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Any of the four large satellites of Jupiter discovered by Galileo in 1610. In order of distance from Jupiter: Io (the most volcanically active body in the solar system), Europa (suspected to contain a liquid ocean under its frozen surface), Ganymede (the largest satellite in the solar system), and Callisto. Both Ganymede and Callisto are larger than Pluto and Mercury.
This entry comes from Encyclopædia Britannica Concise. For the full entry on Galilean satellite, visit Britannica.com.
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