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Space Topics: Jupiter

Ganymede

Jupiter's Moon Ganymede
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Jupiter's Moon Ganymede
Ganymede's surface is broken into large plates of ancient cratered crust, split by wide lanes of lighter-colored grooved terrain. Credit: NASA/JPL/DLR

Diameter: 5,262.4 kilometers -- 0.4125 Earth diameters -- 9th largest solar system body
Orbital distance: 1,070,400 kilometers from Jupiter
Orbital period: 7.154 days
Discovery: 1610 by Galileo Galilei

Ganymede is the largest moon in the solar system, larger even than the planets Mercury and Pluto.  However, Mercury has twice the mass of Ganymede; Mercury is made almost entirely of metal and rock, while Ganymede’s metal and rock interior is surrounded by a thick mantle of ice.  (In fact, a good model for Ganymede would be Io with an icy shell.)  Ganymede’s icy mantle could contain a liquid ocean layer, but evidence for that is not conclusive.

Ganymede’s icy surface displays two disparate terrains.  Broad regions of dark, heavily cratered, Callisto-like terrain are broken up into plates and are separated by lanes of ridged and grooved material.  Although the grooves of Ganymede are clearly younger than the surfaces that they cut, both kinds of terrain are peppered by impact craters, suggesting that Ganymede’s surface is an ancient one.  In fact, there are traces of very large impact craters on Ganymede.  Over time, though, the icy surface has relaxed the originally steep crater topography, leaving circular ghosts of old impact basins called palimpsests.

Unlike the other large moons of Jupiter, whose magnetic fields are induced by Jupiter’s, Ganymede has its own internally generated magnetic field.  It could either be generated in a subsurface ocean or in a liquid outer core.