You Gotta Know
You Gotta Know These Planetary Moons
1.
Charon (Pluto) Named for the mythical boatman of the Greek underworld. Its expected
pronunciation of "KAIR-en" 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 long-term 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.
This list obviously excludes the most famous moon of all, the Earth's moon. While odd, this is in keeping
with the spirit of "You Gotta Know" lists because, for whatever reason, there are relatively fewer questions
about it.