Satellites of the Outer Planets
Crescent Ganymede observed by Voyager 1
The satellites of the giant outer planets are a surprisingly diverse group of bodies. Most are composed of mixtures of water and other ices, including most likely ammonia ice, and rocky material. An amazing variety of unusual and spectacular geologic features are observed on these satellites, including active volcanism on two of them, which are illustrated on these pages. Also discussed are some of the processes that form them and the important issues they raise for geologists.
The Galilean Satellites
The four large Galilean Satellites of Jupiter (from left to right in mosaic: Io, Europa, Ganymede, and Callisto) resemble a miniature solar system. The global density, relative amount of rocky material, and duration and intensity of geologic activity on each satellite all decrease with increasing distance from Jupiter. Each satellite is very distinctive in appearance and geologic history. Io, the densest and innermost major satellite, is volcanically active today. Europa has a highly fractured, young icy outer shell. Ganymede has a complex volcanic and tectonic history, and Callisto, the least dense and outermost satellite, is heavily cratered and rather bland by comparison. The geologic histories of these satellites are related to tidal deformation and heating, which is more intense closer to Jupiter.
The Saturnian Satellites
The Saturnian satellites (left to right in mosaic: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe) are diverse in size and appearance. Most are between 400 and 1500 kilometers across. Cloud-covered Titan, similar in size and density to Ganymede and Callisto, is the sole exception. The smaller satellites have low bulk densities (~1 to 1.5 g/cc) and are mostly water and other ices. A number of small, lumpy satellites orbit near Saturn's massive ring system or in orbits similar to Tethys and Dione.
The Uranian Satellites
There are five major Uranian satellites (left to right in mosaic: Miranda, Ariel, Umbriel, Titania, and Oberon). They range from ~470 to 1600 kilometers across. They are composed of roughly equal mixtures of water and other ices and rocky material. Umbriel and Oberon are heavily cratered and Titania has a few large fractures. Miranda and Ariel have surprisingly complex geologic histories for such small bodies, the reasons for which are not well understood, but are probably related to tidal heating. At least 10 small satellites orbit near Uranus' thin ring system.
Five small satellites orbit within or near the edge of Neptune's ring system. Further out, three major satellites orbit Neptune (left to right in mosaic: Proteus, Triton, and Nereid). Nereid is in a distant retrograde orbit and is probably a captured asteroid. Neptune's largest satellite, Triton, is in a highly inclined retrograde orbit and is believed to have been captured by Neptune early in its history.