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Satellites And Ring

  • Io, with an irregularly shaped crater
    Io, with an irregularly shaped crater (Click on the image for a larger view)
    Voyager 1 identified nine currently active (erupting) volcanoes on Io, probably driven by tidal heating. Many more are suspected. Voyager 2 observed eight of the nine; the largest shut down by the time Voyager 2 arrived at Jupiter. Plumes from the volcanoes reach more than 300 kilometers (190 miles) above the surface. The material was being ejected at velocities up to 1.05 kilometers a second (2,300 miles an hour). By comparison, ejection velocities at Mount Etna, one of Earth's most explosive volcanoes, hit 50 meters a second (112 miles an hour). Volcanism is associated with heating of Io by tidal pumping. Europa and Ganymede, two large satellites nearby, perturb Io in its orbit and Jupiter pulls Io back again. The pumping action causes tidal bulging up to 100 meters (330 feet) on Io's surface, compared with typical tidal bulges on Earth of one meter (three feet).

  • Voyager 1 measured the temperature of a large hot spot on Io associated with a volcanic feature. While the surrounding terrain has a temperature of about 130 Kelvins (-230 degrees Fahrenheit), the hot spot's temperature is about 290 Kelvins (60 degrees Fahrenheit). Scientists believe the hot spot may be a lava lake, although the temperature indicates the surface is not molten; it is, at least, reminiscent of lava lakes on Earth.

  • Europa displayed a large number of intersecting linear features in the distant, low-resolution photos from Voyager 1. Scientists at first believed the features might be deep cracks, caused by crustal rifting or tectonic processes. Closer, high- resolution photos by Voyager 2, however, left scientists puzzled: The features were so lacking in topographic relief that they "might have been painted on with a felt marker," one scientist commented. There is a possibility that Europa may be internally active due to tidal heating at a level one-tenth or less that of Io. Models of Europa's interior show that beneath a thin crust (5 kilometers or 3 miles) of water ice, Europa may have oceans as deep as 50 kilometers (30 miles) or more.

  • Ganymede turned out to be the largest satellite in the solar system. Before the Voyager encounters, astronomers thought that Saturn's satellite, Titan, was the largest. Ground-based observations of Titan, of necessity, had included its substantial atmosphere. Voyager measurements of Ganymede showed it is largerthan Titan. Ganymede had two distinct terrain types --- cratered and grooved, telling scientists that Ganymede's entire, ice-rich crust has been under tension from global tectonic processes.

  • Callisto's icy surface
    Callisto's icy surface (Click on the image for a larger view)
    Callisto has an ancient, heavily cratered crust, with remnant rings of enormous impact basins. The largest craters apparently were erased when the ice-laden crust flowed during geologic time; almost no topographic relief is apparent in ghost remnants of the impact basins, identifiable only by their light color and surrounding subdued rings of concentric ridges.

  • Amalthea is elliptical: 270 kilometers (170 miles) by 165 kilometers (105 miles) by 150 kilometers (95 miles). It is about 10 times larger than Mars' larger satellite, Phobos, and has 1,000 times the volume.

  • Voyager discovered a ring around Jupiter. Its outer edge is 129,000 kilometers (80,000 miles) from the center of the planet, and, though the brightest portion is only about 6,000 kilometers (4,000 miles) wide, ring material may extend another 50,000 kilometers (30,000 miles) downward to the top of Jupiter's atmosphere. Evidence also suggests that diffuse ring material extends as far out as the orbit of Amalthea. The ring is no more than 30 kilometers (20 miles) thick. Thus Jupiter joins Saturn, Uranus, and Neptune as a ringed planet -- although each ring system is unique and distinct from the others.

  • Two new satellites, Adrastea and Metis, only about 40 kilometers (25 miles) in diameter, orbit just outside the ring. A third new satellite, Thebe, diameter about 80 kilometers (50 miles), was discovered between the orbits of Amalthea and Io.

 

 
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