Chapter 34 - Planetary Satellites

Abstract

A planetary satellite is any one of the celestial bodies in orbit around a planet, which is known as the primary body. They range from large, planetlike, geologically active worlds with significant atmospheres, such as Neptune's satellite Triton and Saturn's satellite Titan, to tiny irregular objects tens of kilometers in diameter. 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 solar system, with the exception of Io, all have as major components some type of frozen volatile, primarily water ice, and also methane, ammonia, nitrogen, carbon monoxide, carbon dioxide, or sulfur dioxide existing alone or in combination with other volatiles. As of the end of 2013, the planets have among them a total of 173 known satellites (compare the list of satellites in the appendix). There undoubtedly exist many more undiscovered small satellites in the outer solar system. The relative sizes of the main satellites are illustrated in Figure 34.1. Table 34.1 is a summary of their characteristics. This chapter covers the satellites of Mars, Jupiter, Saturn, Uranus, and Neptune, except the Galilean satellites (the four largest moons of Jupiter), Triton, Titan, and Enceladus (see The volcanic Moon; Galilean Satellites; Titan; Enceladus; Triton; Pluto and Charon).