Wet Past of Jupiter's Largest Moon.

• Position: Ganymede - Brief Article

Bright, flat terrain in long swaths on the surface of Jupiter's icy moon Ganymede may testify that water or slush emerged there about a billion years ago, say planetary scientists who have combined stereo images from NASA's Galileo and Voyager missions to examine provocative features on that moon. This bright terrain, long since frozen over, lies uniformly in troughs about one kilometer (a little over a half-mile) lower than Ganymede's older, darker, cratered terrain.

Ganymede is the largest moon in the solar system and bigger than the planet Mercury. The roles that volcanism and various forms of tectonics have played in molding its complex topography have been hotly debated over the years. The large quantity of Voyager images and the higher resolution of Galileo's point to volcanism as the main impetus behind the troughs.

"What we think we're seeing is evidence of an eruption of water on the surface of Ganymede," indicates William B. McKinnon, professor of earth and planetary sciences at Washington University in St. Louis (Mo.). "We see these long, smooth troughs that step down up to a full kilometer. They're really very much like rift valleys on the Earth and they're repaved with something pretty smooth. The material in the troughs is more like terrestrial lava in terms of its fluidity than relatively stiff glacial ice." He says the material is banked up against the edges of the walls of the trough and appears to have been more fluid than solid ice would have been, even if it were relatively warm ice. These features support the idea that they...