SAN FRANCISCO - Ganymede, the solar system's largest moon, appears to have a liquid, saltwater ocean deep beneath its cratered and fractured surface of solid ice, researchers said Saturday.
With the new findings, Ganymede joins Europa and Callisto as yet another moon of Jupiter suspected to harbor underground water - a key ingredient for life.
The ocean is the latest finding announced by scientists interpreting data from NASA's Galileo spacecraft, which began orbiting Jupiter in December 1995.
''Galileo is sparking a new revolution in our understanding of the solar system by opening our minds to the role played by that precious ingredient to life - water,'' said Robert Pappalardo, a senior research associate at Brown University.
Scientists received the latest data as Galileo made its closest flyby of Ganymede, coming within 503 miles on May 20. They discussed the findings at the fall meeting of the American Geophysical Union.
An instrument that measures magnetic fields detected a subtle change in the moon's internal field - a signature of a global layer of a conducting liquid, such as saltwater.
''After months and months of wrestling with the data ... we believe there is very strong evidence of a layer of melted water beneath Ganymede's icy surface,'' said Margaret Kivelson, a space physicist at the University of California, Los Angeles.
If the interpretation is correct, a layer of water several miles thick lies more than 100 miles below the surface, Kivelson said. Researchers believe the temperature of the water is about 68 degrees below zero but doesn't freeze because of the high pressure deep inside the moon.
''It's not the silver bullet that we would like to have but it looks really good,'' she said.
Galileo scientists also said the latest high-resolution images from the spacecraft suggest Ganymede's surface is active like Earth's.
In Ganymede's case, the surface of solid ice apparently floats on a layer of more malleable ice, which is heated by the decay of uranium and other radioactive elements found toward the icy moon's core.
On parts of Ganymede, regions are pulling apart at the surface. That allows the ductile material to ooze upward and form smooth bands. Scientists previously thought the patterns were formed by volcanoes that spewed water when Ganymede's icy surface heated up.
The Galileo spacecraft, which is now making joint observations of Jupiter with NASA's Saturn-bound Cassini probe, also has provided strong evidence of liquid underground oceans on the moons Europa and Callisto.
Europa remains the leading candidate for life and will be the target of a future NASA orbiter because, unlike Callisto and Ganymede, Europa's suspected ocean lies only a few miles beneath its surface.
Scientists said their understanding of Ganymede has improved dramatically in the last five months thanks to the Galileo data.
''It always had the advantage of being big,'' Kivelson said of Ganymede, a moon larger than the planets Mercury and Pluto. ''Relative to what we knew five months ago, Ganymede has become a great deal more exciting.''
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Galileo Web site: http://galileo.jpl.nasa.gov