| Jupiter's atmosphere is similar to that of the sun, and the composition is mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At further depths, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation (9.8 Earth hours.) At the center, the immense pressure may support a solid core of rock about the size of Earth. Jupiter's Galilean Satellites. Io is the most volcanically active body in the solar system and the surface is covered by sulfur in different multi-colored forms. As Io travels in its slightly elliptical orbit, Jupiter's immense gravity causes "tides" in the solid surface that rise 100 m (300 feet) high on Io, generating enough heat for volcanic activity and to drive off any water. Io's volcanoes are driven by hot silicate magma. Europa's surface is mostly water ice, and there is evidence that it may be covering an ocean of water or ice beneath. Europa is thought to have twice as much water as does Earth, and intrigues scientists because of its potential for having a "habitable zone." Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations that may be analogues to what may exist on Europa. Given the right chance and some basic conditions, life is possible on so many different levels. Ganymede is the largest moon in the solar system (larger than the planet Mercury), and is the only moon known to have its own internally generated magnetic field. Callisto's surface is extremely heavily cratered and ancient, a visible record of events from the early history of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity. |
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