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CHAPTER 19

Planetary Satellites

Bonnie J. Buratti

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

Peter C. Thomas

Cornell University

Ithaca, New York

1. Summary of Characteristics 4. Individual Satellites


2. Formation and Evolution of Satellites Acknowledgments


3. Observations of Satellites Bibliography

A

planetary satellite (ormoon) is any one of the celes-
tial bodies in orbit around a planet, which is known as
the primary body. They range from large, planet-like, geo-
logically active worlds with significant atmospheres such as
Neptune’s satellite Triton and Saturn’s satellite Titan to tiny
irregularly shaped objects as small as a kilometer in diame-
ter. Two satellites are larger than the planet Mercury: Titan
and Jupiter’s Ganymede, the largest moon with a radius
of 2634 km. Six planetary satellites are larger than Pluto.
The large and medium-sized satellites are thought to have
been formed in place around their primaries at the same
time the solar system condensed 4.6 billion years ago, while
many of the smaller satellites are captured objects or rem-
nants of collisions. Small satellites that have been found in
Saturn’s rings help form gaps in ring particles, while other
“shepherd” satellites act to gravitationally define the edges
of the rings. The satellites in the inner solar system—the
two moons of Mars and the Earth’s Moon—are composed
primarily of rocky material. The satellites of the outer so-
lar system—Jupiter and beyond—have as major compo-
nents some type of frozen volatile, primarily water ice, but
also methane, ammonia, nitrogen, carbon monoxide, car-
bon dioxide, or sulfur dioxide existing alone or in combina-
tion with other volatiles. As of July 2006, the planets have
among them a total of 156 known satellites. There undoubt-
edly exist many more undiscovered small satellites in the
outer solar system. The relative sizes of the main satellites
are illustrated in Fig. 1. Table 1 is a summary of the char-

acteristics of the main planetary satellites; a current list of

all satellites and their physical and dynamical properties is

maintained by the National Aeronautics and Space Admin-

istration at http://horizons.jpl.nasa.gov. This chapter covers

the satellites of Mars, Jupiter, Saturn, Uranus, and Nep-

tune, but not the Galilean satellites (the four largest moons

of Jupiter), Triton, and Titan. Mercury and Venus are not

expected to have any large satellites because of solar tides.

[SeeIo:TheVolcanicMoon;Europa;Ganymede and

Callisto;Titan;Triton;Pluto).

1. Summary of Characteristics

1.1 Discovery

None of the satellites of the outer planets was known be-

fore the invention of the telescope. When Galileo turned

his telescope to Jupiter in 1610, he discovered the four

large satellites in the jovian system. His observations of

their orbital motion around Jupiter in a manner analogous

to the motion of the planets around the Sun provided impor-

tant evidence for the acceptance of the heliocentric (Sun-

centered) model of the solar system. These four moons—

Io, Europa, Ganymede, and Callisto—are sometimes called

the Galilean satellites.

In 1655, Christian Huygens discovered Titan, the gi-

ant satellite of Saturn. Later in the 17th century, Giovanni

Cassini discovered the four next largest satellites of Saturn.

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