Jupiter and Saturn

1. Jupiter and Saturn Chapter 23

2. Guidepost As we begin this chapter, we leave behind the psychological security of planetary surfaces. We can imagine standing on the moon, on Venus, or on Mars, but Jupiter and Saturn have no surfaces. Thus, we face a new challenge—to use comparative planetology to study worlds so unearthly we cannot imagine being there. One reason we find the moon and Mars of interest is that we might go there someday. Humans may become the first Martians. But the outer solar system seems much less useful, and that gives us a chance to think about the cultural value of science. This chapter begins our journey into the outer solar system. In the next chapter, we will visit worlds out in the twilight at the edge of the sun’s family.

3. Outline I. Jupiter A. Surveying Jupiter B. Jupiter's Magnetic Fields C. Jupiter's Atmosphere D. Jupiter's Ring E. Comet Impact on Jupiter F. The History of Jupiter II. Jupiter's Family of Moons A. Callisto: The Ancient Face B. Ganymede: A Hidden Past C. Europa: A Hidden Ocean D. Io: Bursting Energy E. The History of the Galilean Moons

4. Outline (continued) III. Saturn A. Planet Saturn B. Saturn's Rings C. The History of Saturn IV. Saturn's Moons A. Titan B. The Smaller Moons C. The Origin of Saturn's Satellites

5. Jupiter Largest and most massive planet in the solar system: Contains almost 3/4 of all planetary matter in the solar system. Most striking features visible from Earth are the multi-colored cloud belts Explored in detail by several space probes: Visual image Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 (1979), Voyager 2 (1979), Galileo (1995-2003) Infrared false-color image

6. The Mass of Jupiter Mass can be determined from the orbit of any of the innermost four Galilean moons Io Moon 1.8 day orbital period 27.3 day orbital period Jupiter Earth Relative sizes and distances are to scale click for orbital animation Using Kepler’s third law: MJupiter = 318 MEarth

7. Jupiter’s Interior From volume and mass, average density of Jupiter is 1.34 g/cm3[compare to Mercury (5.44), Venus (5.24), Earth (5.50), Mars (3.94)] Therefore, Jupiter cannot be made mostly of rock, like earthlike planets, but consists mostly of hydrogen and helium. T = 30,000 K (hotter than sun’s surface!) Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.

8. The Chemical Composition of Jupiter and Saturn Hydrogen gas Helium gas Water/ice Methane Ammonia

9. Jupiter’s Rotation Jupiter is the most rapidly rotating planet in the solar system: Rotation period slightly less than 10 hours. Centrifugal forces stretch Jupiter into a oblate shape (like an M&M).

10. Jupiter’s Magnetic Field Discovered by observations of radio waves and microwaves Magnetic field at least 10 times stronger than Earth’s magnetic field. Magnetosphere over 100 times larger than Earth’s. Intense radiation belts trap very high energy particles (electrons and protons). Radiation doses are 100 times lethal amount for humans!

11. Auroras on Jupiter Just like on Earth, Jupiter’s magnetosphere produces auroras concentrated in rings around the magnetic poles. They are 1000 times more powerful than auroras on Earth!

12. Explorable Jupiter (SLIDESHOW MODE ONLY)

13. The Io Plasma Torus Some of the heavier ions originate from Jupiter’s moon Io. flux tube plasma torus Jupiter’s magnetic field sweeps past Io, creating a donut-shaped plasma torus (donut-shaped ring of ions). Electrical current (over 1 million amps!) flows along the flux tube, creating bright spots in the aurora.

14. 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, so hydrogen atmosphere exists above cloud layers. Only very thin atmosphere above cloud layers. Transition to liquid hydrogen zone about 1000 km below clouds.

15. Jupiter’s Atmosphere (2): Clouds Three layers of clouds: 1. Ammonia (NH3) crystals 2. Ammonia hydrosulfide (NH4SH) 3. Water crystals

16. Planetary Atmospheres (SLIDESHOW MODE ONLY)

17. The Cloud Belts on Jupiter Dark belts and bright zones. Zones are higher and cooler than belts since they are 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 is the The Great Red Spot. It has been visible for over 330 years. Formed by rising gas carrying heat from below the clouds, creating a vast, rotating storm. ~ 2 DEarth

20. The Great Red Spot (2) Structure of Great Red Spot may be determined by circulation patterns in the liquid interior

21. Jupiter’s Ring Not only Saturn, but all four gas giants have rings. Galileo spacecraft image of Jupiter’s ring, illuminated from behind Jupiter’s ring: dark and reddish; only discovered by Voyager 1 spacecraft. 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.

22. Roche Limit (SLIDESHOW MODE ONLY)

23. Comet Impact on Jupiter video clip 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. Visual: Impacts seen for many days as dark spots Impacts released energies equivalent to a few megatons of TNT (Hiroshima bomb was 0.15 megaton)!

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

25. Jupiter’s Family of Moons Over five dozen moons known now and new ones are still being discovered! Four largest moons discovered by Galileo in 1610 are called the Galilean moons Io Europa Ganymede Callisto Each moon has interesting and diverse individual geologies.

26. Callisto: The Ancient Face Tidally locked to Jupiter, like all of Jupiter’s moons. Density is 1.8 g/cm3 Composition is mixture of ice and rocks Dark surface, heavily pocked with craters. No metallic core because it never differentiated to form core and mantle. No magnetic field. Layer of liquid water, about 10 km thick, about 100 km below surface, probably heated by radioactive decay.

27. Ganymede: A Hidden Past Largest of the all moons in the solar system. • Density is 1.9 g/cm3 • Rocky core • Ice-rich mantle • Crust of ice 1/3 of surface old, dark, cratered. 2/3 is bright, young, grooved terrain. Bright terrain probably formed through flooding when surface broke

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

29. Europa: A Hidden Ocean Density is 3,0 g/cm3 Composition is mostly rock and metal with icy surface. Close to Jupiter so should be hit by many meteoroid impacts, but few craters visible. Why? It has an active surface, so impact craters rapidly erased.

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

31. The Interior of Europa Europa is too small to retain its internal heat. Heating mostly from tidal interaction with Jupiter. Core not molten so No magnetic field. Liquid water ocean 15 km below the icy surface.

32. Io: Bursting Energy Most active of all Galilean moons, with no impact craters visible. Over 100 active volcanoes! Geologic activity powered by tidal interactions (heating) with Jupiter. Density is 3.6 g/cm3, sointerior is mostly rock.

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

34. The History of the Galilean Moons • Minor moons are probably captured asteroids • Galilean moons probably formed together with Jupiter. • Moon densities decreasing outward – moons probably formed in a “mini solar nebular disk around Jupiter, similar to how the planets formed around the sun. Galilean moons are probably a second generation of moons (earlier moons spiraled into Jupiter. Io, Europa, and Ganymede are in orbital resonance with 1:2:4 ratio of periods orbit animation

35. Saturn Mass is 1/3 of mass of Jupiter Radius is 16 % smaller than Jupiter Density: 0.69 g/cm3So low it would float in water! • Rotates about as fast as Jupiter, in 10 hr 40 min, but is twice as oblate since it has no large core of heavy elements. • Mostly hydrogen and helium with liquid hydrogen core. • Saturn radiates 1.8 times the energy received from the sun. • Probably heated by liquid helium droplets falling towards center, similar to how sun heats while it contracts.

36. Saturn’s Magnetosphere Saturn’s magnetic field: • driven by dynamo effect • is 20 times weaker than Jupiter’s • has weaker radiation belts • not inclined (tilted) to rotation axis • Auroras are centered around poles of rotation

37. Saturn’s Atmosphere Has zone-belt structure, formed through the same processes as on Jupiter, but not as distinct and colder than on Jupiter since Saturn is twice as far from the Sun.

38. Saturn’s Atmosphere (2) Three-layered cloud structure, just like on Jupiter Main difference to Jupiter is fewer wind zones, but much stronger winds than on Jupiter. Winds up to 1100 mph near the equator!

39. Saturn’s Rings A Ring Ring consists of 3 main segments: A, B, and C ring separated by empty regions called divisions. B Ring C Ring Cassini Division Rings did not form with Saturn because ice material would have been heated at the time of formation. Rings must be replenished by fragments of passing comets & meteoroids.

40. Composition of Saturn’s Rings Rings are composed of ice particles moving at large but equal speeds around Saturn, so the astronaut shown here could “swim” through the ring.

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

42. Divisions and Resonances • Some moons act as “shepherds” that “herd” material into rings with gravitational pull. • Moons can also create divisions (gaps) when the orbital period of a moon is a small ratio of the orbital period of material in the disk (for example “2:3 resonance”).

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

44. Titan’s Atmosphere Because of the thick, hazy atmosphere, surface features are only visible in infrared images. Many of the organic compounds in Titan’s atmosphere may have been precursors of life on Earth. Surface pressure is 50% greater than air pressure on Earth Surface temperature is -290 oF Methane and ethane are liquid! Methane is gradually converted to ethane in the atmosphere Methane must be constantly replenished, probably through breakdown of ammonia (NH3).

45. Saturn’s Smaller Moons Saturn’s smaller moons, formed of rock and ice, are heavily cratered and appear geologically dead. Tethys Heavily cratered and marked by 3 km deep, 1500 km long crack. Iapetus Leading (upper right) side darker than rest of surface because of dark deposits. Enceladus Possibly active with regions of fewer craters, containing parallel grooves, possibly filled with frozen water.

46. Saturn’s Smaller Moons (2) Hyperion is too small to pull itself into spherical shape. video clip All other known moons are large enough to attain a spherical shape.

47. 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.

48. Coorbital Moons (SLIDESHOW MODE ONLY)

49. New Terms oblateness liquid metallic hydrogen decameter radiation decimeter radiation current sheet Io plasma torus Io flux tube critical point belt zone forward scattering Roche limit gossamer rings grooved terrain tidal heating shepherd satellite spoke

50. Discussion Questions 1. Some astronomers argue that Jupiter and Saturn are unusual, while other astronomers argue that all solar systems should contain one or two such giant planets. What do you think? Support your argument with evidence. 2. Why don’t the terrestrial planets have rings?