JUPITER'S MOONS

On Jan. 7, 1610, Galileo Galilei's improvements to the telescope enabled humanity to see Jupiter's four largest moons for the first time.
Io, Europa, Ganymede and Callisto--the so-called Galilean satellites--were seen by the Long Range Reconnaissance Imager on the New Horizons spacecraft during its flyby of Jupiter in late February 2007.
Io is notable for its active volcanism, which New Horizons studied extensively.
On the other hand, Europa's smooth, icy surface likely conceals an ocean of liquid water.
Ganymede, the largest moon in the solar system, has a dirty ice surface cut by fractures and peppered by impact craters.

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