Contact: Tony Fitzpatrick smail.wustl.edu Louis
tony_fitzpatrick@ai
314-935-5272
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
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 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).