The satellites of the giant planets are believed to have formed in two ways: (1) in a circumplanetary accretion disk (accretion disks are discussed in Chapter 2) at the same time as the planet; and (2) later capture of remaining planetesimals in the solar system. Satellites formed by the first mechanism are expected to lie in the equatorial plane of the planet and to have near-circular orbits. Satellites formed by the second mechanism may have any inclination, and are likely to have eccentric orbits.
The satellites of the giant planets show just this dichotomy (as can be seen in Tables 1.2a-d). We will see in Chapter 2 that those satellites that formed directly from the circumplanetary disk should have compositional differences that reflect the temperature distribution of the disk during the period in which the satellites were forming. This is particularly clear for the "Galilean" satellites of Jupiter (Io, Europa, Ganymede, and Callisto), where distinct density and thus compositional differences are observed with the inner satellites containing less water and other volatiles than the outer satellites. To capture interplanetary planetesimals into the eccentric orbits seen requires some kind of friction, which suggests either aerocapture or that the planetesimals were braked by passing through the dense circumplanetary disk and were thus captured early in the planets' formation. Presumably the composition of these captured satellites has not varied much since the beginning of the solar system and so they are very interesting bodies to examine to understand the composition of early planetesimals. The clearest candidate for a captured satellite is Triton, whose physical characteristics are thought to be similar to Pluto and Charon (listed in Table 1.2e) and to the Kuiper Belt objects discussed in Chapter 2.
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