- a large, icy, dark-colored, low-density outer moon of Jupiter 
- 3,000 miles/4800 kilometers in diameterIt  
- the second-largest moon of Jupiter; it is roughly the size of Mercury  
- has the largest known impact crater in the Solar System 
- orbits Jupiter at a mean distance of 1,170,000 miles/1,883,000 kilometers 
- has a mass of 1.08 * 1023 kg 
- takes 400.8 hours/16.7 days to orbit Jupiter in a synchronous orbit  
- independently discovered by Galileo and S. Marius in 1610
- surface is extremely heavily cratered and ancient - a record of events from the early history of the solar system
- the outermost of the Galilean moons
- Every square mile is covered with craters or other signs of bombardment
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