PHYS178 Other Worlds

1. PHYS178Other Worlds A/Prof. Orsola De Marco 9850 4241 orsola@science.mq.edu.au http:/web.science.mq.edu.au/~orsola/teaching/PHYS178/

2. Course Outline • Exploration of the outer solar system. • Jupiter and Saturn. • Atmospheres • Inner composition • Magnetic fields • The rings

3. Jupiter and Saturn

6. The compositions of Jupiter and Saturn • They are mostly made of hydrogen (H) and helium (He). • Hydrogen’s lack of spectral lines – hard to detect H spectroscopically. • Low densities of J. and S. indirect evidence that they are not made of rock. • Presence of CH4 and NH3 indirect evidence for presence of abundant H. • UV spectra have H lines – 1960s confirmation of H composition. • Helium was confirmed in 70s and 80s when spacecrafts measured spectra in detail detecting signature of collisions between H and He. • J. has “cosmic” (also called solar) abundances: 86% H, 14% He • S. has much less helium: 96% H, 3% He. • S. is less massive than J. – it cooled more rapidly. Helium on S. froze and • precipitated to the centre, where it cannot be detected.

9. Missions to the outer planets • Pioneer 10: launch 1972, first through asteroid belt; at Jupiter 1973; lost contact early 2002. • Pioneer 11: launch 1973, Jupiter 1974, Saturn 1979, lost contact 1995. • The Pioneer anomaly … • Voyager 1: launch 1977, Jupiter 1978, Saturn 1980, still going. • Voyager 2: launch 1977, Jupiter 1989, Saturn 1981, Uranus 1986, Neptune 1989, • interstellar mission still active. • Galileo: launch 1989, arrived at Jupiter 1995, deployed probe, orbits Jupiter • Cassini: launch 1997, Jupiter 2000, Saturn 2004, deployed probe on Titan. • New Horizons: launch 2006, will arrive at Pluto 2015!

10. Launched in 1977 Arrived at Jupiter 1979 Arrived at Saturn 1980/81 Arrived at Uranus and Neptune in 1986/89 Still operational – interstellar missions The Voyagers

13. Gas giants spin with different periods at different latitudes

15. The Great Red Spot, the white ovalsand the brown ovals.

17. Why is the GRS long-lived? Zonal winds up to 500 km/s circulate at zone/belt boundaries

19. The three white ovals merged in2000 to become a new storm.

21. Saturn’s rare storms • Rarely, spots are noticed on S. • They are thought to be equivalent to thunderstorms – rising gas which solidifies into crystals (ammonia in the case of S.). • Lightning also detected by Cassini.

22. The atmospheres of Jupiter and Saturn • Heavily influenced by their internal heat. • E-W and W-E “zonal winds” between 500 km/s (Jupiter) and 1800 km/s (Saturn). Below Jupiter’s surface they are faster (650 km/s), showing that the source of the energy is the planet’s heat. • Clouds are made of ammonia (highest), ammonium hydrosulfide (medium) and water crystals (lowest). They are more compressed on Jupiter than on Saturn • (higher gravity). • Clouds on Saturn are deeper and details harder to seehence the overall banding of S. is duller than for J. • The brown clouds are the lowers, the white the medium and the red the highest, but why the colours is not known.

24. The zones are cool high clouds,the belts are warm loser strata We are not sure whether belts or zones are rising

25. Galileo’s probe did not find the expected water • Layer – possibly it dropped in a dry area, equivalentto a desert. • Jupiter has the same composition as the Sun, but has more heavy elements – accumulated through impacts. • Ar, Xe, Kr are also too abundant: clues on origin?

26. Jupiter and Saturn are oblate because theyrotate. • The degree of oblateness depends on the • distribution of mass. • Models of the mass distribution on J. and S. indicate that they have 2.6% and 10% of their mass locked in a small rocky core. For Jupiter this is 11,000 km in diameter, 8 times Earth’s mass with pressures of 70 million bars. • Saturn’s larger core can be inferred from itshigher degree of oblateness (due to lesser gravity). However it would be even more oblatewere it not for its heavier core. • Both planets have a layer of liquid ices whichhave precipitated after collisions. They arelighter and float on the rocky core. • Above the ices is a thick layer of helium and • liquid metallic hydrogen – high pressure! • It is likely that the inner rocky core wasformed first and onto it H and He accreted.

27. Jupiter’s and Saturn’s magnetic fields Jupiter’s magnetosphere: 16 times the area of the Moon • Generated by the spinning liquid metallic hydrogen. 20,000 times Earth’s! • For Saturn’s only 600 times Earth’s: less liquid metallic hydrogen because • less mass, less pressure.

28. Jupiter’s magnetosphere: 16 times the area of the Moon

29. Radio emission from Jupiter Radio emission due to synchrotron radiation: charged particles from Io’s volcanos are accelerated in the magnetic field and emit radiation. Periodic changes reveal inner period. Saturn has a lesser emission – why?

31. Noticed by Galileo ~1610 Disappeared by 1612 Reappeared in 1613 Huygens in 1655 predicted that. This is due to the equinox – how so? 1675 Cassini noticed the gap between A and B rings. 1800s the fainter C ring was found. Saturn’s rings

32. Saturn‘s Equinox • Saturn’s Equinox is only every 15 years, why? • During the Equinox objects embedded in therings cast the longest shadow. • We can therefore spot objects that protrudefrom the flatness of the rings….

33. Saturn‘s Equinox: August 2009 Can anybody spot anything???

35. The composition of Saturn’s rings • Could the rings be a solid sheet? • 1857 Maxwell says no. • 1895 James Keeler sees the inner parts of the rings rotate faster – it is not a rigid sheet. • They reflect 80% of the sunlight that falls on them – might be ice. • Confirmed in the 70s (Gerarg Kuiper) using IR spectroscopy. • Voyager and Cassini spacecrafts found temperatures to range between -180o and • -200o C – ice would be….icy! • Radio absorption determined size of particles to be 1 cm – 5 m. Most particles about 10 cm.

36. The composition of Jupiter’s rings • Reflect only 5% of sunlight that falls on themso they are very faint. • Have only 1/100,000th of the mass of Saturn’s rings, so they are faint. • Mof rocky particles only 1 mm in diameter. • Cannot be seen from Earth. • Discovered by Voyager 1 in 1979. Ring at about 2 Jupiterradii from the centre ofJupiter.

37. The formation of planetary rings

39. Voyager 1 image of the Saturn’s rings

40. Cassini image of the Saturn’s rings’ drapes

41. Backlit Saturn’s rings (Voyager 1) Why is the Cassini division bright?

42. Compare the Cassini division reflecting light and scattering light The F ring is made of 1 mm particles – measured by light scattering

43. Colour variations depend on small amounts of trace elements. These trace elements have not yet been identified. Colour variations suggest that there is little mixing from ring to ring. Rings might have been added at different times and contained traces of different original materials. Cassini image

44. Voyager discovered the D, G and E rings. E and G likely formed from particles spewed out by Enceladus.

45. Small moons can clear gaps and make rings:Pandora and Prometheus shepherd particles into the F ring.

46. Daphnis clears the Keeler gap

48. These spokes maintain the same shape throughout the orbital periods of the rings. Do you find this strange?

49. Key Ideas • Composition and Structure: Jupiter and Saturn are both much larger than Earth. Each is composed of 71% hydrogen, 24% helium, and 5% all other elements by mass. Both planets have a higher percentage of heavy elements than does the Sun. • Jupiter probably has a rocky core several times more massive than the Earth. The core is surrounded by a layer of liquid “ices” (water, ammonia, methane, and associated compounds). On top of this is a layer of helium and liquid metallic hydrogen and an outermost layer composed primarily of ordinary hydrogen and helium. All of Jupiter’s visible features are near the top of this outermost layer.

50. Key Ideas • Saturn’s internal structure is similar to that of Jupiter, but its core makes up a larger fraction of its volume and its liquid metallic hydrogen mantle is shallower than that of Jupiter. • Jupiter and Saturn both rotate so rapidly that the planets are noticeably flattened. The rotation of both planets’ interiors is revealed by variations in the radio emissions from the planets. • Both Jupiter and Saturn emit more energy than they receive from the Sun. Presumably both planets are still cooling.