The Origin and Evolution of the Solar System

22 The structure of the Solar SystemTable 1.7. The satellite system of Uranus. The spin period of Uranus ½Ö½ÑÒ.Inclination Semi-major Averageof orbit to axis Mass diameter DensitySatellite equator (½¼ ¿ km) Eccentricity (½¼ ¾¾ kg) (km) (½¼ ¿ Ñ ¿ )Nine small 50–73 40–80satellitesPuck ¼¼ Æ 86 0.000 170Miranda ¼¼ Æ 130 0.000 0.0075 484 1.26Ariel ¼¼ Æ 191 0.003 0.14 1160 1.65Umbriel ¼¼ Æ 266 0.003 0.13 1190 1.44Titania ¼¼ Æ 436 0.002 0.35 1610 1.59Oberon ¼¼ Æ 583 0.007 0.29 1550 1.50Table 1.8. The satellite system of Neptune. The spin period of Neptune ½ Ö ÑÒ.Inclination Semi-major Averageof orbit to axis Mass diameter DensitySatellite equator (½¼ ¿ km) Eccentricity (½¼ ¾¾ kg) (km) (½¼ ¿ Ñ ¿ )Niaid 48 60Thalassa 50 80Despoina 53 150Galatea 62 160Larissa 74 200Proteus 118 415Triton ½¼ Æ 355 0.000 2.21 2705 2.07Nereid ¾ Æ 5513 0.749 0.0021 340These two, plus another six found by spacecraft observation are listed in table 1.8.Stellar occultation observations had indicated that Neptune should have aring system and, indeed, these were seen and imaged by the Voyager spacecraft.The Earth-bound measurements had suggested that the rings were only partial butit turns out that they are complete but have a rather lumpy structure.The presence of rings accompanying each of the major planets suggests thatthere is some common cause associated with their characteristics as large bodieswith many satellite companions. The most likely origin for a ring system is thatthe orbit of a small orbiting satellite decayed to the extent that it strayed within theRoche limit (section 4.4.2). It would then have been tidally disrupted by the planetto give a vast number of small fragments that would have spread out to form therings. Structure in the rings could then be produced by resonant perturbations bysome of the inner satellites as described in section 1.4.3.