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EART 160: Planetary Science
MESSENGER Flyby of Mercury
This hemisphere never before seen!
Last Time
• Celestial Mechanics
– Newton Proves Kepler’s Laws
– Conservation of Momentum, Angular
Momentum, Energy
– Collisions, Gravitational Slingshot
• Solar System Formation
– Nebular Theory
– Jeans Collapse
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”
Jeans Collapse
• A perturbation will cause the density to increase locally
• Runaway Process
– Increased density  increased gravity  more material gets
sucked in
GM 2
~
R
M
Thermal energy ~ kTN ~ kT
MH
Gravitational
potential energy
M,r
R
Equating these two
and using M~rR3 we get:
M=mass; r=density; R=radius;
k=Boltzmann’s constant; T=temperature (K)
N=no. of atoms; =atomic weight; MH=mass of H atom
r crit
kT
~
GR 2
Does this
make sense?
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
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
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)
Collisional Accretion (104 y)
Inelastic Collisions between
dust grains
Dust grains also accrete onto
chondrules: solidified molten fragments
Forms Planetesimals
R < few km
Vertical Motions canceled out
Disk orientation controlled by angular momentum
Runaway Growth (105 y)
• Slow-moving planetesimals accrete
• Protoplanets grow to size of moon (3500 km)
Fg = GMm / R2
vorbital < vesc
vorbital > vesc
“The rich get richer!”
-- Bender
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.
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)
Volatile
Refractory
• How do we know this?
– Look at the Sun!
– Absorpiton lines indicate
elements
– Discovery of He
Image courtesy N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Condensation in the Nebula
Disk cools by radiation
Polar jets
Dust grains
Cold,
Hot,
low r
high r
Infalling
material Stellar magnetic field
(sweeps innermost disk
clear, reduces stellar spin
rate)
Terrestrial planets
Metals and Rocks
1600 K
Nebula disk
(dust/gas)
Gas giants Ice giants
Ices
180 K
The Frost Line
Terrestrial v. Jovian
• Only refractories in inner SS
– Planets can only grow to Earthsize
– 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?
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).
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 Mars’s
moons) captured later (high e, i)
• What about our own freakishly large
Moon?
Problems with this
• Why exactly four terrestrial planets?
– Numerical models can’t 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
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 Earth’s Moon
Jupiter: The Cosmic Bully
• It’s 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
The Nebular Theory Explains:
• All planets’ orbits in a single
plane.
• Sun’s 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
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.
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
Requirements
• Explain Angular Momentum of System
• Explain Metal depletion of Moon
• Initially different orbits
• Silicates mixed
• Earth’s core untouched
•  Giant Impact!
– Parasite-host relationship?
– Genetic Engineering Experiment?
– Other bad relationship analogy?
Giant Impact Hypothesis
Mars-sized
Planetesimal
Proto-Earth
Asphaug et al., 2001
• 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
Canup and Asphaug, 2001
– At 12 R
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”
Bwa ha ha!
Um,
guys?
!
Cheese it!
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 doesn’t work!
•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
Hot Jupiters
• Less than 0.05 AU from
star
• Problems with forming in
situ
HD209458b
– Not enough material
– No ice, gas at all!
– Atmosphere gets stripped
away?
Image Courtesy ESA/ Alfred Vidal-Madjar / NASA
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
Movie courtesy Phil Armitage
http://jilawww.colorado.edu/~pja/planet_migration.html
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?
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.