2007 Class 23 Astronomy 340 Fall 2007 Outgassing Model Tobie Lunine Sotin 2006 Measuring Composition Poulet et al 2003 Astronomy amp Astrophysics 412 305 Particle Size Distribution ID: 391466
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Slide1
20 November 2007Class #23
Astronomy 340
Fall
2007Slide2
Outgassing Model (Tobie, Lunine,
Sotin
2006)Slide3
Measuring Composition
Poulet et al 2003 Astronomy & Astrophysics 412 305Slide4
Particle Size DistributionPower law
n(r) = n
0
r
-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 Slide5
Ring Composition
Water ice plus impurities
Color variation = variation in abundance of impurities
What could they be?Slide6
Ring Composition
Red slope arises from complex
carbon compounds
Variation in grain size included
in model
90% water ice overallSlide7
Spectra of Saturn’s D ring
3 micron feature
is water ice.Slide8
Comparative Spectroscopy
olivine
Ring particleSlide9
Ring DynamicsInner particles overtake outer particles
gravitational interactionSlide10
Ring DynamicsInner particles overtake outer particles
gravitational interaction
Inner particle loses energy, moves closer to planetOuter particle gains energy, moves farther from planet
Net effect is spreading of the ring
Spreading timescale = diffusion timescaleSlide11
Ring DynamicsSpreading stops when there are no more collisionsIgnores radiation/magnetic effects that are linearly proportional to the size
Exact distribution affected by
Differential rotation
Presence of moons and resonances with those moonsSlide12
Saturn’s RingsD 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: 181000-483000Slide13
Structure in the RingsLet’s look at some pictures and see what there is to see….
Gaps
RipplesAbrupt edges to the rings
Presence of small moonsSlide14
Moons and RingsPerturb orbits of ring particlesConfine 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 PandoraSlide15
Shepherds in Uranus’ ring systemSlide16
Moons and RingsPerturb 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 MimasSlide17
Resonances
ALMOST ALL STRUCTURE IN RINGS IS PROBABLY DUE TO DYNAMICAL INTERACTIONS WITH MOONS
Orbits of the ring particles have:
Orbital frequency
Radial frequency
Vertical frequency
Pattern speed of the perturbing potential vs. orbital frequency of the particles
when they match we get co-rotation
Pattern speed
vs
radial frequency
Lindblad
resonancesSlide18Slide19
“Perturbing Potential”?Gravitational potential
Orbit about main planet
ring particles are in orbit
Potential due to moon that varies with same period as that of the moon rotating reference frame
Net effect
spiral density wave
Exists between inner and outer
Lindblad
resonances
Fluctuations in potential fluctuations in the surface density
azimuthal
variation, tightly wound, shows up looking like an old-fashioned LPSlide20
Vertical resonances – vertical structureSlide21
Moonlets?Slide22
Pan – density wavesSlide23
EnceladusSlide24
Enceladus occultationSlide25
EnceladusSlide26Slide27