Evidence Seen For Wet Past On Ganymede, Jupiter's Largest Moon
Ron Baalke
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
CONTACT: Washington University/Tony Fitzpatrick (314) 935-5272
JPL/Guy Webster (818) 354-6278
NASA Ames Research Center/Kathleen Burton (650) 604-1731
FOR IMMEDIATE RELEASE March 1, 2001
EVIDENCE SEEN FOR WET PAST ON GANYMEDE, JUPITER'S LARGEST 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.
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