The Origin and Evolution of the Solar System

20 The structure of the Solar SystemThe other satellites of Saturn have well-cratered icy surfaces and are obviouslyquite old. Mimas has a very large crater, Herschel, with a diameter almostone-third that of the satellite. It is clearly the scar of an impact that must havebeen close to destroying the satellite. Iapetus, in an extended orbit with semimajoraxis more than ¿ ¢ ½¼ km, has a leading hemisphere that is much darkerthan the trailing hemisphere. The fractions of reflected light, the albedoes, forthe two sides are 0.05 and 0.50 respectively. The reason for this difference hasbeen the subject of much debate. Iapetus is certainly an icy satellite so the darkregion must be due to something covering the ice, which is naturally white andbright. The most favoured explanation is that the dark material has come from theinterior of Iapetus, although the nature of this material is very uncertain.The outermost satellite of Saturn, Phoebe, is quite small and was not wellobserved by either Voyager 1 or Voyager 2. Its main claim to fame is its retrogradeorbit that suggests that it is almost certainly a captured object.The ring system of Saturn is one of the most striking and structurally interestingfeatures of the Solar System. Seen from the Earth there are several prominentbands and divisions, notably the Cassini division, but imaged by Voyager 1 (figure1.10(a)) the ring structure is seen to be very complex. The general structure,seen in figure 1.10(b), has divisions between various rings that correspond to orbitscommensurate with the more massive inner satellites. The broad Cassini divisioncorresponds to a period one-half that of Mimas, one-third that of Enceladusand one-quarter that of Tethys. The division between the B and C rings correspondsto one-third of the period of Mimas while the Encke division correspondsto three-fifths of Mimas’ period. When the orbit of a particle is commensuratewith one of the more massive inner satellites it tends to receive a perturbing kickat the same point or points in its orbit which reinforces the disturbance until theperiod changes to non-commensurability.The F-ring has a peculiar braided structure that, at first, seemed inconsistentwith the mechanics of a Keplerian orbit. However, the particles in this ringare influenced by the so-called shepherd satellites, Prometheus and Pandora, thepositions of which bracket the ring. Not only do these satellites cause the non-Keplerian motions in the ring but they also lead to stability of the F-ring. A particlejust inside the orbit of Pandora will overtake the satellite and be perturbedinto an orbit just outside Pandora. It is then overtaken by Pandora and perturbedinto an orbit inside Pandora and so on. Prometheus exerts a similar influence onthe particles in its vicinity and so the satellites ‘shepherd’ the particles and keepthem within the F-ring region.1.4.4 Satellites of Uranus and NeptuneUranus spins less rapidly than Jupiter and Saturn and its equator is inclined at Æ to its orbital plane, thus making its spin retrograde. The 15 known satellites,shown in table 1.7, all have orbits near the equatorial plane. The five outermostsatellites, including four with diameters over 1000 km, were known from tele-