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       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       Tufts believes Astypalaea Linea is probably no longer active, because large ridges formed more recently crosscut it without interruption.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:,,

       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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