Word: callisto
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Dates: during 1970-1979
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...latest flood of information from these Jovian satellites would have thoroughly awed the great Italian Scientist Galileo, who discovered them 369 years ago. Moving at speeds approaching 45,000 m.p.h., the 1,800-lb. spacecraft swept by Callisto, the oldest, outermost and apparently smoothest of the Galilean moons...
...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...
...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...
...photographs of Callisto, darkest and outermost of the Galilean moons, showed that this gloomy sphere may be at least half water. Ganymede also seems watery, but appears to have an ice crust and possibly a mud core. Remarkably, both moons are dotted with dark spots that some scientists are tentatively calling ice volcanoes-craters that spew ice instead of fire and lava. Still closer to Jupiter, Europa apparently hides a rocky core beneath its bright icy surface...