1. Chapter 9 9-3 thru 9-4 The Jovian Planets Courtesy of NASA/JPL/Space Science Institute

2. Saturn’s Motions 1. Saturn orbits the Sun at an average distance of 9.6 AU; its distance from the Earth varies from 8.5 AU to 10.5 AU. 2.Saturn has an orbital period of 29.5 years. 3. Saturn is tilted 27° with respect to its orbital plane, so over time its rings appear in different orientations when viewed from Earth.

3. Figure 9.20: Saturn seasonal progression Courtesy of NASA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: R.G. French (Wellesley College), J. Cuzzi (NASA/Ames), L. Dones (SWRI), J. Lissauer (NASA/Ames) Figure 9.21: Saturn's orbit around Earth

4. 4. Like Jupiter, Saturn shows differential rotation. Its equatorial rotation rate is 10 h 39 m. 5. Saturn is even more oblate than Jupiter, with its equatorial diameter 10% greater than its polar diameter.

5. Pioneer, Voyager, and Cassini 1. Pioneer 11 passed Saturn in 1979, followed by Voyager 1 in 1980 and Voyager 2 in 1981. Knowledge gained from these probes was used to guide scientists in decisions concerning probes that followed. 2. Saturn’s magnetic field is only 5% as strong as Jupiter’s because Saturn’s liquid metallic hydrogen only extends about half way to its cloud tops. 3. As for the case of Jupiter, Saturn’s interior structure is inferred from models and extrapolation of data from the outer layers.

6. 4. Saturn’s clouds are less colorful than Jupiter’s because the colder temperatures at Saturn’s distance from the Sun inhibit chemical reactions that give Jupiter’s atmosphere its varied colors, and a layer of methane haze above the cloud tops on Saturn blurs out color differences. 5. Saturn has atmospheric features similar to Jupiter’s, but Saturn’s winds reach speeds 3 to 4 times faster.

7. Saturn’s Excess Energy 1. Saturn radiates more energy than it absorbs. It also has less helium in its upper atmosphere than Jupiter has, by a factor of two (by mass). 2. The leading hypothesis in explaining both observations is that the cooling of Saturn’s atmosphere causes helium to condense to liquid and rain downward. As the helium droplets fall, they lose gravitational energy, which is converted to thermal energy.

8. Enceladus and Titan 1.Saturn has 47 moons, most of which consist of dirty ice. Major moons include Titan, Mimas, Enceladus, Dione. 2. Enceladus is covered in water ice and its interior may be liquid today. Active volcanism exists on this object; Cassini images show plumes of water vapor and ice water particles. 3. The atmosphere of Enceladus also includes carbon dioxide, methane, and other simple carbon-based molecules. Figure 9.26a: Cassini image of Enceladus Courtesy of NASA/JPL/Space Science Institute

9. 4. Titan may be the most interesting moon in the solar system because it has an atmosphere, which is composed mostly of nitrogen with a few percent of methane and argon. There are also traces of water and organic compounds. 5.When sunlight breaks down methane in Titan’s upper atmosphere, organic molecules are formed; these molecules then slowly drift down to the surface. This raises the question of whether life might have formed on Titan’s surface.

10. Figure 9.27a: Titan in color-enhanced UV Courtesy of NASA/JPL/Space Science Institute

11. 6.Huygens data show bright highlands, deep channels, and dark lowlands that look like dried lake or river beds on Titan’s surface. All the existing data suggests that Titan resembles Earth, with clouds, rain and seas. 7. Titan is the second largest moon (after Ganymede) in the solar system with a diameter of 5,150 km. 8. Titan’s atmosphere is denser and 10 times more massive than Earth’s because its surface temperature of –180°C is low enough to keep gas molecules from escaping.

12. Figure 9.28b: A composite Huygens image shows many different flows into a river channel. Courtesy of ESA/NASA/JPL/University of Arizona

13. Figure 9.28a: Objects on surface of Titan Courtesy of ESA/NASA/JPL/University of Arizona

14. Planetary Rings 1. Saturn’s rings are very thin, a few tens of meters across. 2. The rings are not solid sheets but are made up of small particles of water ice or rocky particles coated with ice. 3. Each ring particle revolves around Saturn according to Kepler’s laws. 4. Three distinct ring bands are visible from Earth, and named (outer to inner) A, B, C. Figure 9.29: Saturn's rings Courtesy of NASA/JPL/Space Science Institute

15. 5. The largest division between the rings is known as Cassini’s division. This space is caused primarily by the gravity of Mimas and the synchronous relationship between the orbital periods of Mimas and of any particle in the Cassini division. 6. Other features of the rings are explained by the presence of small shepherd moons.

16. The Origin of Rings 1. The origin of Saturn’s rings is not well understood but is thought to be the result of a close-orbiting, icy moon that was shattered by a collision with a passing asteroid. Another possibility is that an object from the outer solar system came too close to Saturn and was torn apart by the planet’s gravity. 2. Tidal forces are greater on a moon in orbit close to a planet than they are on a moon in an orbit farther out.

17. 3. The Roche limit is the minimum radius at which a satellite (held together by gravitational forces) may orbit without being broken apart by tidal forces. 4. Saturn’s rings are inside Saturn’s Roche limit, so no moons can form from the particles in the rings. 5. If all ring particles were to be collected to form a small moon, its mass would be about 1/20,000 the mass of our Moon.

18. 1. Uranus was plotted on star charts as early as 1690, Uranus’s slow orbital motion caused it to go unnoticed until Herschel discovered it in 1781. 9-3 Uranus 2. Uranus’s diameter is difficult to determine from Earth because its angular size is very small and it can’t be seen clearly. The first reliable value for Uranus’ diameter came from a telescope in a high-altitude balloon. 3. An improved determination of Uranus’s diameter was made in 1977 during an occultation of a star by the planet.

19. 4. Uranus has a diameter of 51,000 km (32,000 mi), 4 times that of Earth. 5. Uranus has a density of 1.27 g/cm 3 ; it might have a very small rocky core or no core at all. 6. Uranus’s atmosphere is similar to that of Jupiter and Saturn: mostly hydrogen and helium with some methane.

21. 7. Uranus does not have cloud layers, so the methane in its atmosphere, which absorbs red light, makes the planet appear blue. 8. Occultation data from 1977 showed that Uranus has a system of thin rings that contain very little material. 9. Uranus’s rings only reflect 5% of the sunlight that hits them so they cannot be seen from Earth. (Saturn’s rings reflect 80% of incident sunlight.)

22. Question 1 (9-3 thru 9-4 PPT Questions) Why don’t we see the rings of the planets Jupiter, Uranus, and Neptune? What might be the reason behind your answer?

23. Uranus’s Orientation and Motion 1.Uranus’s equatorial plane is tilted 98° to its plane of revolution. This results in a retrograde rotation, as seen from far above the Sun’s north pole. It also implies extreme seasons since during each revolution, the planet’s north pole at one time points almost directly to the Sun and at another time faces nearly away from the Sun. Figure 9.35: Tilted axis of Uranus

24. 2. Uranus has an orbital period of 84 years. 3. Uranus has a fairly uniform temperature over its surface (about –200°C), indicating that the atmosphere is continually stirred up. 4. Uranus has cloud bands that rotate differentially—16 hours at the equator and 28 hours at the poles.

25. 5. Uranus’s magnetic field is comparable to Saturn’s. –It probably originates in electric currents within the planet’s layer of water. –The magnetic field’s axis is tilted 59° with respect to its rotation axis. –No other planet has such a large angle between the two axes (though Neptune’s at 47° is close).

26. 6. Five moons were known before Voyager; we now know of 27 moons. All are low-density, icy worlds. –The innermost, Miranda appears as if it were torn apart by a great collision and then reassembled. 7. Two of Uranus’s moons are shepherd moons. Material in Uranus’ rings is very sparse; all of it together is less than the material in Cassini’s division! Courtesy of NASA/JPL-Caltech Figure 9.38: Miranda

27. Question 2 (9-3 thru 9-4 PPT Questions) Speculate what possibly could have happened to Miranda.

28. 9-4 Neptune 1. Neptune is similar to Uranus, slightly smaller at 49,500 km in diameter. Neptune’s composition matches that of Uranus. Neptune’s color is also blue (because of methane in its upper atmosphere).

29. 2. Unlike the nearly featureless Uranus, Neptune exhibits weather patterns in its atmosphere. It has parallel bands around it and its Great Dark Spot is similar in appearance to Jupiter’s Great Red Spot. Figure DP10.02: Neptune Courtesy of NASA/JPL-Caltech

30. 3. Neptune radiates more internal energy than Uranus, although the cause is unknown. –This energy drives the weather on Neptune and results in winds that reach speeds of 700 miles/hr. 4. The wispy white clouds seen on Neptune are thought to be crystals of methane. 5. Neptune exhibits the most extreme differential rotation of any of the Jovian planets: 18 hours at the equator and 12 hours at the poles. –However, these differences are confined to the upper few percent of the atmosphere.

31. 6.Neptune’s magnetic field rotates with a period of 16 h 7 m, which is taken as the planet’s basic rotation rate. 7. Neptune’s temperature is remarkably uniform at –216°C and its axis is tilted less than 30° to its orbit. 8. Neptune’s density is greater than Uranus’; this is probably due to a somewhat larger rocky core.

33. Neptune’s Moons and Rings 1.Before Voyager Neptune was known to have 2 moons (Triton and Nereid); 11 moons are now known. 2. Triton, Neptune’s largest moon, is the only major moon to revolve around a planet in a clockwise (retrograde) direction. 3. Nereid has the most eccentric orbit of any moon in the solar system.

34. 4. Triton has a light-colored surface composed of water ice with some nitrogen and methane frost. Its surface appears young, with active geyser-type volcanoes and very few craters. 5. Triton’s density is about the same as Pluto’s. 6. The leading hypothesis in explaining the properties of both Triton and Nereid is that these moons were captured by Neptune after the initial formation of the solar system. Triton’s active volcanism is probably due to internal heating from tidal forces caused by Neptune’s gravity.

35. Figure 9.42: Triton Courtesy of NASA/JPL-Caltech

36. 7. Stellar occultations observed in 1984 revealed that Neptune has rings. They are “lumpy,” perhaps as a result of undiscovered moons orbiting with them. Courtesy of NASA/JPL-Caltech Figure 9.43: Rings of Neptune

37. Question 3 (9-3 thru 9-4 PPT Questions) List some unique features about Neptune’s moons Triton and Nereid.