Science: There's a Ring, By Jupiter

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And some startling moons too

He may have had his hands full with more down-to-earth problems last week, but even President Carter took time out to watch an otherworldly show as the Voyager 1 spacecraft made its closest approach to the giant planet Jupiter. Coming within 278,000 km (172,400 miles) of the swirling Jovian cloud tops, the robot survived intense radiation, peered deep into the planet's storm-tossed cloud cover, provided startling views of the larger Jovian moons and, most surprising of all, revealed the presence of a thin, flat ring around the great planet. Said University of Arizona Astronomer Bradford Smith: "We're standing here with our mouths open, reluctant to tear ourselves away."

There was every reason for exhilaration. As Voyager curved around the sun's largest planet at speeds up to 104,600 km (65,000 miles) per hour, the craft performed nearly flawlessly, its probing eyes and instruments shifting between Jupiter and its moons. As one startling picture after another flashed onto the screens at Caltech's Jet Propulsion Laboratory, even Cornell's irrepressible Carl Sagan was left nearly speechless. Said he: "This is almost beyond interpretation. There's different chemistry, different physics, different forces at work out there."

The close encounter lasted 39 tense hours, during which Voyager sent back enough data to fill up miles of magnetic tape and keep scientists busy for years ahead. But Voyager has already opened up new worlds for them. Ablaze with colors of every shade and hue, speckled with strange, often puzzling features, the Jovian moons prompted oohs and aahs from even the most seasoned scientists.

As the center of a kind of mini-solar system, Jupiter is surrounded by at least 13 moons, and possibly a 14th. The four largest—lo, Europa, Ganymede and Callisto—are the so-called Galilean moons (named after their discoverer). Like the earth's moon, they are large enough to be considered small planets, but appeared as little more than fuzzy blobs in earth-bound telescopes. Now, Voyager's cameras have found that these moons are not only complex but also markedly different, their surfaces varying greatly in age, composition and appearance. Observed the U.S. Geological Survey's Laurence Soderblom: "There is no such thing as a boring Galilean moon."

The surface of Callisto, the outermost of these moons, is riddled with craters, apparently the result of pummeling by meteorites for some 4 billion years. Although it is mountainless, Callisto has a feature never before seen in the solar system: a huge, smooth, circular basin rimmed with concentric ridges that look almost like a frozen tsunami (tidal wave). Appearances may not be entirely deceiving: the scientists speculated that these ridges were created when a particularly large meteorite hit, melted subsurface ice and caused the water to spread out from the place of impact, only to freeze rapidly again.

Neighboring Ganymede, like Callisto, is at least half composed of water and ice. It shows sinuous ridges and crisscrossing fractures that look like earthly fault lines—possibly caused by what Soderblom calls "water quakes." Ganymede's surface is less cratered than Callisto's and only a fourth its age, about 1 billion years.

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