Early in 1979, a seemingly routine issue of America’s premier general scientific periodical hit the shelves. That issue of Science magazine included a research paper by Stanton Peale of the University of California, and his two NASA colleagues, Pat Cassen and Ray Reynolds. The team spent several months studying Jupiter's four big satellites, and they calculated the amount of tidal heating inside them. On Io (pronounced eye-oh), the innermost of the four satellites, they noticed something odd. The gravity of the other large satellites forces Io into a slightly elliptical orbit. As a result, tidal heating is unusually strong inside Io. The researchers predicted that the heating in Io's interior might cause ""widespread and recurrent surface volcanism. This was a totally unexpected prediction, because astronomers had previously assumed that in the region of distant Jupiter, five times farther from the Sun than the Earth, satellites would all be icy, geologically dead worlds.
The research team&quo;s article might have faded into obscurity, except for its fortuitous timing. As that issue of Science hit the stands on March 2, 1979, a spindly spacecraft called Voyager 1 was approaching Jupiter and Io after a voyage of a year and a half. This would be humanity’s first clear look at the satellites Galileo discovered so long ago. On March 3, the spacecraft was close enough to Io to reveal its odd, splotchy surface with orange colored markings — unlike those of any other planetary surface ever seen. Strangely, there were no impact craters, which showed that the surface must be geologically young. Photos taken in the next few days revealed a world that looked like a pizza, in the words of one investigator. Patchy deposits of sulfur compounds stained the surface white, orange, and black. Still no impact craters were visible. Voyager 1 sailed past Io as scientists struggled to explain these surface features, which were unlike those seen on any other world.
On March 8, Voyager 1 photographed Io in crescent phase, backlit by the Sun. Later that day, Linda Morabito, an engineer using the images to help navigate Voyager 1, discovered a faint bulge projecting from the edge of Io. Within a few days, excited scientists recognized this as a cloud of volcanic debris shooting up from the surface. They soon located several more such clouds. Thermal infrared data from other instruments onboard Voyager 1 revealed hot spots at these erupting sites that were 200 C hotter than their surroundings. Instead of being a dormant, icy world, Io had the first erupting volcanoes ever discovered beyond the Earth. In fact, Io is the most active volcanic world in the solar system! The theoretical prediction by Peale, Cassen, and Reynolds was dramatically proven correct.
The story of the discovery of volcanoes on Io is one of the most impressive successes of the scientific method. Using only theoretical tools — the laws of gravity and ideas about internal tidal heating — humans on Earth had been able to predict the existence of active volcanoes on a distant world. The prediction was confirmed by direct observation a few weeks later. Without the photographic evidence from the Voyager spacecraft, the theory of Ionian volcanism might have remained no more than a crazy idea that contradicted current beliefs about the outer solar system. But the evidence supported the theory, and the scientific method again proved to be a powerful tool in investigating the nature of the universe.