Jupiter's moon Ganymede is the largest in the solar system with
a diameter of 5,262 km. Titan, the largest moon of Saturn, is a
close second at 5,150 km, and was once thought to be larger than
Ganymede. Third position goes to Ganymede's neighbour at
Jupiter, Callisto. Both Ganymede and Callisto are larger than the
planet Mercury (diameter 4878 km).
Ganymede owes its status as largest moon to the thick mantle of
ice that overlies its rocky interior. The rocky cores of both
Ganymede and Callisto are probably similar in size to the two
inner and smaller Galilean moons of Jupiter, Io (3,630 km) and
Europa (3,138 km). However, due to their proximity to Jupiter's
warming influence, Io has no ice mantle and Europa has only a
thin crust of ice, possibly with water or slush just below the
surface. By contrast, Ganymede's composition is about half ice
and half rocky material.
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