Saturn is the second largest planet. Its diameter is about 120, 000 km, ten times the diameter of the Earth, and the mass, 95 Earth masses. The density is only 700 kg m-3, less than the density of water. The rotation axis is tilted about 27° with respect to the orbital plane, so every 15 years, the northern or the southern pole is well observable.

Fig. 7.43. Saturn and its rings. Three satellites (Tethys, Dione, and Rhea) are seen to the left of Saturn, and the shadows of Mimas and Tethys are visible on Saturn's cloud tops. (NASA/JPL)

The rotation period is 10 h 39.4 min, determined from the periodic variation of the magnetic field by the Voyager spacecraft in 1981. However, Cassini spacecraft observed in 2004 the period of 10 h 45 min. The reason for the change is unknown. Due to the rapid rotation, Saturn is flattened; the flattening is 1/10, which can be easily seen even with a small telescope.

The internal structure of Saturn resembles that of Jupiter. Due to its smaller size, the metallic hydrogen layer is not so thick as on Jupiter. The thermal radiation of Saturn is 2.8 times that of the incoming solar flux. The heat excess originates from the differentiation of helium. The helium atoms are gradually sinking inward and the released potential energy is radiated out as a thermal radiation. The abundance of helium in Saturn's atmosphere is only about half of that on Jupiter.

The winds, or jet streams, are similar to those of Jupiter but Saturn's appearance is less colourful. Viewed from the Earth, Saturn is a yellowish disk without any conspicuous details. The clouds have fewer features than those on Jupiter, because a haze, composed of hydrogen, ammonium and methane floats above the cloud tops. Furthermore, Saturn is farther from the Sun than Jupiter and thus has a different energy budget.

The temperature at the cloud tops is about 94 K. Close to the equator the wind speeds exceed 400 m/s and the zone in which the direction of the wind remains the same extends 40° from the equator. Such high speeds cannot be explained with external solar heat, but the reason for the winds is the internal flux of heat.

Saturn's most remarkable feature is a thin ring system (Fig. 7.44, 7.45), lying in the planet's equatorial plane. The Saturnian rings can be seen even with a small telescope. The rings were discovered by Galileo Galilei in 1610; only 45 years later did Christian Huygens establish that the formation observed was actually a ring, and not two oddly behaving bulbs, as they appeared to Galileo. In 1857 James Clerk Maxwell showed theoretically that the rings cannot be solid but must be composed of small particles.

The rings are made of normal water ice. The size of the ring particles ranges from microns to truck-size chunks. Most of the particles are in range of centimetres to metres. The width of the ring system is more than 60,000 km (about the radius of Saturn) and the

Fig. 7.44. A schematic drawing of the structure of the Saturnian rings
Fig. 7.45. At a close distance, the rings can be seen to be divided into thousands of narrow ringlets. (JPL/NASA)

thickness, at most 100 m, and possibly only a few metres. Cassini spacecraft discovered also molecular oxygen around the rings, probably as a product of the disintegration of water ice from the rings.

According to Earth-based observations, the rings are divided into three parts, called simply A, B, and C. The innermost C ring is 17,000 km wide and consists of very thin material. There is some material even inside this (referred to as the D ring), and a haze of particles may extend down to the clouds of Saturn.

The B ring is the brightest ring. Its total width is 26, 000 km, but the ring is divided into thousands of narrow ringlets, seen only by the spacecraft (Fig. 7.45). From the Earth, the ring seems more or less uniform. Between B and A, there is a 3000 km wide gap, the Cassini division. It is not totally void, as was previously believed; some material and even narrow ringlets have been found in the division by the Voyager space probes.

The A ring is not divided into narrow ringlets as clearly as the B ring. There is one narrow but obvious gap, Encke's division, close to the outer edge of the ring. The outer edge is very sharp, due to the "shepherd" moon, some 800 km outside the ring. The moon prevents the ring particles from spreading out to larger orbits. It is possible that the appearance of B is due to yet-undiscovered moonlets inside the ring.

The F ring, discovered in 1979, is about 3000 km outside A. The ring is only a few hundred kilometres wide. On both sides there is a small moon; these shepherds prevent the ring from spreading. An interior moon passing a ring particle causes the particle to move to a larger orbit. Similarly, at the outer edge of the ring, a second

moon forces the particles inward. The net result is that the ring is kept narrow.

Outside the F ring, there are some zones of very sparse material, sometimes referred to as the G and E rings. These are merely collections of small particles.

Fig. 7.46a-e. Saturnian moons photographed by the Cassini spacecraft in 2005-2006 (a) Hyperion, (b) Enceladus, (c) Iapetus and (d) Tethys. (e) A radar picture of the northern latitudes of Titan, taken by Cassini in summer 2006. The black patches are probably methane lakes. The width of the picture is about 450 km. (Photo NASA)

Fig. 7.46a-e. Saturnian moons photographed by the Cassini spacecraft in 2005-2006 (a) Hyperion, (b) Enceladus, (c) Iapetus and (d) Tethys. (e) A radar picture of the northern latitudes of Titan, taken by Cassini in summer 2006. The black patches are probably methane lakes. The width of the picture is about 450 km. (Photo NASA)

The Saturnian rings were possibly formed together with Saturn and are not debris from some cosmic catastrophe, like remnants of a broken moon. The total mass of the rings is 10-7 of the mass of Saturn. If all ring particles were collected together, they would form an ice ball, 600 km in diameter.

A total of 56 moons (late 2006) of Saturn are known. Many of the large Saturnian moons (Fig. 7.46) were observed by Pioneer 11 and Voyager 1 and 2 spacecrafts. Large moons (excluding Titan) are composed mainly of ice. The temperature of the primeval nebula at the distance of Saturn was so low that bodies of pure ice could form and survive.

Some moons are dynamically interesting; some have an exotic geological past. Outside the F ring, there are two moonlets, Epimetheus and Janus, almost in the same orbit; the difference of the semimajor axes is about 50 km, less than the radii of the moons. The inner moon is gaining on the outer one. The moons will not collide, since the speed of the trailing moon increases and the moon moves outward. The speed of the leading moon decreases and it drops inward. The moons exchange their roles roughly every four years. There are also several shepherding satellites, like Atlas, Prometheus and Pandora that keep rings in their place. Their gravitational pull prevents ring particles from drifting away.

The innermost of the "old" moons is Mimas. There is a huge crater on Mimas' surface with a diameter of 100 km and a depth of 9 km (Fig. 7.46). Bigger craters exist in the solar system, but relative to the size of the parent body, this is almost the biggest crater there could be room for (otherwise the crater would be bigger than

Mimas itself). On the opposite side, some grooves can be seen, possibly signifying that impact has almost torn the moon apart.

The surface of the next moon, Enceladus, consists of almost pure ice, and one side is nearly craterless. Craters and grooves can be found on the other hemisphere. Tidal forces result in volcanic activity where water (not lava or other "hot" material) is discharged to the surface.

Titan is the largest of the Saturnian moons. Its diameter is 5150 km, so it is only slightly smaller than Jupiter's moon Ganymede. Titan is the only moon with a dense atmosphere. The atmosphere is mainly nitrogen (98-4%) and methane, and the pressure at the surface is 1-5-2 bar. The temperature is about 90 K. Reddish clouds form the visible surface some 200 km above the solid body. Measurements and images of Huy-gens probe landing on Titan in 2005 did not reveal liquid methane lakes. However, Cassini orbiter radar data in 2006 strongly indicate the presence of the lakes (Fig. 7.44). An independent proof of liquids in the recent past can be seen in several surface features which possibly have been formed by flowing fluids.

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