PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Franklin O'Donnell
FOR IMMEDIATE RELEASE
January 17, 1997
ICE VOLCANOES RESHAPE EUROPA'S CHAOTIC SURFACE
Ice-spewing volcanoes and the grinding and tearing of
tectonic plates have reshaped the chaotic surface of Jupiter's
frozen moon Europa, images from NASA's Galileo spacecraft reveal.
The images, captured when Galileo flew within just 692
kilometers (430 miles) of Europa on Dec. 19, were released at a
news briefing today at NASA Headquarters, Washington, DC.
Although the images do not show currently active ice
volcanoes or geysers, they do reveal flows of material on the
surface that probably originated from them, said Galileo imaging
team member Dr. Ronald Greeley of Arizona State University,
"This is the first time we've seen actual ice flows on any
of the moons of Jupiter," said Greeley. "These flows, as well
as dark scarring on some of Europa's cracks and ridges, appear to
be remnants of ice volcanoes or geysers."
The new images appear to enhance Europa's prospects as one
of the places in the solar system that could have hosted the
development of life, said Greeley.
"There are three main criteria to consider when you are
looking for the possibility of life outside the Earth -- the
presence of water, organic compounds and adequate heat," said
Greeley. "Europa obviously has substantial water ice, and
organic compounds are known to be prevalent in the solar system.
The big question mark has been how much heat is generated in the
"These new images demonstrate that there was enough heat to
drive the flows on the surface. Europa thus has a high potential
to meet the criteria for exobiology," Greeley added.
"This doesn't prove that there is an ocean down there under
the surface of Europa, but it does demonstrate that it is a
scientifically exciting place," said Galileo imaging team member
Dr. Robert Sullivan, also of Arizona State University.
The images also reveal a remarkable diversity in the
geological age of various regions of Europa's surface. Some
areas appear relatively young, with smooth, crater-free terrain,
while others contain large craters and numerous pits, suggesting
that they are much older.
The icy crust bears the signs of having been disrupted by
the motion of tectonic plates. "There appear to be signs of
different styles of tectonism," said Greeley. "In many areas we
see that the crust was pulled apart in a spreading similar to the
processes on the sea floor on Earth. This is different from the
tectonic processes at work on, say, Jupiter's moon Ganymede.
This suggests that Europa's interior may be different from
Galileo scientists will have a better chance to understand
Europa's interior when the spacecraft gathers gravity data on
another flyby next November. The gravity field is measured by
tracking how the frequency of Galileo's radio signal changes as
it flies past the moon. This was not possible during the recent
flyby because radio conditions were degraded as Jupiter passed
behind the Sun from Earth's point of view.
Europa is crisscrossed by an amazingly complex network of
ridges, said Sullivan. "Ridges are visible at all resolutions,"
he explained. "Closely paired ridges are most common. With
higher resolution, ridges seen previously as singular features
are revealed to be double."
Some of the ridges may have formed by tension in the icy
crust: as two plates pull apart slightly, warmer material from
below might push up and freeze to form a ridge. Other ridges may
have been formed by compression: as two plates push together,
the material where they meet might crumple to form the ridge.
In addition to ice flows and tectonics, Greeley and Sullivan
noted that some areas on Europa seem to have been modified by
unknown processes that scientists are still debating.
Greeley said that some areas, for example, seem to have been
modified by "sublimation erosion" -- the evaporation of water and
other volatiles such as ammonia and methane into the vacuum of
space. "Something is destroying the topography," said Greeley,
"and this sublimation erosion is a good candidate for what is at
During last month's encounter, Galileo flew more than 200
times closer to Europa than the Voyager 2 spacecraft did in 1979.
After a swing past Jupiter next week in what mission engineers
call a "phasing orbit," Galileo's next targeted flyby will take
it again past Europa as it passes within 587 kilometers (364
miles) on February 20.
The Jet Propulsion Laboratory, Pasadena, CA, manages the
Galileo mission for NASA's Office of Space Science, Washington,