Evidence Seen for Wet Past on Ganymede. Jupiter’s Large Moon

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
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 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,” said 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. “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 said 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 report’s other authors are Dr. Paul Schenk of the
Lunar and Planetary Institute, Houston, Texas; Dr. David Gwynn
of the University of California, Los Angeles; and Dr. Jeffrey
Moore of NASA’s Ames Research Center, Moffet Field, Calif.
Images from the report are available online from NASA’s Jet
Propulsion Laboratory, Pasadena, at
http://www.jpl.nasa.gov/pictures/jovianmoons .

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
terrains. Maps were then generated from the stereo images.
“This is a new kind of stereo topographical information over
hundreds of kilometers across Ganymede,” McKinnon said. The
images provide new clues about what happened on Ganymede long
ago and how that moon 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
said. “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,” said
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 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,” McKinnon
said. “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.”

Galileo has been orbiting Jupiter since 1995. Its 12
scientific experiments have enhanced researchers’
understanding of Jupiter’s atmosphere, large moons and vast
magnetic field. It carried the first atmospheric probe to
enter Jupiter’s atmosphere. In other firsts, it was the first
mission to discover a satellite of an asteroid (Ida’s
satellite Dactyl), the first to go into orbit around Jupiter,
the first to make a close flyby of an asteroid (Gaspra), and
the first to provide direct observations of a comet hitting a
planet (Shoemaker-Levy 9). Galileo has also 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.

The Voyager 1 and Voyager 2 spacecraft each passed near
Jupiter in 1979 and then explored more distant parts of the
solar system. The Jet Propulsion Laboratory, a division of the
California Institute of Technology in Pasadena, manages the
Galileo and Voyager missions for NASA’s Office of Space
Science, Washington, D.C.