1. Charon (Pluto) Named for the mythical boatman of the Greek underworld. Its expected pronunciation of "KAIRen" is not the correct one, which is actually "SHAHR-en", in honor of Charlene Christy, wife of Jim Christy, its
discoverer. The largest moon relative to the size of its orbiting planet, Charon not only is in synchronous orbit
with Pluto, but the two show the same face toward each other at all times. The relative sizes of the two bodies has
led some to call Charon and Pluto a double planet system. Charon's surface is believed to be water ice.
2. Deimos and Phobos (Mars) Named for two sons of Ares and Aphrodite. Phobos and Deimos (Greek for "fear"
and "panic") are the two moons of Mars and both were discovered in 1877 by Asaph Hall. Phobos orbits closer to
the planet and has as its most prominent feature the crater Stickney (Hall's wife's maiden name). Unlike the
Earth's moon, it rises in the west and sets in the east, about twice per Martian day. This is due to it being below
the radius for synchronous orbit. This position also means it will either impact Mars or break into a ring in around
50 million years. Deimos is the smallest moon in the solar system. It was discovered two days before Phobos.
Deimos was likely an asteroid brought into Mars' orbit after being disturbed by Jupiter. Like Phobos, Deimos is
heavily cratered, rich in carbon, and believed to have water ice.
3. Europa (Jupiter) One of the Galilean moons, discovered in 1610 by Galileo (the others are Callisto, Ganymede,
and Io). It resembles Io, and to a degree, Earth, in its composition of silicate rocks. However, it is coated in a thin
layer of ice, which causes it to be exceedingly smooth. This ice layer may also provide a thin atmosphere as
hydrogen and oxygen are released when the planet is exposed to sunlight. There is the possibility of an active sea
of liquid water beneath the surface. The most striking feature of the surface is a series of dark streaks that may be
due to geysers or volcanic eruptions.
4. Ganymede (Jupiter) The largest satellite in the solar system, this Galilean moon is larger than Mercury, but has
only half its mass. Based on the observations of the Galileo spacecraft, it is thought to have a three-layer structure
of a molten iron core, silicate mantle, and ice exterior. Its surface is marked by older, dark, highly cratered
regions, mixed with lighter, grooved regions. These grooves indicate tectonic activity, but Ganymede does not
appear to have undergone recent tectonic shifts.
5. Io (Jupiter) Like Europa, Io (named for a lover of Zeus) is primarily formed of silicate rock. Its surface, however,
is unlike any other satellite. Rather than craters, Io is dotted with active volcanoes, calderas, and other signs of
geological activity. The eruptions are believed to consist of sulfurous compounds that comprise Io's thin
atmosphere. The tremendous activity is due to tidal warming from the gravity of Jupiter and other satellites.
Additionally, as Io orbits it is heated electrically from currents produced by Jupiter's magnetic field. This action
strips material from Io, producing a radiation field and increasing Jupiter's magnetosphere.
6. Nereid (Neptune) Discovered by Gerard Kuiper (who also discovered Miranda, Titan's atmosphere, and an
asteroid belt), Nereid (named for the daughters of Nereus and Doris) has the most eccentric orbit of any known
satellite, ranging from 1.3 million kilometers to 9.6 million. The oddity of this orbit indicates it is likely a
captured asteroid.
7. Oberon (Uranus) Named for the King of the Fairies in A Midsummer Night's Dream (all of Uranus' satellites are
named for literary, rather than mythological, characters), Oberon is both the second largest of Uranus' satellites,
and the outermost of its large satellites. Like all large Uranian moons, its structure is about half water ice, half
rock. Large faults are visible across its southern hemisphere, but its surface is heavily cratered, indicating longterm tectonic stability. Some craters have dark floors that could possibly indicate post-impact upwellings of water.
8. Titan (Saturn) The largest of Saturn's satellites, Titan might be the largest satellite in the solar system, but this
awaits more accurate measurements. Those measurements are difficult because of Titan's major characteristic: It
is the only satellite to have a substantial atmosphere. Its significant atmosphere, a mix of nitrogen (80%), methane
(20%), and argon (trace), also makes it unique among satellites.
9. Titania (Uranus) Another of Herschel's discoveries, Titania is named for Oberon's wife, the Queen of the Fairies,
and is the largest of the Uranian satellites. Its surface is an odd mix of craters and valleys. One theory regarding
this is that it began as a liquid, then cooled surface first. Once ice had formed, the interior, freezing forced surface
cracks which formed the valleys. This also accounts for the appearance of some craters, where ice appears to have
melted and filled in.
10. Triton (Neptune) By far the largest of Neptune's satellites, Triton is also unusual for its retrograde orbit, which
indicates that it was not part of the natural formation of Neptune's other moons. It also features seismic activity in
the form of ice volcanoes, a tenuous nitrogen-methane atmosphere, and a southern hemisphere "ice cap" of
nitrogen and methane. All of these may be caused by Triton's odd rotational axis, which tends to alternate polar
and equatorial regions facing the sun.