February 2000 was a high-water month for Io, that most unusual moon of the planet Jupiter. The February issue of Scientific American had Io on its cover, and so did the more buttoned-down Physics Today. Is it just a pretty face?
Io in the News
This was the third time Io has made the news. The first was in December 1974, when Pioneer 11 took a couple of distant pictures of it. It was pretty obscure news outside the scientific community—I remember newscasters talking about "Jupiter's moon Ten." (You have to remember that newswire copy comes in all capitals, so they were trying to read "IO.")
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. That was 21 years before the latest burst of Iolatry. So far, Io is still the only heavenly body known to have active volcanism.
Io's third wave of popularity 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 Web site did a huge business, a standout example of science education for the public.
What We Learned About Io
Io is 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. And that creates a huge amount of heat. (Here's a classroom lesson that shows this effect.)
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.
PS: For even more of a stretch, see the mythological story of Io.
