File image of Ganymede as seen by Galileo. Photo: NASA/JPL

Planetary scientists studying Jupiter's icy moon Ganymede have combined stereo images from the Galileo mission with Voyager images from the 1970s and found provocative features on the moon. They have mapped long swathes of bright flat terrain that they think is evidence of water or slush that emerged one billion years or so ago.

This bright terrain, long since frozen over, lies uniformly in troughs about 1 kilometer (a little over a half mile) beneath Ganymede's older, darker, cratered terrain.

Ganymede, the largest moon in the solar system, is an icy satellite of Jupiter and is larger than the planet Mercury. The roles that volcanism and various forms of tectonics have played in molding the complex topography of Ganymede have been hotly debated over the years. But the newly created images, taking advantage of the quantity of the Voyager images and the higher resolution of the Galileo ones, point to volcanism as the main impetus behind the troughs.

"This is a new kind of stereo topographical information over hundreds of kilometers across Ganymede, " said William B. McKinnon, Ph.D., professor of earth and planetary sciences at Washington University in St. Louis and co-author of the study published in Nature on March 1st, 2001.

"What we think we're seeing is evidence of an eruption of water on the surface of Ganymede, " said McKinnon. "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. We can see this material is banked up against the edges of the walls of the trough and appears to have been pretty fluid, much more so than solid, albeit warm, ice. These features directly support the idea that they were created by volcanism."

The researchers used stereoimaging -- a method where three- dimensional objects are reproduced by combining two or more images of the same subject taken from slightly different angles -- to reconstruct the physical topography of Ganymede's terrains. Maps were then generated from the stereoimages.

McKinnon says the images provide fundamental new information about what really happened long ago on Jupiter's large satellite and also illuminates an essential mystery about the way the body reworks its older, darker material.

One trough extends an estimated 900 kilometers, roughly 600 miles, the approximate distance between St. Louis and New Orleans.

"The long trough is probably a billion years old, but it's actually one of the younger volcanic features," McKinnon says. "It's the last gasp of the process that made the bright terrain."

According to McKinnon, the geological explanation for such long lanes of flatness is that they occurred by the extending and opening up of Ganymede's crust. And then that portion of the crust became flooded with some sort of lava. The high- resolution Galileo images show that material that flooded the lanes 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. "

Moreover, the images reveal depressions that resemble volcanic calderas along the edges of the bright terrains. On Earth, calderas are large, more-or-less circular craters usually caused by the collapse of underground lava reservoirs.

"The caldera-like features make a pretty good circumstantial case for volcanism causing this topography, " says McKinnon. "We think these particularly bright terrains were formed by volcanism , which means that most or all the other bright terrains started out this way, and became fractured, or grooved, over time through tectonic forces."

The earliest proposal about Ganymede is that there was water on the Jovian moon billions of years ago. An alternate theory proposed that the bright features were glacier ice erupted from Ganymede's mantle. A third theory proposes that Ganymede's rifts were caused by a process similar to seafloor spreading seen on Earth. While crustal spreading could conceivably operate on Ganymede, it cannot account for the smooth swaths McKinnon studied.

"In the places we have looked at, the two edges of the trough simply cannot be matched up."

The Galileo Mission will orbit around Jupiter and fly by some of its moons for another two years before coming to an end. It has gathered valuable images of the outer solar system and enhanced researchers' understanding of Jupiter and its many moons. While it is not the first mission to explore Jupiter -- there were four before it -- a number of "firsts" have been documented. Among them: it is the first atmospheric probe to enter Jupiter's atmosphere; it is the first mission to discover a satellite of an asteroid (Ida's satellite Dactyl); it is the first spacecraft to go into orbit around Jupiter; it provided the first multispectral study of the Moon; it is the first mission to make a close flyby of an asteroid (Gaspra); it provided the first direct observations of a comet impacting a planet (Shoemaker-Levy 9) and of active extraterrestrial volcanoes (Io); and it provided the first strong evidence for an extraterrestrial ocean (Europa).