The Saturnian satellites (left to right in mosaic:
Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe)
are diverse in size and appearance.
With the exception of cloud-covered Titan,
the major Saturnian satellites are between 400 and 1500 kilometers across.
These eight satellites have low bulk densities (~1 to 1.5 g/cc) and are composed mostly of water
and other ices, including probably ammonia ice. There
is no obvious relationship between satellite density, geologic activity, or
distance from Saturn. The level of geologic complexity ranges from virtually
none (heavily cratered Rhea) to intense (volcanically resurfaced Enceladus).
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Mimas & Enceladus
The two inner satellites of Saturn are similar in
size and density but have very different geologic histories. These two
views (Mimas at top and Enceladus at bottom) were obtained by Voyager
at similar resolution and solar illumination. Mimas is heavily
cratered. Enceladus, however, is covered by smooth plains, ridge belts,
and some oddly-shaped craters. Enceladus has been volcanically resurfaced and
tectonically deformed. Melting of ammonia-water ice (at 175 K
(-98° C) may be involved but the causes of such extensive
resurfacing on this tiny world are still a source of mystery.
Dione has regions that are both heavily cratered (like Rhea), and areas
that are relatively smooth and not so heavily cratered (such as on Enceladus). These
smooth areas have probably been resurfaced in the past, probably
by volcanic flows of molten ices. The Cassini mission to Saturn,
scheduled for the early years of the next century, will examine these smooth
areas to determine their origin.
The large 150-kilometer wide crater near the bottom is over 4 kilometers deep.
Iapetus, similar in size and density to Rhea, is one of the
most unusual bodies in the solar system. It is divided into two very
different terrains. One hemisphere is very bright, heavily cratered, and
resembles Rhea. The other hemisphere is one of the darkest terrains in the solar system
and is darker than asphalt. The origin of this dark material is unclear.
It may have formed by the raining of dark debris from Phoebe, a small satellite orbiting Saturn
beyond Iapetus. On the other hand, it might consist of volcanic deposits. This Voyager 2 view has
a resolution of 9 kilometers.
All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.
©Lunar and Planetary Institute, 1997
Supplemental Reading Materials
Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.
Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.
Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.
Moore J. et al. (1985) Geomorphology of Rhea. Journal of Geophysical Research, 90, C785-C795.
Moore J. (1984) The tectonic and volcanic history of Dione. Icarus, 59, 205-220.