Chapter 14 Comparative Planetology of the Jovian Planets Saturn, Uranus, Neptune & their satellite systems

1. Chapter 14 Comparative Planetology of the Jovian Planets Saturn, Uranus, Neptune & their satellite systems Dwarf planets: Trans-Neptunians

2. Voyager 2 flew past each of the outer planets in the 1970s and 1980s. • The Galileo spacecraft circled Jupiter dozens of times in the late 1990s. • The Cassini/Huygens orbiter and probe arrived at Saturn in 2004. • Throughout this discussion, you will find images and data returned by these robotic explorers.

3. Saturn • The Roman god Saturn was celebrated in a weeklong Saturnalia at the time of the winter solstice in late December • Romans took over that tradition from Mesopotamia (god Marduk’s festival) • Early Christians took over the Roman holiday to celebrate Xmas.

4. Saturn • Saturn is most famous for its beautiful rings. • These are easily visible through the telescopes of modern amateur astronomers. • Amateurs recently discovered a storm on Saturn

5. Saturn • Saturn’s orbital radius is almost twice that of Jupiter (9.5 AU) and Saturn takes almost 30 years to orbit the Sun. • Earth is shown for comaprison of sizes

6. Saturn • Interestingly, Saturn’s density is ρ = 0.69 g/cm3, One often hears that Saturn would float on water Actually, that’s not true, the lake would fall toward and wrap around Saturn.. Low density means it’s made mostly of hydrogen + helium gas

7. Saturn • The two Voyager spacecraft flew past Saturn in 1979. • The Cassini spacecraft went into orbit around Saturn in 2004 on an extended exploration of the planet, its rings, and its moons.

8. Saturn the Planet • Saturn shows only faint belt-zone circulation. • However, Voyager, Hubble Space Telescope, and Cassini images show that belts and zones are present and that the associated winds blow up to three times faster than on Jupiter.

9. Saturn the Planet • Belts and zones on Saturn are less visible because they occur deeper in the cold atmosphere – below a layer of methane haze (CH4)

10. Saturn the Planet • The low density of Saturn suggests that it is, like Jupiter, rich in hydrogen and helium. • Photos show that Saturn is the most oblate of the planets. • That evidence shows that its interior is mostly liquid with a small core of heavy elements.

11. Saturn the Planet • As its internal pressure is lower, Saturn has less liquid metallic hydrogen than Jupiter. • Perhaps this is why its magnetic field is 20 times weaker than Jupiter’s. • Like Jupiter, Saturn radiates more energy than it receives from the Sun. • Models predict that it has a very hot interior. • Scientists suspect that He rains down toward the center of Saturn in the form of droplets.

12. Saturn’s Rings • There are several important points to note about the icy rings of Saturn. #1 Cassini division at 2:1 resonance with moon Mimas

13. Saturn’s Rings • #2. The rings are made up of billions of ice particles, each in its own orbit around the planet. • Particles you observe now can’t be as old as Saturn. They must be replenished now and then by impacts on Saturn’s moons or other processes. • The same is true of the rings around the other Jovian planets.

14. Saturn’s Rings • #3. The gravitational effects of small moons can confine some rings in narrow strands or keep the edges of rings sharp. • Moons can also produce waves in the rings that are visible as tightly wound ringlets. Daphnis, 8 km moon Opens the Encke gap

15. Saturn’s Rings • #4. The ring particles are confined in a very thin plane (10 m only!!!) spread among small moons and confined by gravitational interactions with larger moons.

16. Saturn’s Rings & moons • The rings of Saturn & other Jovian worlds are created by and controlled by the planet’s moons. • Without the moons, there would be no rings.

17. Saturn’s Family of Moons • Saturn currently (2012) has 62 known moons. • They contain mixtures of ice and rock. Many are small. Many are probably captured objects.

18. Saturn’s Family of Moons: Titan • The largest of Saturn’s moons is Titan. • It is a bit larger than Mercury. • Its density suggests that it must contain a rocky core under a thick mantle of ices. • Titan is so cold that its gas molecules do not travel fast enough to escape.

19. Saturn’s Family of Moons: Titan • It has an atmosphere that is about 1.5 times denser than Earth’s but is composed mostly of N2, with traces of argon (Ar) and methane (CH4).

20. Saturn’s Family of Moons • Sunlight converts some of the methane into complex carbon-rich molecules. • These collect into small particles, filling the atmosphere with orange smog.

21. Titan • These particles settle slowly downward to coat the surface with what has been described as dark organic goo, composed of carbon-rich molecules. Dunes on Earth (upper) & Titan (lower)

22. Saturn’s Family of Moons: Titan • Titan’s surface is mainly composed of ices of water and methane at T= -180°C (93 K).

23. Saturn’s Family of Moons • The Cassini spacecraft dropped the Huygens probe into Titan’s atmosphere. • It photographed dark drainage channels – rivers of liquid methane (CH4) • It falls as rain

24. Saturn’s Family of Moons • Such methane downpours may be rare, though. No direct evidence of liquid methane was detected as the probe descended. But the orbiter found lakes!

25. Saturn’s Family of Moons • Later radar images made by the Cassini orbiter have detected what appear to be lakes presumably containing liquid methane: the first extraterrestrial river discovered flows into lake Ontario on Titan. • Infrared images suggest the presence of methane volcanoes that replenish the methane in the atmosphere. • So, Titan must have some internal heat source to power the activity.

26. Saturn’s Family of Moons: Iapetus Iapetus close-up by Cassini space probe

27. Saturn’s Family of Moons • Most of the remaining moons of Saturn: • Are small and icy, • Have no atmospheres, • Are heavily cratered, and • Have dark, ancient surfaces (are not active)

28. Saturn’s Family of Moons: Pandora (diameter 84km) • Most of the remaining moons of Saturn: • Are small and icy, • Have no atmospheres, • Are heavily cratered, and • Have dark, ancient surfaces. Pandora Diameter 84 km

29. Saturn’s Family of Moons • The moon Enceladus, however, shows signs of recent geological activity • Some parts of its surface contain 1000 times fewer craters than other regions. It also has ice volcanoes!

30. Saturn’s Family of Moons: Enceladus • Infrared observations show that its south polar region is unusually warm and venting water and ice geysers, which have been directly imaged.

31. Saturn’s Family of Moons • Evidently, a reservoir of liquid waters lies only tens of metres below the surface. • At some point in its history, the moon must have been caught in a resonance with another moon and was warmed by tidal heating.

32. Planet Uranus • Now that you are familiar with the gas giants in our solar system, you will be able to appreciate how weird the ice giants – Uranus and Neptune – are.

33. Uranus • Uranus was discovered in 1781 serendipitously (accidentally) by astronomer William Herschel, a German expatriate living in England. • He named it George’s Star – after his patron the English King George III. • Continental astronomers – especially the French – refused to accept a planet named after an English king. They called it Herschel.

34. Planet Uranus • Years later, the name Uranus was suggested – the oldest of the Greek gods (god of the heavens)

35. Uranus • Uranus orbits the Sun at an average of 19.2 AU, almost twice as far away as Saturn. • Once again, we see how greatly spaced the Jovian planets are compared to the inner Terrestrial planets, and that the next one is about 1.8-2 times further than its inner neighbour

36. Planet Uranus • Uranus is only a third of the diameter of Jupiter and only 1/20 as massive. • Being four times farther from the Sun, its atmosphere is over 100°C colder than Jupiter’s. [1 K = 1°C (temp. difference), but Kelvin scale starts from absolute zero or T = 0 K = -273°C] • T (Jupiter) ~ 150 K (110 K w/o internal heat) • T (Saturn) ~ 70 K • T (Uranus) ~ 50 K • T (Neptune) ~ 40 K

37. Planet Uranus • Uranus never grew massive enough to capture large amounts of gas from the nebula as Jupiter and Saturn did. • So, it has much less hydrogen and helium. • Its internal pressure is enough lower than Jupiter’s that it should not contain any liquid metallic hydrogen.

38. Planet Uranus • Oblateness and mean density => a small core of heavy elements and a deep mantle of partly solid water.

39. Planet Uranus • Although referred to as ice, this material would not be anything like ice on Earth – given the temperatures and pressures inside Uranus. • The internal structure and composition of Uranus and Neptune are similar

40. Planet Uranus • The mantle also contains rocky material and dissolved ammonia and methane. • Circulation in this electrically conducting mantle may generate the planet’s peculiar magnetic field, which is highly inclined to its axis of rotation. • Above the mantle lies the deep hydrogen and helium atmosphere.

41. Planet Uranus • Uranus rotates on its side with its equator inclined 98°to its orbit, meaning that it actually rotates backwards (backwards and forwards and sideways) while it appears to “roll” along or across its orbit. With an orbital period of 84 years, each of its four seasons lasts 21 years. • The winter–summer contrast is extreme. • During a season when one of its poles is pointed nearly at the Sun (a solstice), a inhabitants of Uranus would never see the Sun rise or set.

42. Planet Uranus • Uranus’s odd rotation may have been produced when it was struck by a very large planetesimal late in its formation. • Alternatively, it could due to tidal interactions with the other giant planets, as it migrated outward early in the history of the solar system.

43. Planet Uranus • Voyager 2 photos show a nearly featureless ball. • The atmosphere is mostly hydrogen and helium. • However, traces of methane absorb red light, making the atmosphere look green-blue.

44. Planet Uranus • Computer enhancement revealed a few clouds and bands around the south pole.

45. Planet Uranus • In the decades since Voyager 2 flew past Uranus, spring has come to the northern hemisphere of Uranus and autumn to the southern hemisphere.

46. Planet Uranus • Images made by the Hubble Space Telescope and modern Earth-based telescopes reveal changing clouds and cloud bands in both hemispheres.

47. Planet Uranus • Infrared measurements show that Uranus is radiating about the same amount of energy that it receives from the Sun. • Thus, it has much less heat flowing out of its interior than Jupiter or Saturn (or Neptune).

48. Planet Uranus • This may account for its limited atmospheric activity. • Astronomers are not sure why it differs in this respect from the other Jovian worlds.

49. The Uranian Moons • The five major moons of Uranus are smaller than Earth’s Moon and have old, dark, cratered surfaces. • A few have deep cracks produced, perhaps, when the interior froze and expanded. • In some cases, liquid water “lava” appears to have erupted and smoothed over some regions.

50. The Uranian Moons: Miranda • Miranda, the innermost moon, has 1/7 of the diameter of Earth’s moon. • Its surface is marked • by grooves called ovoids. • It looks like it was broken • apart and reassembled • by a giant collision. But • it could just be ice geology.