EART 160: Planetary Science

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EART 160 Planetary Science
MESSENGER Flyby of Mercury This hemisphere never
before seen!
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Last Time

• Celestial Mechanics
• Newton Proves Keplers Laws
• Conservation of Momentum, Angular Momentum,
  Energy
• Collisions, Gravitational Slingshot
• Solar System Formation
• Nebular Theory
• Jeans Collapse

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Today

• Solar System Formation
• Runaway and Oligarchic Growth
• Distribution of solar system materials
• Planetary composition, structure
• Late-stage accretion
• Formation of the Moon
• Planetary Migration
• Late Heavy Bombardment
• Extrasolar Planets Hot Jupiters

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Jeans Collapse

• A perturbation will cause the density to increase
  locally
• Runaway Process
• Increased density ? increased gravity ? more
  material gets sucked in

Gravitational potential energy
M,r
Thermal energy
R
Equating these two and using MrR3 we get
Does this make sense?
Mmass rdensity Rradius kBoltzmanns
constant Ttemperature (K) Nno. of atoms
matomic weight MHmass of H atom
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Proplyds in the Orion Nebula
Disks radiate in the infrared All very young few
My
Beta Pictoris 50 ly
Bipolar Outflow
HH-30 in Taurus
HST Images Courtesy NASA/ESA/STSci
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Minimum Mass Solar Nebula
Density drops off with distance. COINCIDENCE?!?!?
!

• We can use the present-day observed planetary
  masses and compositions to reconstruct how much
  mass was there initially

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Timeline of Planetary Growth

• 1. Nebular disk formation
• 2. Initial coagulation (10km, 104 yrs)
• 3. Runaway growth (to Moon size, 105 yrs)
• 4. Oligarchic growth, gas loss(to Mars size,
  106 yrs)
• 5. Late-stage collisions (107-8 yrs)

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Collisional Accretion (104 y)
Inelastic Collisions between dust grains
Dust grains also accrete onto chondrules
solidified molten fragments
Forms Planetesimals R lt few km
Vertical Motions canceled out Disk orientation
controlled by angular momentum Disks gravity
also draws material toward midplane
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Runaway Growth (105 y)

• Slow-moving planetesimals accrete
• Protoplanets grow to size of moon (3500 km)

Fg GMm / R2
vorbital lt vesc
vorbital gt vesc
The rich get richer! -- Bender
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Oligarchic Growth (105 y)

• Cosmic Feudal System
• Only a few dozen big guys left (oligarchs)
• And a lot of very small stuff (serfs?)
• Oligarchs sweep up everything in their feeding
  zones
• Gas drag slows large objects down, circularizes
  orbits
• Brightening sun clears away nebular gas.

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Composition

• Solar Nebula
• 98.4 gas (H, He)
• 1.1 ices (e.g. H2O, NH3, CH4)
• 0.4 rock (e.g. MgSiO4)
• 0.1 metal (mostly Fe, Ni)
• How do we know this?
• Look at the Sun!
• Absorpiton lines indicate elements
• Discovery of He

Volatile
Refractory
Image courtesy N.A.Sharp, NOAO/NSO/Kitt Peak
FTS/AURA/NSF
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Condensation in the Nebula
Metals and Rocks Ices 1600 K 180 K
The Frost Line
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Terrestrial v. Jovian

• Only refractories in inner SS
• Planets can only grow to Earth-size
• Too small to hold onto gas
• Ices also available beyond frost line
• Much more material
• Ice-rock planets up to 20 M? possible
• Big enough to accrete H, He ? can get huge, 300
  M?
• Why no giant planets farther out than Neptune?

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Final Compositions
Io
Ganymede

• Terrestrial Planets
• Iron Core (Red), Silicate Mantle (Grey)
• Mercury has v. thin mantle. Why?
• Very few volatiles, thin atmospheres?
• Jovian Planets
• Rock (Grey) and Ice (Blue Cores)
• Gas envelope (Red, Yellow)
• Jupiter and Saturn mostly H, He
• Uranus, Neptune mostly ice

Guillot, Physics Today, (2004).
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Satellites

• Satellites formed from mini-accretion disks about
  giant planets
• Explains why they all orbit the same way and in
  the same plane
• Irregular satellites (including Marss moons)
  captured later (high e, i)
• What about our own freakishly large Moon?

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Problems with this

• Why exactly four terrestrial planets?
• Numerical models cant do this.
• What is up with the Moon?
• Gas Loss Timing
• As star heats up, gas in disk is blown away
• Gas causes planets to spiral in
• Gas must stick around long enough to form giant
  planets
• Why are Uranus and Neptune so shrimpy?
• Why are extrasolar planets so close in?
• Alan Boss
• Rapid giant planet formation by disk instability
  (100s of years)
• Planets tend to spiral into Sun
• Hard to explain heavy elements abundances
• Migration

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Late-stage accretion (107-108 y)

• Oligarchic growth results in dozens of
  planetesimals
• Oligarchy is unstable!
• Perturb each other until orbits cross
• Giant Impacts
• Large basins on all planetary bodies
• Retrograde rotation of Venus
• Obliquity of Uranus
• Formation of the Earths Moon

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Jupiter The Cosmic Bully

• Its huge! Perturbs anything nearby
• Disrupted accretion at 2-3 AU
• No planet here where we expected one.
• Location of the asteroid belt

• Ejected icy planetesimals
• Gravitational slingshot effect
• Scattered in all directions ? The Oort Cloud

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The Nebular Theory Explains

• All planets orbits in a single plane.
• Suns rotation in same plane.
• Prograde orbits of all planets
• Planetary orbits nearly circular
• Angular momentum distribution
• Some meteorites contain unique inclusions
• Correlation of planetary composition with solar
  distance.

• Meteorites different from terrestrial and lunar
  rocks
• Spacing of the planets
• Giant impacts on all planetary bodies
• Prograde rotation, low obliquity of most planets
• Similar rotation periods for many planets
• Spherical distribution of comets
• Satellite systems of giant planets

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Formation of the Moon

• Co-accretion (sibling)
• ? and ? formed together from Solar Nebula
• Capture (spouse)
• ? made a close pass to ?, captured into orbit
• Fission (child)
• Fast-spinning ?, a blob tore away
• Apollo mission to determine which one is real.

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None of Them!

• ? similar to ?s mantle. Depleted in Fe,
  siderophiles, volatiles.
• Cannot form from same assemblage
• O, Si-isotopes in ? and ? rocks IDENTICAL.
• Meteorites all different
• Implies common origin of the silicates.
• Angular Momentum of ? - ? too small for fission.
  
• ?-orbit not in equatorial plane.
• Implies different trajectories

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Requirements

• Explain Angular Momentum of System
• Explain Metal depletion of Moon
• Initially different orbits
• Silicates mixed
• Earths core untouched
• ? Giant Impact!
• Parasite-host relationship?
• Genetic Engineering Experiment?
• Other bad relationship analogy?

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Giant Impact Hypothesis
Mars-sized Planetesimal
Proto-Earth
Asphaug et al., 2001
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• Oblique impact, rotation increases
• 5 hour day!
• Impactor destroyed, Mantle stripped away
• Cores merge, silicates form accretion disk
• Some silicates fall back onto planet
• Rest forms the Moon
• At 12 R?

Canup and Asphaug, 2001
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Migration

• Do planets have to stay where they formed?
• Why are Uranus and Neptune so small?
• Extrasolar gas giants have TIGHT orbits!
• Hot, hot, hot! WAY inside frost line

Cheese it!
Um, guys?
!
Bwa ha ha!
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Gas Giant Formation

• Beyond frost line, planets accrete rock AND ice
• Grow to 10-15 M?
• Accrete Gas
• Uranus and Neptune have little gas
• Failed cores
• BUT nebula too sparse that far out to even get
  cores!
• Standard formation model doesnt work!

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• Four 15 M? cores between 4 and 10 AU.
• Jupiter forms where nebula is the densest, gets
  big.
• All three other cores scatter off Jupiter, flung
  outward
• Saturn still close enough to accrete a bunch of
  gas.
• What happens to Joop?

Conservation of Angular Momentum!
Thommes et al., 1999
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Hot Jupiters

• Less than 0.05 AU from star
• Problems with forming in situ
• Not enough material
• No ice, gas at all!
• Atmosphere gets stripped away?

HD209458b
Image Courtesy ESA/ Alfred Vidal-Madjar / NASA
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Inward Migration

• Type I Dynamical Friction
• Small Planets drive spiral density waves in disk
• Outer wave imparts torque, planet loses L.
• Moves inward.
• Type II Coevolution
• Growing planet clears a gap in the disk
• Relay station for L-transport
• Moves L outward, planet and gap move inward

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Movie courtesy Phil Armitage http//jilawww.colora
do.edu/pja/planet_migration.html
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Consequences

• Hot Jupiters probably were Regular Jupiters that
  got Type II Migration
• Giant moves in
• What does Conservation of Angular Momentum say?
• Terrestrial Planets move out. Wayyyy out!
• Why did we escape this fate?
• Atmosphere stripped off by solar wind?
• Chthonian planet?

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Next Time

• Paper Discussions
• Asphaug et al. (2006)
• Thommes et al. (1999)
• Meteorites
• Asteroids
• The Late Heavy Bombardment
• You should now have everything you need to
  complete the homework. Really. I mean it this
  time.

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