Ganymede is the largest and brightest of the four Galilean satellites of Jupiter. They are so called because they were the first moons to be discovered in orbit around Jupiter when Galileo trained his telescope on the giant planet in 1610.
Ganymede is the third of the four in distance from Jupiter, orbiting about 665,000 miles out, which is between two and three times the distance between our own Moon and Earth. However, Ganymede’s orbit is considerably quicker, at just over seven Earth days.
Not only is Ganymede the largest moon of Jupiter, but – with a diameter of 3275 miles – it is the largest moon in the whole solar system and is even larger than the planet Mercury (our Moon’s diameter is 2160 miles and that of Mercury is 3000 miles).
Ganymede is the only natural satellite the Solar System witha substantial magnetic field of its own, which suggests that it has a significant amount of molten iron at its core.
There is also evidence of a saltwater ocean layer some 125 miles beneath the moon’s crust.
The surface shows two distinct types of terrain. There are dark regions that are heavily cratered and lighter, less cratered, areas that must therefore be of more recent origin. The two types of terrain intermingle, thus giving a complex surface.
(A boundary between dark and light terrain on Ganymede. Image released into the public domain by NASA)
The younger areas are characterized by long parallel grooves and ridges that wander across the surface for thousands of miles. The ridges comprise high mountain ranges that are separated by plains up to ten miles wide.
Ganymede differs from the inner moons Io and Europa in that it is not subject to gravitational tidal forces that cause constant surface movement. However, there is evidence to suggest that this was certainly the case in the past, leading to a form of plate tectonics that allowed older portions of crust to move apart and a mixture of rock and ice to well up through the gaps.
There is also evidence that Ganymede has a tenuous atmosphere that contains oxygen. This would have been formed when water molecules in the icy crust were broken down by charged solar particles.
The name Ganymede derives from Greek mythology but is unusual in being the name of a male person rather than that of a female who was seduced by Zeus (i.e. Jupiter). Ganymede was, however, abducted by Zeus in the form of an eagle so that he could serve as cup-bearer to the gods. This was on account of his high degree of male beauty.