Contact: Tony Fitzpatrick
Washington University in St.


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


“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 Ganymedeís

mantle. A third theory proposes that Ganymedeís 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 ñ 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).