Ganymede’s Liquid Past


Ganymede topography

This computer-generated image reveals that younger, smoother terrains are lower than older, heavily faulted terrains. The lower areas may have been flooded by liquid water or water-ice slush, which then froze over.

Long swaths of bright, flat terrain 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. NASA scientists have combined stereo images from NASA’s Galileo and Voyager missions to examine these provocative features on the 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 larger 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. But the newly created images, taking advantage of the quantity of 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," says Dr. William B. McKinnon, professor of Earth and planetary sciences at Washington University in St. Louis and co-author of the study published in Nature on March 1, 2001. His co-authors are Dr. Jeffrey Moore of NASA’s Ames Research Center, Moffet Field, CA; Dr. Paul Schenk of the Lunar and Planetary Institute, Houston, TX; and Dr. David Gwynn of the University of California, Los Angeles.

"What we think we’re seeing is evidence of an eruption of water on the surface of Ganymede," says 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."

Elevations on Ganymede

(Image a) A band of grooved terrain called Erech Sulcus intersects a smooth swath called Sippar Sulcus.
(Image b) The smooth terrain of Sippar Sulcus is at a low elevation (shown in blue), while the rough surrounding terrain is at a higher elevation (green, yellow, and magenta).

McKinnon 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 were created by volcanism.

The researchers used stereo imaging 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 terrain. Maps were then generated from the stereo images. McKinnon says the images provide fundamental new information about what really happened long ago on Jupiter’s large satellite, and illuminate a mystery about the way Ganymede reworks its older, darker material.

One trough extends an estimated 900 kilometers (about 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. The lanes then 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," says McKinnon. "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 terrain. On Earth, calderas are large, more-or-less circular craters usually caused by the collapse of underground lava reservoirs.

Boundary between smooth and rough terrain
Smooth terrain penetrates into the grooves between ridges, creating "bays" (indicated by arrows) along the "coastline." Stiff glacial ice probably would not have filled in the troughs as completely. This image suggests that the smooth areas may have been created by fluid water or ice slush rather than glacial ice.

"The caldera-like features make a pretty good circumstantial case for volcanism causing this topography," McKinnon says. "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."

Since there seems to be evidence for both liquid water and volcanoes on Ganymede, can the discovery of life there be far behind? Many scientists now believe that life only needs three conditions to emerge: liquid water, organic chemicals, and heat energy. But according to Ron Greeley, professor at Arizona State University and head of the NASA Astrobiology Institute’sEuropa Focus Group, it is more likely that life could exist on Jupiter’s moon Europa than on Ganymede.

For one thing, Ganymede experiences less tidal stress than Europa. With less gravitational tugging from Jupiter, Ganymede probably has much less heat energy.

"When we look at the geophysical models for heat production on Europa," says Greeley, "potentially there’s much more heat produced on Europa than there is on Ganymede."

Possible caldera

This image shows a possible caldera from which liquid water or slush may have once flowed.

Ganymede also has less evidence than Europa of biologically important chemistry.

"Magnesium salts and other materials that are important for exobiology exist on Europa in greater variety and greater abundance than on Ganymede," says Greeley.

Our understanding of moons like Europa and Ganymede and the possibility of finding life there has vastly improved thanks to the Galileo spacecraft. In addition to this most recent finding about Ganymede, Galileo has provided extensive information about active volcanism on the moon Io, and the possibility of a subsurface ocean on the moon Europa. Later this year, it will make close approaches to the moons Callisto and Io.