Natural Satellites

The natural satellites of planets in the solar system display a rich variety of orbital configurations and surface characteristics that have intrigued astronomers, physicists, and mathematicians for several centuries. As detailed information about satellite properties have become available from close spacecraft reconnaissance, geologists and geophysicists have also joined the study with considerable enthusiasm. This paper summarizes our ideas about the origin of the satellites in the context of the origin of the solar system itself, and it highlights the peculiarities of various satellites and the configurations in which we find them as a motivation for constraining the satellites’ evolutionary histories. Section 2 describes the processes involved in forming our planetary system and points out how many of these same processes allow us to understand the formation of the regular satellites (those in nearly circular, equatorial orbits) as miniature examples of planetary systems. The irregular satellites, the Moon, and Pluto’s satellite, Charon, require special circumstances as logical additions to the more universal method of origin. Section 3 gives a brief description of tidal theory and a discussion of various applications showing how dissipation of tidal energy effects secular changes in the orbital and spin configurations and how it deposits sufficient frictional heat into individual satellites to markedly change their interior and surface structures. These consequences of tidal dissipation are used in the discussions of the evolutions of satellite systems of each planet, starting with the Earth-Moon system in Section 4. Additional processes such as collisional breakup and reassembly of some of the smaller satellites associated with the ubiquitous equatorial rings of small particles are discussed, although ring properties and evolution are excluded (see Nicholson 1999). There is no attempt to include technical details of published explanations of the various phenomenologies. Rather, the explanations are described and the uncertainties in the assumptions and resulting conclusions are emphasized. The