1.2 Satellite systems and their origins

The satellite systems of the giant planets have several features in common. Most satellites are in synchronous rotation, always keeping the same face towards their planet. Irregularly shaped moonlets associated with the ring system orbit closest to the planet. They travel in near-circular prograde orbits in the planet's equatorial plane. ('Prograde' in this sense means orbiting in the same direction as the planet's spin.) These moonlets (like the rings) are believed to be fragments of larger satellites that were destroyed by collisions or tidal forces (Figures 4 and 5). Most are bright and presumed to be icy in composition.

Figure 4 Five of Saturn's innermost satellites as imaged by the Voyager spaceprobes, shown at their correct relative sizes. From left to right: Atlas, Pandora (above) and Prometheus (below), Janus (above) and Epimetheus (below). Janus is 99 km in length. The dark line across Epimetheus is the shadow of one of the narrowest of Saturn's rings. (NASA)

Figure 5 A Voyager 2 image showing Uranus's outermost and most prominent ring, which is kept in shape by the small satellites Ophelia and Cordelia (each less than about 30 km across), one of which orbits 2000 km inside the ring and the other 2000 km beyond it. The ring consists of millions of dark, dusty boulders, mostly 10 cm to 10 m in size. Four fainter rings are just visible within the orbit of Cordelia. (NASA)

Orbiting further from each planet come all the satellites large enough to be spherical (or nearly so) in shape, typically in near-circular prograde orbits close to the planet's equatorial plane. These satellites probably grew within a disc of gas and dust that surrounded the planet in the later stages of its growth, mimicking in miniature the birth of the terrestrial planets from the solar nebula. Neptune's large satellite, Triton, is an exception (Figure 6). This has a retrograde orbit, and may be a Pluto-like Kuiper Belt object that was captured into orbit around Neptune some billions of years ago. (We have used the term 'Kuiper Belt' but you may also see it called the 'Edgeworth-Kuiper Belt'. Kenneth Edgeworth, a British astronomer, published similar ideas a few years prior to Kuiper, but this only really came to light after the term 'Kuiper Belt' had become widely established.)

Figure 6 A Voyager 2 image of the sunlit part of the Neptune-facing hemisphere of Triton. The south polar cap of bright nitrogen-ice is marked by streaks of sooty (carbon-rich) material erupted from geysers. The rugged surface beyond the polar cap is methane-rich ice, contaminated by nitrogen, carbon dioxide, carbon monoxide and water. (NASA)

Beyond its large satellites each giant planet has a second collection of small irregular-shaped satellites, travelling in elongated, inclined and in many cases retrograde orbits. Most are dark bodies, rich in silicates and/or carbon compounds. These satellites are likely to be captured comets or asteroids (Figure 7).

Figure 7 Some of the outer small satellites of Saturn shown at their correct relative sizes. From left to right: Telesto (above) and Calypso (below), Helene, Hyperion and Phoebe. Hyperion is 185 km in length. (NASA)