1. Comparative Planetology of Jupiter and Saturn Chapter 23

2. As you begin this chapter, you leave behind the psychological security of planetary surfaces. You can imagine standing on the moon, on Mars, or even on Venus, but Jupiter and Saturn have no surfaces. Here you face a new challenge — to use comparative planetology to study worlds so unearthly you cannot imagine really being there. On the other hand, Jupiter and Saturn also have extensive systems of moons and rings. Someday humans may walk on some of the moons and watch erupting volcanoes or stroll through methane rain storms, and then journey to the rings and float among the ring particles. As you study these worlds you will find answers to four essential questions: Guidepost

3. How do the outer planets compare with the inner planets? How did Jupiter and Saturn form and evolve? How is Saturn different from Jupiter? How did Jupiter’s and Saturn’s systems of moons and rings form and evolve? After learning about the two largest Jovian planets, in the next chapter you will continue your trip away from the sun and visit their two smaller, and in some ways even stranger, siblings, Uranus and Neptune. It will be interesting, but there is no place like home. Guidepost (continued)

4. I.A Travel Guide to the Outer Planets A.The Outer Planets B.Atmospheres and Interiors C.Satellite Systems II. Jupiter A. Surveying Jupiter B. Jupiter's Magnetic Field C. Jupiter's Atmosphere D. Jupiter's Ring E. Giant Impacts in the Outer Solar System F. The History of Jupiter Outline

5. III. Jupiter's Family of Moons A. Callisto: The Ancient Face B. Ganymede: An Obscure Past C. Europa: A Hidden Ocean D. Io: Roaring Volcanoes E. The History of the Galilean Moons IV. Saturn A. Surveying Saturn B. Saturn's Rings C. The History of Saturn V. Saturn's Moons A. Titan B. The Smaller Moons C. The Origin of Saturn's Moons Outline (continued)

6. A Travel Guide to the Outer Planets Visual image All are much larger than the terrestrial planets, have thick atmospheres, but are very inhospitable: No solid surfaces Mostly Hydrogen and Helium Strong atmospheric circulation (winds/storms) Cloud belt patterns Rings Multiple moons

7. Jupiter Largest and most massive planet in the solar system: Contains almost 3/4 of all planetary matter in the solar system Explored in detail by several space probes: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo Most striking features visible from Earth: Multi- colored cloud belts Visual image Infrared false- color image

8. The Mass of Jupiter Mass can be inferred from the orbit of Io, the innermost of the 4 Galilean Moons: Using Kepler’s third law  M Jupiter = 318 M Earth

9. Jupiter’s Interior From radius and mass  Average density of Jupiter ≈ 1.34 g/cm 3  Jupiter can not be made mostly of rock, like earthlike planets.  Jupiter consists mostly of hydrogen and helium. Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state. T ~ 30,000 K

10. The Chemical Composition of Jupiter and Saturn

11. Jupiter’s Rotation Jupiter is the most rapidly rotating planet in the solar system: Rotation period slightly less than 10 hr. Centrifugal forces stretch Jupiter into a markedly oblate shape.

12. Jupiter’s Magnetic Field Discovered through observations of decimeter (radio) radiation Magnetic field at least 10 times stronger than Earth’s magnetic field Magnetosphere over 100 times larger than Earth’s Extremely intense radiation belts: Very high energy particles can be trapped; radiation doses corresponding to ~ 100 times lethal doses for humans!

13. Aurorae on Jupiter Just like on Earth, Jupiter’s magnetosphere produces aurorae concentrated in rings around the magnetic poles. ~ 1000 times more powerful than aurorae on Earth

14. The Io Plasma Torus Some of the heavier ions originate from Jupiter’s moon Io. Inclination of Jupiter’s magnetic field against rotation axis leads to wobbling field structure passing over Io  Acceleration of particles to high energies. Visible UV Io flux tube

15. Jupiter’s Atmosphere Jupiter’s liquid hydrogen ocean has no surface: Gradual transition from gaseous to liquid phases as temperature and pressure combine to exceed the critical point Jupiter shows limb darkening  hydrogen atmosphere above cloud layers Only very thin atmosphere above cloud layers; transition to liquid hydrogen zone ~ 1000 km below clouds

16. Jupiter’s Atmosphere (2): Clouds Three layers of clouds: 1. Ammonia (NH 3 ) crystals 3. Water crystals 2. Ammonia hydrosulfide

17. The Cloud Belts on Jupiter Dark belts and bright zones Zones higher and cooler than belts; high-pressure regions of rising gas

18. The Cloud Belts on Jupiter (2) Just like on Earth, high-and low-pressure zones are bounded by high-pressure winds. Jupiter’s Cloud belt structure has remained unchanged since humans began mapping them.

19. The Great Red Spot Several bright and dark spots mixed in with cloud structure Largest and most prominent: The Great Red Spot Has been visible for over 330 years Formed by rising gas carrying heat from below the clouds, creating a vast, rotating storm For the first time, very recently, a new, similar red storm system has been observed.

20. Jupiter’s Ring Not only Saturn, but all four gas giants have rings Jupiter’s ring: dark and reddish; only discovered by Voyager 1 spacecraft Galileo spacecraft image of Jupiter’s ring, illuminated from behind Composed of microscopic particles of rocky material Location: Inside Roche limit, where larger bodies (moons) would be destroyed by tidal forces. Ring material can’t be old because radiation pressure and Jupiter’s magnetic field force dust particles to spiral down into the planet. Rings must be constantly re-supplied with new dust.

21. Giant Impacts in the Outer Solar System Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb: ~ 0.15 megaton)! Visual: Impacts seen for many days as dark spots Impact of 21 fragments of comet SL-9 in 1994 Impacts occurred just behind the horizon as seen from Earth, but came into view about 15 min. later. Impact sites appeared very bright in the infrared.

22. The History of Jupiter Formed from cold gas in the outer solar nebula, where ices could condense Rapid growth Soon able to trap gas directly through gravity Heavy materials sink to the center In the interior, hydrogen becomes metallic (very good electrical conductor) Rapid rotation  strong magnetic field Rapid rotation and large size  belt-zone cloud pattern Dust from meteorite impacts onto inner moons trapped to form ring

23. Jupiter’s Family of Moons Over two dozen moons are known now; new ones are still being discovered. Four largest moons already discovered by Galileo: The Galilean moons IoEuropaGanymedeCallisto Interesting and diverse individual geologies

24. Callisto: The Ancient Face Tidally locked to Jupiter, like all of Jupiter’s moons. Av. density: 1.79 g/cm 3  composition: mixture of ice and rocks Dark surface, heavily pocked with craters. No metallic core: Callisto never differentiated to form core and mantle  No magnetic field Layer of liquid water, ~ 10 km thick, ~ 100 km below surface, probably heated by radioactive decay

25. Ganymede: A Hidden Past Largest of the 4 Galilean moons Av. density = 1.9 g/cm 3 Rocky core Ice-rich mantle Crust of ice 1/3 of surface old, dark, cratered; rest: bright, young, grooved terrain Bright terrain probably formed through flooding when surface broke

26. Jupiter’s Influence on its Moons Presence of Jupiter has at least two effects on the geology of its moons: 1. Tidal effects: possible source of heat for interior of Gany- mede 2. Focusing of meteoroids, exposing nearby satellites to more impacts than those further out

27. Europa: A Hidden Ocean Av. density: 3 g/cm 3  composition: mostly rock and metal; icy surface Close to Jupiter  should be hit by many meteoroid impacts; but few craters visible  Active surface; impact craters rapidly erased

28. The Surface of Europa Cracked surface and high albedo (reflectivity) provide further evidence for geological activity.

29. The Interior of Europa Europa is too small to retain its internal heat  Heating mostly from tidal interaction with Jupiter. Core not molten  No magnetic field. Europa has a liquid water ocean ~ 15 km below the icy surface.

30. Io: Roaring Volcanoes Most active of all Galilean moons; no impact craters visible at all Over 100 active volcanoes! Activity powered by tidal interactions with Jupiter Av. density = 3.55 g/cm 3  Interior is mostly rock

31. Interaction with Jupiter’s Magnetosphere Io’s volcanoes blow out sulfur-rich gasses  tenuous atmosphere, but gasses can not be retained by Io’s gravity  gasses escape from Io and form an ion torus in Jupiter’s magnetosphere

32. The History of the Galilean Moons Minor moons are probably captured asteroids. Densities decreasing outward  Probably formed in a disk around Jupiter, similar to planets around the sun Earliest generation of moons around Jupiter may have been lost and spiraled into Jupiter; Galilean moons are probably a second generation of moons. Galilean moons probably formed together with Jupiter.

33. Saturn Mass: ~ 1/3 of mass of Jupiter Radius: ~ 16 % smaller than Jupiter Av. density: 0.69 g/cm 3  Would float in water! Rotates about as fast as Jupiter, but is twice as oblate  No large core of heavy elements Mostly hydrogen and helium; liquid hydrogen core Saturn radiates ~ 1.8 times the energy received from the sun Probably heated by liquid helium droplets falling towards center

34. Saturn’s Magnetosphere Magnetic field ~ 20 times weaker than Jupiter’s  weaker radiation belts Magnetic field not inclined against rotation axis  Aurorae centered around poles of rotation

35. Saturn’s Atmosphere Cloud-belt structure, formed through the same processes as on Jupiter, but not as distinct as on Jupiter; colder than on Jupiter

36. Saturn’s Atmosphere (2) Three-layered cloud structure, just like on Jupiter Main difference to Jupiter: Fewer wind zones, but much stronger winds than on Jupiter: Winds up to ~ 500 m/s near the equator!

37. Saturn’s Rings Ring consists of 3 main segments: A, B, and C Ring separated by empty regions: divisions A Ring B Ring C Ring Cassini Division Rings must be replenished by fragments of passing comets & meteoroids. Rings can’t have been formed together with Saturn because material would have been blown away by particle stream from hot Saturn at time of formation.

38. Composition of Saturn’s Rings Rings are composed of ice particles moving at large velocities around Saturn, but small relative velocities (all moving in the same direction).

39. Shepherd Moons Some moons on orbits close to the rings focus the ring material, keeping the rings confined.

40. Divisions and Resonances Moons do not only serve as “Shepherds”. Where the orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material in the disk (“resonance”), the material is cleared out  Divisions

41. Titan About the size of Jupiter’s moon Ganymede Rocky core, but also large amount of ice Thick atmosphere, hiding the surface from direct view

42. Titan’s Atmosphere Explored in detail by the Cassini spacecraft and the Huygens probe, which landed on Titan’s surface in 2005 Surface pressure: 50% greater than air pressure on Earth Surface temperature: 94 K (-290 o F)  methane and ethane can condense and lead to rain of methane and ethane Methane is gradually converted to ethane in the Atmosphere  Methane must be constantly replenished, probably through breakdown of ammonia (NH 3 ). Atmosphere consists mostly of nitrogen, methane and ethane

43. … and grapefruit sized rocks on the surface. Huygens discovered outflow channels, possibly of liquid methane … Titan’s Surface

44. Saturn’s Smaller Moons Hyperion: Too small to pull itself into spherical shape All other known moons are large enough to attain a spherical shape.

45. Saturn’s Smaller Moons (2) Saturn’s smaller moons are formed of rock and ice; heavily cratered and appear geologically dead Phoebe and Tethys: Heavily cratered, ancient surfaces Iapetus: Leading (upper right) side darker than rest of surface because of dark deposits Enceladus: Possibly active; regions with fewer craters, containing parallel grooves, possibly filled with frozen water

46. The Origin of Saturn’s Satellites No evidence of common origin, as for Jupiter’s moons Probably captured icy planetesimals Moons interact gravitationally, mutually affecting each other’s orbits Co-orbital moons (orbits separated by only 100 km) periodically exchange orbits! Small moons are also trapped in Lagrange points of larger moons Dione and Tethys