Image: NASA

In taking a new look at Jupiter's largest moon, scientists have found compelling evidence of wateror at least slushon its surface. William McKinnon of Washington University and colleagues combined photographs taken by the Voyager and Galileo spacecrafts of Ganymede (right) into three-dimensional stereoimagesand in doing so mapped long swaths of bright flat terrain. "What we think we're seeing is evidence of an eruption of water on the surface of Ganymede," McKinnon says. "We see these long, smooth troughs that step down up to a full kilometer. They're really very much like rift valleys on Earth, and they're repaved with something pretty smooth."

The scientists, who present their findings in today's issue of Nature, note that the stereoimages provide fresh insight into Ganymede's past. The troughsone of which extends for roughly 600 mileslikely formed as Ganymede's crust extended and opened up. Some sort of watery lava then flooded these lanes about one billion years ago. The Galileo images show that the material is "no less liquid than a slush," McKinnon says. "But it is not glacial ice, which would have big moraines and big round edges like a flowing glacier does." The scientists also used this stereoimaging technique to study the formation of towering mountains on Io, another Jovian moon.