EART 160: Planetary Science

1. EART 160: Planetary Science 25 February 2008

2. Homework 4 Graded • Wikipedia is not a peer-reviewed journal • I gave an incorrect value for H in problem 3, but it didn’t seem to trip anybody up. • Convection in mantle, Conduction in lithosphere • Max: 50, Min: 21, Mean 36, St. Dev. 10

3. Last Time • Coriolis Force Demo • Planetary Atmospheres • Thermal Balance • Origin / Geochemistry • Climate Change

4. Jeans Escape • This expression for Jeans Escape IS correct after all. • Escape velocity ve= (2 g R)1/2 • Recall: g = GM / R2

5. Today • Giant Planets • Atmospheres • Interiors • Magnetospheres? • Questions to Ponder: • What determines their internal structure? • How did they form and evolve? • What controls their atmospheric dynamics? • Moons: Tour of Icy Satellites

6. Giant Planets Image not to scale!

7. Basic Parameters Data from Lodders and Fegley 1998. Surface temperature Ts and radius R are measured at 1 bar level.

8. Compositions • We’ll discuss in more detail later, but briefly: • (Surface) compositions based mainly on spectroscopy • Interior composition relies on a combination of models and inferences of density structure from observations • We expect the basic starting materials to be similar to the composition of the original solar nebula • Surface atmospheres dominated by H2 or He: (Lodders and Fegley 1998)

9. Pressure • Hydrostatic approximation • Mass-density relation • These two can be combined (how?) to get the pressure at the centre of a uniform body Pc: • Jupiter Pc=7 Mbar, Saturn Pc=1.3 Mbar, U/N Pc=0.9 Mbar • This expression is only approximate (why?) (estimated true central pressures are 70 Mbar, 42 Mbar, 7 Mbar) • But it gives us a good idea of the orders of magnitude involved

10. Equation of State • If parcel of gas moves up/down fast enough that it doesn’t exchange energy with surroundings, it is adiabatic • In this case, the energy required to cause expansion comes from cooling (and possible release of latent heat); and vice versa • For an ideal, adiabatic gas we have two key relationships: Ideal Gas Law Polytropic Law Here P is pressure, r is density, R is gas constant (8.3 J mol-1 K-1), T is temperature, m is the mass of one mole of the gas, g is a constant (ratio of specific heats, ~ 3/2) • So: • Adiabatic Lapse Rate At 1 bar level on Jupiter:T=112 K, g=23 ms-2, Cp = 25 J mol-1 K-1,m=0.002 kg mol-1dT/dz = 1.4 K/km

11. Hydrogen phase diagram Hydrogen undergoes a phase change at ~100 GPa to metallic hydrogen (conductive) It is also theorized that He may be insoluble in metallic H. This has implications for Saturn. Interior temperatures are adiabats • Jupiter – interior mostly metallic hydrogen • Saturn – some metallic hydrogen • Uranus/Neptune – molecular hydrogen only

12. Compressibility & Density • As mass increases, radius also increases • But beyond a certain mass, radius decreases as mass increases. • This is because the increasing pressure compresses the deeper material enough that the overall density increases faster than the mass • The observed masses and radii are consistent with a mixture of mainly H+He (J,S) or H/He+ice (U,N) radius Constant density mass

13. From Guillot, 2004

14. Giant Planet Formation • Recall the first week of class • Initially solid bodies (rock + ice; beyond snow line) • When solid mass exceeded ~10 M, gravitational acceleration sufficient to trap an envelope of H and He • Process accelerated until nebular gas was lost • So initial accretion was rapid (few Myr) • Uranus and Neptune didn’t acquire so much gas because they were further out and accreted more slowly • Planets will have initially been hot (gravitational energy) and subsequently cooled and contracted • We can investigate how rapidly they are cooling at the present day . . .

15. After Hubbard, in New Solar System (1999) All units in W/m2 48 1.4 3.5 14 reflected incident 13.5 0.6 0.6 4.6 8.1 0.3 0.6 2.6 5.4 0.3 2.0 Neptune Jupiter Uranus Saturn Energy budget observations • Incident solar radiation much less than that at Earth • So surface temperatures are lower • We can compare the amount of solar energy absorbed with that emitted. It turns out that there is usually an excess. Why?

16. Sources of Energy • One major one is contraction – gravitational energy converts to thermal energy. Helium sinking is another. • Gravitational energy of a uniform sphere is • So the rate of energy release during contraction is e.g.Jupiter is radiating 3.5x1017 W in excess of incident solar radiation. This implies it is contracting at a rate of 0.4 km / million years • Other sources? • Tidal dissipation • Radioactive decay small compared to grav. energy

17. Uranus – What The Hell? • Why is Uranus’ heat budget so different? • Perhaps due to compositional density differences inhibiting convection at levels deeper than ~0.6Rp .May explain different abundances in HCN,CO between Uranus and Neptune atmospheres. • This story is also consistent with generation of magnetic fields in the near-surface region (see earlier slide) • Why is Uranus tilted on its side? • Nobody really knows, but a possible explanation is an oblique impact with a large planetesimal (c.f. Earth-Moon) • This impact might even help to explain the compositional gradients which (possibly) explain Uranus’ heat budget

18. Atmospheric Structure • Lower atmosphere (opaque) is dominantly heated from below and will be conductive or convective (adiabatic) • Upper atmosphere intercepts solar radiation and re-radiates it • There will be a temperature minimum where radiative cooling is most efficient; in giant planets, it occurs at ~0.1 bar • Condensation of species will occur mainly in lower atmosphere radiation Temperature (schematic) Theoretical cloud distribution mesosphere 80 K CH4 (U,N only) stratosphere 140 K NH3 ~0.1 bar tropopause clouds 230 K NH3+H2S troposphere adiabat 270 K H2O

19. Giant planet atmospheric structure • Note position and order of cloud decks

20. Bands and Zones Jupiter’s Clouds – New Horizons Different colors indicate different gases You’re seeing to different depths in the atmosphere Zonal winds affect opacity Galileo went into a boring spot – crushed before it got to anything interesting

21. Magnetic fields

22. How are they generated? • Dynamos require convection in a conductive medium • Jupiter/Saturn – metallic hydrogen (deep) • Uranus/Neptune - near-surface convecting ices (?) • Terrestrial Planets – convection in liquid iron core • Icy Satellites – salty ocean

23. Van Allen Belts • Torus of charged particles trapped in Earth’s magnetic field • Shield Earth’s surface against high-energy particles • Hazard to satellite navigation, manned space missions • Similar radiation belts observed at other planets • Jupiter’s belt poses problem for dedicated Europa mission

24. Aurora • Collisions between charged particles with the atmosphere. • Gas controls color (Red: N, Green: O) • Magnetic reconnection between solar magnetic field and Earth’s magnetosphere at poles

25. Summary • Jupiter - mainly metallic hydrogen. Rock-ice core ~10 ME. • Saturn - mix of metallic and molecular hydrogen; helium may have migrated to centre due to insolubility. Similar rock-ice core to Jupiter. Mean density lower than Jupiter because of smaller self-compression effect (pressures lower). • Uranus/Neptune – thin envelope of hydrogen gas. Pressures too low to generate metallic hydrogen. Densities (and moment of inertia data) require large rock-ice cores in the interior. • All four planets have large magnetic fields, presumably generated by convection in either metallic hydrogen (J,S) or conductive ices (U,N)

26. Moons • What are they? • Natural things that orbit about other natural things that aren’t stars • Terrestrial planets have few if any • Jovian planets have whole bunches • Even some asteroids and comets have them.

27. The Magnificent Seven 1. Ganymede 3. Callisto 4. Io 6. Europa 2. Titan 5. The Moon 7. Triton

28. Earth: The Moon Maria: Lava plainsOnly on near side Terrae: Cratered Highlands

29. Mars: Phobos and Deimos Captured Asteroids Crater: Stickney

30. Jupiter: The Inner Moons Metis Adrastea Thebe Amalthea

31. Jupiter: Io Tvashtar Plume The Volcanic Moon The lavas of violent Io, Though they may look like pico de gallo, Erupt and then rain On the sulphurous plain, Looking nothing at all like Ohio.

32. Jupiter: Europa ALL THESE WORLDS ARE YOURS EXCEPT EUROPA ATTEMPT NO LANDING THERE USE THEM TOGETHER USE THEM IN PEACE A promising place is Europa Where astrobiologists hope a Critter or three May swim in the sea Far beneath the icy dystopia

33. Jupiter: Callisto

34. Jupiter: The Outer Moons • Seven in prograde orbits • Themisto, Leda, Himalia, Lysithia, Elara • 46 in retrograde orbits • Probably Captured • Ananke, Came, Pasiphae, Sinope

35. Saturn MET DR THIP For a sickeningly huge number of awesome images, go to: http://ciclops.org Some share orbits!

36. Saturn: Mimas Now Mimas has one giant crater That’s sitting right on the equator Because it’s a hole It should be at the pole. It could maybe reorient later. Herschel That’s no moon . . . it’s a space station!

37. Saturn: Enceladus South Polar Plumes Tiger-Stripes Possible liquid ocean Tidally heated? There once was a moon called Enceladus, Whose Tiger-stripes have cast a spell at us. The south polar plume Like a vapor mushroom, Has poked its way through the ice shell at us.

38. Saturn: Tethys • Heavily cratered • Active early on • Some Resurfacing • Dark Belt • Polar caps Odysseus Calypso and Telesto on same orbit

39. Saturn: Dione and Rhea Wispy streaks Eruptions of Snow? The ice shell of distant Dione Lies over a core that is stony. Its wispy terrain In extensional strain Is as brittle as dry macaroni. Shares orbit with Helene

40. Saturn: Titan Xanadu Haze Volcano? The tholins surrounding old Titan Raise the following question. “Might an Airborne balloon Get a look at this moon And see if the dark patches brighten?”

41. Saturn: Iapetus Black on White or White on Black? 20 km highridge! Cassini RegioHuge Fracture

42. Uranus • 11 small inner moons • Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck • MAUTO • Miranda, Ariel, Umbriel, Titania, Oberon • 5 small outer moons • Caliban, Sycorax, Stephano, Prospero, Setebos

43. Uranus: Miranda Iapetus said to Miranda “You’re no place at all for a lander. Your canyons have rocks Like the teeth on some crocs Whereas I’m black and white like a panda.” Broken apart and reassembled

44. Uranus: Ariel and Umbriel Fluorescent Cheerio (north polar crater) Interconnected valleys 100s km long, 10 km deep

45. Uranus: Titania and Oberon Queen and King of the Faeries Chasms all over 6 km high mountain, craters

46. Neptune: Moons • 5 small inner moons • Proteus • Triton, the big retrograde moon • Captured KBO? • Going to collide – more on this tomorrow • Nereid (medium size moon) • 3 small retrograde outer moons

47. Neptune: Triton • Orbits retrograde • Unstable orbit • Going to impact Neptune A big KBO in disguise or A moon with a nitrogen geyser? The retrograde moon Of sea-king Neptune Triton thinks we’ll all be none the wiser. Ice Lava

48. Next time • Icy Satellites • Ring Systems of Giant Planets • Tidal Interactions • Paper Discussions • Namouni and Porco (2002) • Porco et al. (2006)