PUBLIC INFORMATION OFFICE
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: Jane Platt
FOR IMMEDIATE RELEASE
April 9, 1997
NEW IMAGES HINT AT WET AND WILD HISTORY FOR EUROPA
Chunky ice rafts and relatively smooth, crater-free patches
on the surface of Jupiter's frozen moon Europa suggest a younger,
thinner icy surface than previously believed, according to new
images from Galileo's spacecraft released today.
The images were captured during Galileo's closest flyby of
Europa on February 20, when the spacecraft came within 586
kilometers (363 miles) of the Jovian moon. These features, which
lend credence to the idea of hidden, subsurface oceans, are also
stirring up controversy among scientists who disagree about the
age of Europa's surface.
Dr. Ronald Greeley, an Arizona State University geologist
and Galileo imaging team member, said the ice rafts reveal that
Europa had, and may still have, a very thin ice crust covering
either liquid water or slush.
"We're intrigued by these blocks of ice, similar to those
seen on Earth's polar seas during springtime thaws," Greeley
said. "The size and geometry of these features lead us to
believe there was a thin icy layer covering water or slushy ice,
and that some motion caused these crustal plates to break up."
"These rafts appear to be floating and may, in fact, be
comparable to icebergs here on Earth," said another Galileo
imaging team member, Dr. Michael Carr, a geologist with the U.S.
Geological Survey. "The puzzle is what causes the rafts to
rotate. The implication is that they are being churned by
convection."
The new images of Europa's surface have also sparked a
lively debate among scientists. Galileo imaging team member Dr.
Clark Chapman is among those who believe the smoother regions
with few craters indicate Europa's surface is much younger than
previously believed. In essence, Chapman, a planetary scientist
at Southwest Research Institute, Boulder, CO, believes the fewer
the craters, the younger the region. Chapman based his estimate
on current knowledge about cratering rates, or the rate at which
astronomical bodies are bombarded and scarred by hits from comets
and asteroids.
"We're probably seeing areas a few million years old or
less, which is about as young as we can measure on any planetary
surface besides Earth," said Chapman. "Although we can't
pinpoint exactly how many impacts occurred in a given period of
time, these areas of Europa have so few craters that we have to
think of its surface as young."
Chapman added, "Europa's extraordinary surface geology
indicates an extreme youthfulness -- a very alive world in a
state of flux."
However, Carr sees things differently. He puts Europa's
surface age at closer to one billion years old.
"There are just too many unknowns," Carr said. "Europa's
relatively smooth regions are most likely caused by a different
cratering rate for Jupiter and Earth. For example, we believe
that both Earth's moon and the Jovian moon, Ganymede, have huge
craters that are 3.8 billion years old. But when we compare the
number of smaller craters superimposed on these large ones,
Ganymede has far fewer than Earth's moon. This means the
cratering rate at Jupiter is less than the cratering rate in the
Earth-moon system."
Scientists hope to find answers to some of the questions
surrounding Europa and its possible oceans as the Galileo
spacecraft continues its journey through the Jovian system.
"We want to look for evidence of current activity on Europa,
possibly some erupting geysers," Greeley said. "We also want to
know whether Europa's surface has changed since the Voyager
spacecraft flyby in 1979, or even during the time of the Galileo
flybys."
The craft will return for another Europa flyby on November
6, 1997, the final encounter of Galileo's primary mission.
However, eight more Europa flybys are planned as part of
Galileo's two-year extended mission, which will also include
encounters with two other Jovian moons, Callisto and Io.
The Jet Propulsion Laboratory manages the Galileo mission
for NASA's Office of Space Science, Washington, DC.
Images and other data received from Galileo are posted on
the Galileo mission home page on the World Wide Web at
http://www.jpl.nasa.gov/galileo.
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