Astronomy 340 Fall 2007

1. Astronomy 340Fall 2007 13 November 2007 Class #21

2. Review/Announcements • HW #4 due now • HW #5 handed out on Thursday, due Nov 29 • Last Time • Moons of Jupiter • Io • Europa

3. Titan • Discovered in 1655 by Huygens • 2nd largest moon • R = 2575 km • Density = 1.9 gm cm-3 • Orbital properties • 4:3 resonance with Hyperion • Somewhat eccentric  effect of tidal forces?

4. Titan’s Atmosphere • 1st detected 1944 • Composition (pre-Cassini) • CH4 plus some C2H6, CH3D, C2H2, HCN • Voyager detected N2 98% N2 • Pressure comparable to Earth’s • CH4 cycle • Liberated from ethane/methane oceans • Some migrates upwards • Dissociation & photolysis  H2 + hydrocarbons • H2 may escape • CxHy haze/clouds remain • Possible methane/ethane snow/rain

5. Titan’s Atmosphere

6. Titan’s Clouds (Griffith et al. 2000) • Techniques • Variations in brightness = variation in albedo = presence of clouds • Spectral windows = different depths • Stratosphere reflects at 2.17-2.4 μm • Deep into atmosphere, but not surface 2.12-2.17 μm • Surface 2.0-2.11 μm • Results • Variations in brightness in 2.12-2.17 μm window • Low covering factor (<5%) • Altitude ~ 25-30 km

7. Titan’s Clouds (Samuelson et al 1983) • Effects of surface temperature variation • Tsurf ~90-95K, lows ~70 K, highs ~180 K • Atmospheric processes • Greenhouse effect • Absorption via hydrocarbons • Photolysis of CH4 reservoir

8. Titan’s Surface

9. Titan’s Surface – Radar Mapping

10. Titan’s Surface

11. Titan’s Surface Note variation in color  variation in surface features Bright spot is of particular interest  CO2 ice?

12. Titan’s Surface

13. Cassini Trajectory

19. Surface Liquid on Titan? Hydrocarbon lake? Old river beds?

20. The surface of Saturn's largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion1. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system2. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package3, 4. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing. Zarnecki et al. 2005 Nature 438 792

21. Structure of Titan • Surface landing site was firm  possible liquid below • Evidence of some liquid on surface • Surface pressure is sufficient (1.5 bar)

22. Methane on Titan • D/H ratio • 80-90 ppm higher than solar, less than ocean water, comets, chondrites (300 ppm) • Methane likely not from comets • Lifetime • Photolyzed in 10 million years • Requires replenishing • Surface ocean? • Interior outgassing? Implies D/H ratio reflects initial composition

23. CO, N2 CH4, NH3 (Lewis & Prinn 1980) • CH4 present in the feeding zone at 10 AU • Satellites form via accretion, methane locked in interior • Requires periodic (constant?) outgassing

24. Outgassing Model (Tobie, Lunine, Sotin 2006)

25. Planetary Rings • A Little History • 1610  Galileo discovered rings; they disappeared in 1612 • 1659  Christian Huygens discovered disk-like nature • 1977  Uranus’ rings discovered during occultation • 1979  Voyager 1 discovered Jupiter’s rings • 1980s  Neptune’s arc-like rings discovered via occulation

28. Cassini-Huygens Science - Rings Study configuration of the rings and dynamic processes responsible for ring structure Map the composition and size distribution of ring material Investigate the interrelation of Saturn’s rings and moons, including imbedded moons Determine the distribution of dust and meteoroid distribution in the vicinity of the rings Study the interactions between the rings and Saturn’s magnetosphere, ionosphere, and atmosphere

29. Basic Properties • Solid disk vs particles • Too big to rotate as solid body • Maxwell  rings are particles • 100m thick vs 105 km wide (Saturn) • Just one ring? No • All reside within Roche limit of planet

30. Ring Comparison • Jupiter • Main ring – bigger particles, more scattering • Halo – interior component, orbits scattered by interaction with Jupiter’s magnetic field • Gossamer – very low density, farther out  more inclined orbit so its fatter • Uranus • 6 identified rings • Thickness ~10 km, width ~250000 km • Very close to being in circular orbit

33. Rings: Uranus & Neptune Uranus’ rings Neptune rings

34. Particle Size Distribution • Power law  n(r) = n0r-3  number of 10 m particles is 10-9 times less than # of 1 cm particles • Total mass distribution is uniform across all bins • Collisions • Net loss of energy  flatten ring • Fracture particles  power law distribution of particle size • But, ring should actually be thinner and radially distribution should gradually taper off, not have sharp edges

35. Measuring Composition Poulet et al 2003 Astronomy & Astrophysics 412 305

36. Ring Composition • Water ice plus impurities • Color variation = variation in abundance of impurities • What could they be?

37. Ring Composition Red slope arises from complex carbon compounds Variation in grain size included in model  90% water ice overall

38. Comparative Spectroscopy olivine Ring particle

39. Ring Dynamics Inner particles overtake outer particles  gravitational interaction

40. Ring Dynamics • Inner particles overtake outer particles  gravitational interaction • Inner particle loses energy, moves closer to planet • Outer particle gains energy, moves farther from planet • Net effect is spreading of the ring • Spreading timescale = diffusion timescale

41. Ring Dynamics • Spreading stops when there are no more collisions • Ignores radiation/magnetic effects that are linearly proportional to the size • Exact distribution affected by • Differential rotation • Presence of moons and resonances with those moons

42. Saturn’s Rings • D ring: 66900-74510 • C ring: 74568-92000 • Titan ringlet 77871 • Maxwell Gap: 87491 • B ring: 92000-117580 • Cassini division • A ring: 122170-136775 • F ring: 140180 (center) • G ring: 170000-175000 • E ring: 181000-483000

43. Structure in the Rings Let’s look at some pictures and see what there is to see….

44. Structure in the Rings • Let’s look at some pictures and see what there is to see…. • Gaps • Ripples • Abrupt edges to the rings • Presence of small moons

45. Moons and Rings • Perturb orbits of ring particles • Confine Uranus’ rings, create arcs around Neptune • Shepherding – two moons on either side of ring • Outer one has lower velocity  slows ring particle, particle loses energy • Inner one has higher velocity  accelerates ring particle, particle gains energy • Saturn’s F ring is confined between Prometheus and Pandora

46. Shepherds in Uranus’ ring system

47. Moons and Rings • Perturb orbits of ring particles • Confine Uranus’ rings, create arcs around Neptune • Shepherding – two moons on either side of ring • Outer one has lower velocity  slows ring particle, particle loses energy • Inner one has higher velocity  accelerates ring particle, particle gains energy • Saturn’s F ring is confined between Prometheus and Pandora • Resonances • Similar to Kirkwood Gap in asteroid belt  2:1 resonance with Mimas