EART 160: Planetary Science

EART 160: Planetary Science

MESSENGER Flyby of MercuryThis 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

Gravitational potential energy R

GM 2

~

R

M,Thermal energy

HM

MkTkTN

~~

Equating these two and using M~R3 we get:

2~

RG

kTcrit

Does this make sense?

M=mass; =density; R=radius;k=Boltzmann’s constant; T=temperature (K)N=no. of atoms; =atomic weight; MH=mass of H atom

Proplyds in the Orion Nebula

HST Images Courtesy NASA/ESA/STSci

Beta Pictoris – 50 ly

HH-30 in Taurus

Bipolar Outflow

Disks radiate in the infraredAll very young; few My

Minimum Mass Solar Nebula

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

Density drops off with distance.

COINCIDENCE?!?!?!

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

Vertical Motions canceled outDisk orientation controlled by angular momentumDisk’s gravity also draws material toward midplane

Dust grains also accrete onto chondrules: solidified molten fragments

Forms PlanetesimalsR < few km

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)

• 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

Condensation in the Nebula

Polar jets

Stellar magnetic field (sweeps innermost disk clear, reduces stellar spin rate)

Disk cools by radiation

Dust grains Infallingmaterial

Nebula disk(dust/gas)

Hot, high

Cold, low

Metals and Rocks Ices1600 K 180 K

The Frost Line

Gas giants Ice giants Terrestrial planets

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?

Final Compositions

•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).

Io

Ganymede

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

Asphaug et al., 2001

Proto-Earth

Mars-sized Planetesimal

Canup and Asphaug, 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– 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”

Cheese it!Bwa ha ha! Um, guys? !

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!

Thommes et al., 1999

•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!

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?

Image Courtesy ESA/ Alfred Vidal-Madjar / NASA

HD209458b

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 Armitagehttp://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.