The reason is that Jupiter 's moon Ganymede is not only the largest moon in the Solar System , it is larger than planets Mercury and Pluto . The robot spacecraft Galileo currently orbiting Jupiter has been able to zoom by Ganymede several times and snap many close-up pictures. Ganymede, shown above in its natural colors, sports a large oval dark region known as Galileo Regio . In general, the dark regions on Ganymede are heavily cratered, implying they are very old, while the light regions are younger and dominated by unusual grooves . The origin of the grooves is still under investigation .
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