MEDIA RELATIONS 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
December 7, 1998
EUROPA FAULT GIVES CALIFORNIA'S SAN ANDREAS A RUN FOR ITS MONEY
New pictures from NASA's Galileo spacecraft show a closeup
view of a fault, or fracture, on Jupiter's icy moon Europa that
stretches as long as the California segment of the infamous San
Andreas fault.
The Europan fault, known as Astypalaea Linea (pronounced
ast-ipp-uh-LAY-uh LINN-ee-uh) was first discovered in 1996 when
Dr. Randy Tufts, Galileo imaging team affiliate and research
associate at the University of Arizona, Tucson, AZ, reviewed
distant images taken years earlier by NASA's Voyager spacecraft.
The new mosaic of Galileo images released today captures a 290-
kilometer-long (180-mile) portion of the fault in Europa's icy
surface. Scientists calculate its full length at about 810
kilometers (more than 500 miles), about the same distance as the
part of the San Andreas fault that runs from the
California-Mexico border north to the San Francisco Bay.
"Comparisons between this Europan fault and faults on Earth
may generate ideas we can use in studying earth movements here on
our planet," said Tufts. "In addition, Astypalaea Linea is
simply a beautiful structure."
The new Galileo images show that about 50 kilometers (more
than 30 miles) of movement, or "displacement," has taken place
along the fault, which is located near Europa's South Pole.
Bends in the fault have allowed the surface to be pulled apart as
this movement took place along Astypalaea Linea, which is the
largest known strike-slip fault on Europa and one of the largest
strike-slip faults known to exist anywhere. A strike-slip fault
is one in which two crustal blocks move horizontally past one
another, somewhat like two opposing traffic lanes.
This pulling-apart along the fault's bends created openings
through which warmer, softer ice from below Europa's brittle ice
shell surface, or frozen water from a possible subsurface ocean,
could reach the surface. This upwelling of material formed large
areas of new ice within the boundaries of the original fault. A
similar pulling-apart phenomenon can be observed in the
geological trough surrounding California's Salton Sea, and in
Death Valley and the Dead Sea. However, in those cases, the
pulled-apart regions can include upwelled materials, but may be
mostly composed of sedimentary and erosional material deposited
from above.
Tufts believes Astypalaea Linea is probably no longer
active, because large ridges formed more recently crosscut it
without interruption. Opposite sides of the fault can be
reconstructed in puzzle-like fashion, matching the shape of its
sides as well as individual older lined areas that had been
broken by its movements. The overall motion along the fault seems
to have followed a continuous narrow break along the entire
length of the feature, with a path resembling steps on a
staircase crossing the pulled-apart zones. Between the zones,
this break coincides with ridges that separate them.
Tufts and fellow University of Arizona researchers, in a
group led by Dr. Richard Greenberg, suspect that the fault motion
is induced by the pull of variable daily tides generated by
Jupiter's gravitational tug on Europa's icy crust. This tidal
effect produces a phenomenon they call "walking."
"In walking, tidal tension opens the fault, subsequent tidal
stress causes it to move lengthwise in one direction, and then
the tidal forces close the fault up again. This prevents the
area from moving back to its original position; it may move
forward with the next daily tidal cycle," Tufts explained. "The
walking analogy describes perfectly what we think happens at the
fault, resulting in a steady accumulation of these lengthwise
offset motions. Walking may explain the appearance of many other
faults and areas of cracks and ridges on Europa."
Unlike Europa, here on Earth, large strike-slip faults such
as the San Andreas are set in motion by plate tectonic forces
from the planet's mantle. Based on the Europa findings, Tufts
said, "The data may teach us more about the detailed structure
that develops at bends in Earth's faults, including the San
Andreas."
The latest Galileo images of Astypalaea Linea are available
on the Internet at the following websites:
http://www.jpl.nasa.gov/galileo,
http://www.jpl.nasa.gov,
http://photojournal.jpl.nasa.gov
Galileo has been in orbit around Jupiter and its moons for
the past three years. Its primary mission ended in December 1997,
and the spacecraft is currently in the midst of a two-year
extension known as the Galileo Europa Mission. Galileo is managed
by JPL for NASA's Office of Space Science, Washington, DC. JPL
is a division of Caltech, Pasadena, CA.
For information about animation of Astypalaea Linea, contact
the JPL Media Relations Office at (818) 354-5011.
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JP #98117
12/7/1998
