The Galileo probe is sending surprises from the biggest planet in our solar system.
Any time anyone takes a close look at Jupiter, the experience is humbling and unsettling. This has been true since 1610, when Galileo Galilei wrote "The Starry Messenger," a brief treatise concerning four bright objects he saw circling the planet. (A man named Simon Marius actually discovered the moons a year earlier and gave them their names: Io, Europa, Ganymede and Callisto.) "Starry Messenger" marked the beginning of Galileo's tragic battle with the Roman Catholic Church, casting doubt that all celestial bodies circled the Earth and eventually costing him his life.
Now, nearly 400 years later, an American spacecraft bearing his name again is peering at the Jovian neighborhood. The Galileo probe has returned to Earth 127 new images (of an expected 3,000 by mission's end) and is providing discoveries that shake conventional wisdom -- especially about the four biggest moons (now known as the Galilean satellites).
Knowledge about Jupiter and its moons had been vastly expanded by the data and images transmitted in 1979 when the twin Voyager spacecrafts sailed past the planet on their first-ever tour of the outer solar system. But Galileo's orbit permits repeated flybys of the moons -- in some cases within 500 miles -- yielding pictures with a resolution hundreds of times greater than either of the Voyagers. "When you talk about two orders of magnitude improvement, you're in a different realm," according to Steven Squyres, a planetary geologist with Cornell University.
Different, perhaps, but not more certain. The closer Galileo looks, the more it seems to replace old mysteries with new ones. Take Ganymede, for example. Earlier photos by Voyager of the solar system's biggest moon suggested a young and dynamic crust altered by ice flows and volcanism. But Galileo's powerful digital camera portrays an ancient, cratered surface. "Each ridge or furrow seen at the Voyager resolution turns out to have five or six additional ones parallel to it," says James Head of Brown University, a member of the Galileo mission team. Ganymede's dry, rocky ridges, scientists now believe, only could have been created by tectonic effects, the same forces that cause continental drift on Earth.
This isn't supposed to happen on planetary bodies as small as Ganymede. Multiple ridges are possible on Earth-sized objects, with an egglike crust encompassing a mass large enough to keep the subsurface molten. Ganymede, however, contains more internal heat than expected, and scientists already have developed an intriguing theory: The moon may possess a magnetic core, possibly composed of solid iron, similar to Earth's. …