286 SOLAR SYSTEM/Jupiter, Saturn and Their Moons Figure Diagram of Saturn and its ring system There are some less prominent rings behind the main system Figure Diagram showing the main ring system The bright rings are A and B, C is the Crepe or Dusky Ring D is not well defined Reproduced from Kennod R (1990) The Journeys of Voyager NASA reaches for the planets, BDD, New York Within a few days the spot had been spread out by Saturn’s strong equatorial winds, and by October had been transformed into a bright zone all round the equator Extra outbreaks were seen in it, clearly indicating an uprush of material from below Saturn has a strong magnetic field, though less powerful than that of Jupiter; the rotational axis and the magnetic axis are almost coincident There are three main rings (Figure 5); A and B, separated by a gap known as the Cassini Division is honour of its discoverer, and an inner semitransparent ring, C (the Crepe Ring) (Figure 6) Details of the ring system are given in Table The rings are made up of ice particles, from grains up to several metres across; the brightest ring, B, shows curious ‘spokes’, presumably particles elevated away from the main ring plane by magnetic or electrostatic forces The irregular F ring, outside the main system, is stabilized by two small ‘shepherd’ satellites, Prometheus and Pandora Though the system is very extensive, with a total diameter of 270 000 km, it cannot be more than 200 m thick, and if all the particles could be combined they would make up an icy satellite less than 300 km across Three space-craft have now encountered Saturn; Pioneer 11, and the two Voyagers Data are given in Table The Cassini/Huygens probe, launched in 1997, was scheduled to reach its target in late 2004 Satellites Table The ring system of Saturn Feature Distance from centre of Saturn, km Inner edge of ring D Outer edge of ring D Inner edge of ring C Outer edge of ring C Inner edge of ring B Outer edge of ring B Centre of Cassini division Inner edge of ring A Centre of Encke division Outer edge of ring A Centre of ring F Centre of ring G Inner edge of ring E Brightest part of ring E Outer edge of ring E 66 900 73 150 74 510 92 000 92 000 117 500 119 000 122 200 135 700 136 800 140 210 168 000 180 000 230 000 480 000 The satellite system of Saturn is very different from that of Jupiter There is one very large satellite, Titan, and seven icy satellites with diameters between 200 and 1600 km; the rest are much smaller By May 2003, the total number of known satellites had risen to 31 Data are given in Table Titan Apart from Ganymede, Titan is the largest, satellite in the Solar System Its surface is permanently hidden by its dense atmosphere, made up chiefly of nitrogen with appreciable amounts of methane and ethane; organic compounds are plentiful The atmospheric pressure on the surface is 1.6 times that of the Earth’s air at sea level; the surface temperature is À178 C Table Space craft to Saturn Name Launch date Encounter date Closest approach, km Pioneer 11 Voyager Voyager Cassini/Huygens Apr 1973 Sept 1977 20 Aug 1977 15 Oct 1997 11 Sept 1979 12 Nov 1980 25 Aug 1981 2004 20 880 124 200 101 300 Preliminary results Images included Titan Went on to Uranus and Neptune Scheduled orbiter and landing on Titan (Huygens)