SATELLITES OF THE OUTER PLANETS
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.
Click on each image to view at full resolution or to
access individual Web pages. A few of these images are
stereo (three-dimensional) images, which are highlighted by the stereo symbol.
Additional 3-D images of the satellites are available on the
3-D Tour of the Solar System.
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
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.
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All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, unless otherwise noted.
Reproduction of these images by permission only.
©Lunar and Planetary Institute, 1997