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Astronomy 340 Fall 2007. 13 November 2007 Class # 21. Review/Announcements. HW #4 due now HW #5 handed out on Thursday, due Nov 29 Last Time Moons of Jupiter Io Europa. Titan. Discovered in 1655 by Huygens 2 nd largest moon R = 2575 km Density = 1.9 gm cm -3 Orbital properties

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astronomy 340 fall 2007

Astronomy 340Fall 2007

13 November 2007

Class #21

review announcements
Review/Announcements
  • HW #4 due now
  • HW #5 handed out on Thursday, due Nov 29
  • Last Time
    • Moons of Jupiter
      • Io
      • Europa
titan
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?
titan s atmosphere
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
titan s clouds griffith et al 2000
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
titan s clouds samuelson et al 1983
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
titan s surface2
Titan’s Surface

Note variation in color  variation in surface features

Bright spot is of particular interest  CO2 ice?

surface liquid on titan
Surface Liquid on Titan?

Hydrocarbon lake?

Old river beds?

slide20

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

structure of titan
Structure of Titan
  • Surface landing site was firm  possible liquid below
  • Evidence of some liquid on surface
  • Surface pressure is sufficient (1.5 bar)
methane on titan
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
slide23

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
planetary rings
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
cassini huygens science rings
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

basic properties
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
ring comparison
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
rings uranus neptune
Rings: Uranus & Neptune

Uranus’ rings

Neptune rings

particle size distribution
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
measuring composition
Measuring Composition

Poulet et al 2003 Astronomy & Astrophysics 412 305

ring composition
Ring Composition
  • Water ice plus impurities
  • Color variation = variation in abundance of impurities
  • What could they be?
ring composition1
Ring Composition

Red slope arises from complex

carbon compounds

Variation in grain size included

in model  90% water ice overall

comparative spectroscopy
Comparative Spectroscopy

olivine

Ring particle

ring dynamics
Ring Dynamics

Inner particles overtake outer particles  gravitational interaction

ring dynamics1
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
ring dynamics2
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
saturn s rings
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
structure in the rings
Structure in the Rings

Let’s look at some pictures and see what there is to see….

structure in the rings1
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
moons and rings
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
moons and rings1
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