Chapter 12: Saturn Spectacular Rings and Mysterious Moons

1. Chapter 12: SaturnSpectacular Rings and Mysterious Moons

2. Saturn

3. Saturn: View from Earth • Saturn reaches opposition every 378 days. • Saturn orbits the Sun at distance of ~ 9.5 AU. • Saturn’s solar year is ~ 29.5 years long. • It moves very slowly through the Zodiac constellations, taking about two years to cross each constellation. • Saturn rotates on its axis once every 10.2 hours. • The rapid rotation flattens Saturn at the poles by about10%, making it the most oblate planet.

4. Saturn’s Rings from Earth • From outside in, the three rings are known as A, B, and C rings. • The Cassini Division lies between rings A and B. • Much narrower Encke gap (some 300 km wide) is found in outer part of the A ring.

5. Saturn’s Rings • Twice during each orbit the plane of Saturn's rings pass through the Earth's orbital plane. • The Voyager spacecraft found that the rings are only 10-50 meters thick. • The rings are translucent, so stars can be seen shining through them. • Because the rings are so thin, they become invisible at these times, and Earth-based observers often look to discover small moons at this time.

6. Rings: Edge View

7. Saturn: Vital Facts

8. Saturn’s Atmosphere

9. Atmospheric Composition • Earth-based and Pioneer and Voyager spacecraft studies indicate that Saturn’s atmosphere consists of • hydrogen 92.4% • helium 7.4% • methane 0.2% • ammonia 0.02% • Similar to Jupiter, except missing about half the helium found in Jupiter’s atmosphere.

10. Circulation in Saturn’s Atmosphere • Zones, belts, and spots are similar to Jupiter's, but much less obvious, probably because • the colder temperature produces a high level haze, • its weaker gravitational field allows the clouds to be spread out over a much greater distance. • Both effects tend to mute Saturn's cloud features. • Strong east-west winds also occur in Saturn's atmosphere (~4 x stronger than Jupiter's). • Because of the tilt of its axis (27o), Saturn has more pronounced seasonal changes than Jupiter.

11. Saturn’s Atmosphere: Clouds • Above clouds lies a layer of haze formed by action of sunlight on upper atmosphere. • Clouds are arranged in three distinct layers by composition: ammonia, ammonium hydrosulfide, water ice. • Total thickness of three cloud layers is roughly 200 km. • 80 km on Jupiter • Colors of cloud layers due to same basic cloud chemistry as on Jupiter. • Saturn's clouds are thicker; fewer holes and gaps in top layer.

12. Saturn’s Jet Stream • Saturn’s zonal flow is considerably faster than Jupiter’s and shows fewer east—west bands. • Equatorial eastward jet stream moves at 1500 km/hr (~400 km/hr on Jupiter) and extends to much higher latitudes. • Not until latitudes 40° N and S of equator are first westward flows found. This latitude also marks strongest bands and most obvious ovals and turbulent eddies. • Reasons for differences between Jupiter's and Saturn's flow patterns not fully known.

13. Storms on Saturn • Saturn has atmospheric wind patterns similar to Jupiter’s. • Similar overall east-west zonal flow, which is quite stable. • Computer-enhanced images clearly show the existence of bands, oval storm systems, and turbulent flow patterns . • Scientists believe that Saturn's bands and storms have essentially the same cause as does Jupiter's weather. Earth-sized storm on Saturn

14. Storms: The Great White Spot • The Great White Spot reoccurs on Saturn about once every 30 years (about the length of Saturn's orbital period). • It was recorded in 1876, 1903, 1933, 1960, and 1990. • Remains visible for a few months and then gradually fades. • Appears to be a seasonal phenomenon.

15. Saturn’s Hydrosphere • Just as with Jupiter, there is probably a layer below the cloud tops where liquid water is stable in the atmosphere of Saturn. • Water (mostly ice) is quite abundant inthe outer Solar System.

16. Saturn’s Biosphere • None is suspected, but just as with Jupiter, some have speculated that layers in Saturn’s atmosphere may be hospitable to life.

17. Saturn's Internal Structure • Probably similar to Jupiter's. It may have • a less dense rocky core, • more molecular hydrogen, and • less liquid metallic hydrogen. • Its low density may be explained by its smaller rocky/icy core with a correspondingly relative higher abundance of hydrogen and helium. • Saturn also radiates more energy into space (2 x 1017 watts) than it receives from the Sun: about 3 x more.

18. Saturn: Internal Heating • Since Saturn radiates about 3 times more energy into space than it receives from the Sun, it must have an internal heat source. • Jupiter’s excess energy is thought to come from left-over heat from formation and contraction. • Saturn is much smaller; should cool more rapidly. • The source of Saturn’s excess energy may be linked to the observed helium deficiency its atmosphere. • Lower T and P conditions allow helium to condense and “rain” into Saturn’s interior, releasing gravitational energy. • Known as “helium precipitation”.

19. Saturn’s Interior • Same basic internal composition as Jupiter, but different relative proportions: • Metallic hydrogen layer is thinner (~1/3 x Jupiter’s). • Core is larger than Jupiter’s. • Less extreme core T, density, and P than Jupiter.

20. Saturn’s Magnetosphere • Similar to Jupiter's but not as strong. • Its radiation belts are more similar to Earth's. • The magnetic axis of Saturn is almost exactly parallel to its rotation axis. • Variations in the flow of the solar wind cause size of Saturn's magnetosphere to fluctuate. • Sometimes the moon Titan is within the magnetosphere, and sometimes it orbits just outside the magnetic field.

21. Saturn’s Magnetic Field • Magnetic field strength: 1/20 x Jupiter’s, 1000 x Earth’s. • Aligned with rotation axis. • Extends ~1 million km • contains rings and 16 innermost moons, • no significant plasma torus, • Titan (orbit =1.2 million km) • Produces AM radio waves • cannot be detected from Earth-based telescopes • Aurora, whistler, radio frequency ES discharge

22. Comparison of Saturn & Jupiter

23. Saturn’s Rings

24. FAQ’s about Saturn’s Rings • What are the rings? Solid, liquid, gas? • Great number of small particles, in independent orbits. • What is the composition of the particles? • Primarily water ice, some ice coated rocky material. • Reflects >80% of incident sunlight. • How big are the particles? • Fractions of mm to tens of meters. • Most are the size of large snowballs. • Spaced by ~2 m. • moving 37,000-50,000 miles/hr around Saturn. • How thick are the rings? • Only a few meters in places (paper, 1 km or 8 blocks, 80-stories)

25. Why are there rings around planets? Roche Limit • Increasing tidal field of planet first distorts, and then destroys, a moon that strays too close. • This critical distance, inside of which the moon is destroyed, is known as the tidal stability limit, or the Roche limit. • The Roche limit is 2.4 x radius of the planet. • For Saturn, no moon can survive within a distance of 144,000 km of the planet's center.

26. Roche Limit for Jovian Planets The rings of Jupiter, Saturn, Uranus, and Neptune are shown above. All distances are expressed in planetary radii. The red line represents the Roche limit. In all cases, the rings lie within the Roche limit of the parent planet.

27. Tilt of the Rings • Over time, Saturn's rings change their appearance to terrestrial observers as the tilted ring plane orbits the Sun. • At times during Saturn's 29.5-year orbital period, the rings seem to disappear altogether as Earth passes through their plane and we view them edge-on.

28. Ring Inclination versus Time(as seen from Earth)

29. Views of the Rings HST images, captured from 1996 to 2000, show Saturn's rings open up from just past edge-on to nearly fully open as it moves from autumn towards winter in its Northern Hemisphere. (Space Telescope Science Institute)

30. Unusual Viewof Rings • Rare view of Saturn's rings seen just after the Sun has set below the ring plane, taken with the HST on Nov. 21, 1995. Unusual perspective because Earth is slightly above Saturn's rings and the Sun is below them. Photograph shows three bright ring features: the F Ring, the Cassini Division, and the C Ring (from the outer rings to inner). The low concentration of material in these rings allows light from the Sun to shine through them. The A and B rings are much denser, which limits the amount of light that penetrates through them. Instead, they are faintly visible because they reflect light from Saturn's disk. • Credit: Phil Nicholson (Cornell University), Steve Larson (University of Arizona), and NASA April 26, 1996

31. How did Saturn get its Rings? • The rings may be the remains of a satellite that wandered too close to Saturn or matter that was prevented from forming into a moon by tidal disruption. • Another view states that the particles gradually accreted from the solar nebula. • More recent studies based on the dynamics of the ring particles favor the idea that the rings are relatively young and are constantly being replenished from the debris of impacts constantly occurring within the rings and moon system of Saturn. • In any case, the mass of the rings is only one millionth the mass of the Earth's Moon.

32. Saturn’s Famous Rings from Voyager

33. Saturn’s A-Ring

34. Spokes within Saturn’s B-ring

35. Saturn’s C-Ring

36. Rings of Saturn: Dimensions • RING INNER RADIUS(km)OUTER RADIUS(km)WIDTH(km) • D 67,000 74,700 7,700 • C 74,700 92,000 17,300 • B 92,000 117,500 25,500 • Cassini 117,500 122,300 4,800 Division • A 122,300 136,800 14,500 • Encke gap* 133,400 133,700 300 • F 140,300 140,400 100 • E 180,000 480,000 300,000 *The Encke gap lies within the A ring.

37. Ring Structures • RINGLETS • The rings are composed of thousands of individual ringlets that look like the grooves on a phonograph record. • Shepherd satellites control the shape of some of the ringlets. • BRAIDED STRUCTURE • This structure is very difficult to explain by gravitational forces alone. • Possibly an optical illusion caused by differing viewing angles. • SPOKES • These features resemble the spokes on a wagon wheel. They are probably caused by electromagnetic forces that suspend the very find ring particles.

38. Saturn’s F-ring • Outside the A ring lies strangest ring of all, Saturn’s F-ring. • Just inside Saturn's Roche limit, and, unlike the inner major rings, the F ring is narrow (< 100 km wide). • Its oddest feature is that it looks as though it is made up of several separate strands braided together. • The ring's intricate structure, as well as its thinness, arise from the influence of two small moons, known as shepherd satellites, that orbit on either side of it.

39. Shepherd Satellites • The F-ring's thinness, and possibly its other peculiarities too, can be explained by the effects of two shepherd satellites that orbit a few hundred kilometers inside and outside the ring. • The F-ring shepherd satellites operate by forcing the F-ring particles back into the main ring. • As a consequence of Newton's third law of motion, the satellites themselves slowly drift away from the ring.

40. Saturn’s Ring Structure and Shepherd Moons Cassini division: Mimas - 2:1 (orbital resonance) F-ring: Pandora and Prometheus (shepherd satellites) Enke division: Pan (gap produced by embedded satellite)

41. Cassini Mission Joint effort of USA, ESA, and Italy scheduled arrival July, 2004; to study Saturn’s atmosphere, magnetosphere, rings, moons; probe to parachute through Titan’s atmosphere.

42. Cassini Mission Goals

43. The Moons of Saturn

44. Moon Facts • The satellite system is dominated by large moon Titan. • In addition there are at least 27 more small to moderate sized icy moons. • The moons are predominantly icy and some have curious dark and light hemispheres. • Some satellites actually share the same orbit (co-orbital moons). • Small shepherd satellites confine the ring material into narrow ringlets. • The innermost satellites actually orbit within the outermost rings.

46. The Moons of Saturn SatelliteOrbit(1000 km)Radius(km)Mass(kg)DiscovererDate Pan 134 10 ? Showalter 1990 Atlas 138 14 ? Terrile 1980 Prometheus 139 46 2.70e17 Collins 1980 Pandora 142 46 2.20e17 Collins 1980 Epimetheus 151 57 5.60e17 Walker 1980 Janus 151 89 2.01e18 Dollfus 1966 Mimas 186 196 3.80e19 Herschel 1789 Enceladus 238 260 8.40e19 Herschel 1789 Tethys 295 530 7.55e20 Cassini 1684 Telesto 295 15 ? Reitsema 1980 Calypso 295 13 ? Pascu 1980 Dione 377 560 1.05e21 Cassini 1684 Helene 377 16 ? Laques 1980 Rhea 527 765 2.49e21 Cassini 1672 Titan 1222 2575 1.35e23 Huygens 1655 Hyperion 1481 143 1.77e19 Bond 1848 Iapetus 3561 730 1.88e21 Cassini 1671 Phoebe 12952 110 4.00e18 Pickering 1898

47. Four New Moons for Saturn • Four new outer moons have been discovered orbiting Saturn at a distance of at least 15 million km. • The new moons are • irregular in shape, • between 10 and 50 km across, • in eccentric orbits, and • probably captured after formation.

48. Nine “Classical” Moons of Saturn • Observed and identified before 1900. • In order of distance from Saturn (mnemonic: MET DR THIP) • Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Pheobe • Of group, only Titan considered to be a large moon.

49. Moon Comparison Titan is similar in size to the other large moons in the Solar system, but the only one that possesses an atmosphere.

50. Titan: Saturn’s Largest Satellite