Jupiter & its system Saturn & its system

1. Lecture 21 ASTA01 • Jupiter & its system • Saturn & its system Chapter 14Comparative Planetology of the Jovian Planets

2. Lecture 21 ASTA01 • The worlds of the outer solar system can be studied from Earth. • However,

3. Space probes • Much of what scientists know has been radioed back to Earth from robot spacecraft. • 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.

4. A Travel Guide to the Outer Planets • You are about to visit five worlds that are truly unearthly. • This travel guide will warn you about what to expect.

5. The Outer Planets • The outermost planets in our solar system are Jupiter, Saturn, Uranus, and Neptune. • These are often called the “Jovian planets,” meaning that they are like Jupiter (named after a Roman god also known as Jove).

6. The Outer Planets • The figure below compares these four worlds.

7. The Outer Planets • Jupiter is the largest of the Jovian worlds. • It is over 11 times the diameter of Earth.

8. The Outer Planets • Saturn is a bit smaller than Jupiter. • Uranus and Neptune are quite a bit smaller than Jupiter.

9. The Outer Planets • The other feature you will notice immediately is Saturn’s rings. • They are bright and beautiful and composed of billions of ice particles.

10. The Outer Planets • Jupiter, Uranus, and Neptune have rings too. • However, they are not easily detected from Earth and are not visible here.

11. Atmospheres and Interiors • The four Jovian worlds have hydrogen-rich atmospheres filled with clouds. • On Jupiter and Saturn, you can see that the clouds form stripes that circle each planet. • You will find traces of these same types of features on Uranus and Neptune, but they are not very distinct.

12. Atmospheres and Interiors • Models based on observations indicate that, below their atmospheres, Jupiter and Saturn are mostly liquid. • So, the old fashioned term for these planets – the gas giants – should probably be changed to the liquid giants.

13. Atmospheres and Interiors • Uranus and Neptune are sometimes called the ice giants. • They are rich in water in both solid and liquid forms.

14. Atmospheres and Interiors • Only near their centres do the Jovian planets have cores of dense material with the composition of rock and metal. • Composition, but not the solid state… • None of the worlds has a definite solid surface on which you could walk.

15. Atmospheres and Interiors • You have learned that the Jovian planets have low density because they formed in the outer solar nebula where water vapour could freeze to form ice particles. • The ice accumulated into proto-planets with density lower than the rocky terrestrial planets and asteroids. • Once these planets grew massive enough, they could draw in even lower-density hydrogen and helium gas directly from the nebula by gravitational collapse.

16. Satellite Systems • You can’t really land your spaceship on the Jovian worlds. • You might, however, be able to land on one of their moons. • All the outer solar system planets have extensive moon systems (as known in 2012): • Jupiter has 67+ moons, • Saturn has 62+ moons • Uranus has 37+ moons • Neptune has 13+ moons

17. Satellite Systems • In many cases, the moons interact gravitationally. • They mutually adjust their orbits. • They also affect the planetary ring systems.

18. Satellite Systems • Some of the moons are geologically active now. • Others show signs of past activity. • Of course, geological activity depends on heat flow from the interior. • So, you might ponder what could be heating the insides of these small objects. • In fact, it is the mutual interaction of moons with themselves and with their host planets via gravitation that causes the heating

19. Jupiter • Jupiter, named for the Roman king of the gods, can be very bright in the night sky. • Its cloud belts and four largest moons can be seen through even a small telescope.

20. Jupiter • Jupiter is the largest and most massive of the Jovian planets. • It contains 71 percent of all the planetary matter in the entire solar system.

21. Jupiter • You used Earth, the largest of the terrestrial planets, as the basis for comparison with the others. • Similarly, you can examine Jupiter in detail as a standard in your comparative study of the other Jovian planets.

22. The Interior • Jupiter is only 1.3 times denser than water. • For comparison, Earth is more than 5.5 times denser than water. • This gives astronomers a clue about the average composition of the planet’s interior.

23. The Interior • Jupiter’s shape also gives information about its interior. • Jupiter and the other Jovian planets are all flattened. • A world with a large solid, rocky core and mantle would not be flattened much by rotation. • A largely liquid planet, though, would flatten significantly (independent of chemical composition)

24. Polar radius 66854+-10 km < Equatorial rad 71492+-4 km 93.5 : 100

25. Jupiter’s orbit and the zodiac Jupiter orbits the Sun at a distance of 5.2 AU, completing each orbit in about 11.9 years – which means that it goes through each zodiacal region in one year. That may be why we have 12 signs of zodiac, and 2*12 hours in a day, but we don’t really know… The zodiac with 12 signs appeared in Chaldea (Babilonian) astrology in ~7th century BCE

26. The Interior • Despite the fact that Jupiter is the most massive of all the planets, its sidereal rotation period is merely 10 hours, making its spinning rate the fastest of all the planets. • There is nowhere to stand on Jupiter’s surface, a fact that is easily forgotten when viewing Jupiter through a telescope or binoculars. • Models indicate that the interior is mostly liquid hydrogen.

27. The Interior • However, if you jumped into Jupiter carrying a rubber raft expecting an ocean, you would be disappointed. • The base of the atmosphere is so hot and the pressure is so high that there is no sudden boundary between liquid and gas. • As you fell deeper through the atmosphere, you would find the gas density increased around you until you were sinking through a liquid. • You would, however, never splash into a distinct liquid surface because the transition from gas to liquid takes place very gradually.

28. The Interior • Under very high pressure, liquid hydrogen becomes liquid metallic hydrogen. • This is a very good conductor of electricity. • Most of Jupiter’s interior is composed of this unusual material.

29. The Interior • That large mass of conducting liquid is stirred by convection currents and spun by the planet’s rapid rotation. • As a result, it drives the dynamo effect and generates a powerful magnetic field. • Jupiter’s field is over 10 times stronger than Earth’s.

30. The Interior • A planet’s magnetic field deflects the solar wind and dominates a volume of space around the planet called the magnetosphere.

31. The Interior • The strong magnetic field around Jupiter traps particle streams from the solar wind in radiation belts a billion times more intense than the Van Allen belts that surround Earth.

32. The Interior • The spacecraft that have flown through these regions received over 1000 times the radiation that would have been lethal for a human.

33. The Interior • At Jupiter’s centre, a so-called rocky core contains heavier elements, such as iron, nickel, and silicon. • With a temperature four times hotter than the surface of the Sun (25000 K) and a pressure of 50 million atm (Earth’s air pressure at sea level), this material is unlike any rock on Earth. • The term rocky core refers to the chemical composition, not to the properties of the material.

34. The Interior • Careful infrared measurements of the heat flowing out of Jupiter reveal that the planet emits about twice as much energy as it absorbs from the Sun. • This energy appears to be due to heat left over from the formation of the planet. Slow contraction (about 2 cm per year)

35. Jupiter’s Complex Atmosphere The red spot, observed by Galileo is a permanent(?) hurricane

36. Jupiter’s Complex Atmosphere • #1. Jupiter’s extensive magnetosphere is responsible for auroras around the magnetic poles. • #2. Jupiter’s rings, discovered in 1979 by the Voyager I space probe, are relatively close to the planet. • #3. the pattern of coloured cloud bands circling the planet is called belt–zone circulation. This pattern is related to the high- and low-pressure areas found in Earth’s atmosphere.

37. Jupiter’s Complex Atmosphere #4. The belt–zone circulation is related to the high- and low-pressure areas found in Earth’s atmosphere. The positions of the cloud layers are at certain temperatures within the atmosphere where ammonia (NH3), ammonium hydrosulphide (NH4SH), and water (H2O) can condense.

38. Jupiter’s Complex Atmosphere #5. It is not yet known whether the Great Red Spot is long-lived or will disappear in a 100 years, whether it is deeply rooted or a shallow circulation like earth hurricanes.

39. Jupiter’s Ring • Astronomers have known for centuries that Saturn has rings. • Jupiter’s ring, though, was not discovered until 1979, when the Voyager 1 spacecraft sent back photos.

40. Jupiter’s Rings • Jupiter’s rings were not discovered until 1979, when the Voyager 1 spacecraft sent back photos. Less than 1 percent as bright as Saturn’s icy rings, Jupiter’s rings. This indicates that they are rocky rather than icy. Ring particles are mostly microscopic, very dark (1/100 brightness of Saturn’s rings) and reddish.

41. Jupiter’s Rings

42. Jupiter’s Rings • Photos show that they are very bright when illuminated from behind. • That is, it is scattering light forward. • Large particles do not scatter light forward. So, a ring filled with basketball-size particles would look dark when illuminated from behind.

43. Jupiter’s Ring • Forward scattering of visible light shows you that the ring is mostly made of tiny grains, with diameters approximately equal to the wavelengths of visible light (0.5 micrometers or microns) • This would be about the size of particles in cigarette smoke.

44. Jupiter’s Ring • The rings orbit inside the Roche limit. • This is the distance from a planet within which a moon cannot hold itself together by its own gravity. • If a moon comes inside the Roche limit, the tidal forces overcome the moon’s gravity and pull the moon apart. • Also, raw material for a moon cannot coalesce inside the Roche limit.

45. Jupiter’s Ring • The Roche limit is about 2.4 times the planet’s radius, depending somewhat on the relative densities of the planet and the moon material. • Jupiter’s rings lie inside the limit for the planet. • Those of Saturn, Uranus, and Neptune too lie within the respective planetary limits.

46. Jupiter’s Ring • Now you can understand Jupiter’s dusty rings. • If a dust speck gets knocked loose from a larger rock inside the Roche limit, the rock’s gravity cannot hold the dust speck. • Also, the billions of dust specks in the ring can’t pull themselves together to make a new moon because of tidal forces inside the Roche limit.

47. Jupiter’s Ring • We can be sure that Jupiter’s ring particles are not old. • The pressure of sunlight and the planet’s powerful magnetic field alter the orbits of the particles. • Images show faint ring material extending down toward the cloud tops – evidently dust specks spiralling into the planet. • Dust is also destroyed by the intense radiation around Jupiter that grinds the dust specks down to nothing in a century or so.

48. Jupiter’s Ring • The rings you see today, therefore, can’t be material left over from the formation of Jupiter. • The rings of Jupiter must be continuously resupplied with new dust. • Observations made by the Galileo spacecraft provide evidence that the source of ring material is micrometeorites eroding small moons orbiting near, or within, the rings.

49. Jupiter’s Ring • The rings around Saturn, Uranus, and Neptune are also known to be short-lived. • They too must be resupplied by new material, probably eroded from nearby moons.

50. Jupiter’s Ring • In addition to supplying the rings with particles, moons: • Confine the rings gravitationally, • Alter their shapes. • For comparison: Mimas, Cassini gap 1:2 resonance with Mimas