite a kneading. Its neighboring moons, Ganymede and Europa and Callisto, also contribute to the strain by locking Io into orbital resonance. All of that activity creates a huge amount of heat.
Lessons for Earth?
Does volcanism on Earth work this way? Not in the slightest. In fact, while there is gravitational wringing going on with Earth and the Moon, it creates a negligible amount of heat. In our case the result is only a gentle flexing of the crust and the slow sloshing of the oceans that we know as the tides. And despite the claims of certain Web sites, the tide has no relation to volcanic eruptions, or earthquakes either.
The Moon is immune to this effect too. First, the gravity of Earth is nothing in comparison to Jupiter's. Second, the Moon is much farther away relative to Earth than Io is to Jupiter. If Io were much closer, the gravitational forces would tear it apart into little pieces, and Jupiter would have a splendid ring around it like Saturn's. Third, the Moon's orbit is much slower, and much more circular, than Io's. As a result, the Moon is a very cold, geologically dead body that hasn't seen much action in the last few billion years.
Some of the scientists studying Io say that the very hot lavas there—hotter than anything Earth has seen for some 3 billion years—are a model for early Earth volcanism. That's a bit of a stretch, but you never know. (For even more of a stretch, see the mythological story of Io.)
Io in the News
Io was discovered when Galileo pointed his homemade telescope at Jupiter in 1610. However, he did not give it a name beyond roman numeral I. His contemporary Simon Marius named it after one of the mythological lovers of Jupiter. Galileo's discovery created a profound sensation at the time, proving to anyone with a telescope that the heavenly bodies are imperfect objects like Earth: worlds of their own.
Io has had three modern-day waves of popularity. The first was in December 1974, when Pioneer 11 took a couple of distant pictures of it. That was pretty obscure news outside the scientific community—I remember newscasters talking about "Jupiter's moon Ten." (You have to remember that newswire copy used to come in all capitals, so they were trying to read IO, pronounced "Eye-oh.") It was a funny throwback to Galileo's original nomenclature.
The second was five years later, when Voyagers 1 and 2 took pictures showing the dramatic presence of actively erupting volcanoes, the first ever found beyond Earth.
Io's third wave of popularity peaked in February 2000, when both Scientific American and the more buttoned-down Physics Today had Io on the cover, and ended in September 2003. The Galileo mission, for eight years, brought spectacle after spectacle like a fireworks show, and each thrilling fly-by showed us new closeups of this volcano world. And the Galileo website did a huge business, a standout example of science education for the public. Since then, planetary geologist Jason Perry has carried a torch for Io at his Io-dedicated blog The Gish Bar Times.
The U.S. Geological Survey is in charge of mapping all the planets, not just Earth. In 2012 it released the first global geologic map of Io, based on the combination of Voyager and Galileo images plus the years of scientific conversation since those missions. This map will be a standard for many decades to comeuntil a new spacecraft mission or until eruptions erase today's features, whichever comes first.