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PTYS 554
Evolution of Planetary Surfaces
Forming Planetary Crusts IForming Planetary Crusts I
PYTS 554 – Forming Planetary Crusts I 2
Forming Planetary Crusts I Tour of planetary surfaces Terrestrial planet formation Differentiation and timing constraints
Forming Planetary Crusts II Giant impacts and the end of accretion Magma oceans and primary crust formation KREEP Late veneers and terrestrial planet water
Forming Planetary Crusts III One-plate planets vs. plate tectonics Recycling crust Plate tectonic changes over the Hadean and Archean
PYTS 554 – Forming Planetary Crusts I 3
Overall solar system structure Inner rocky planets
Mercury 0.39 AU Venus 0.72 AU Earth 1.00 AU Mars 1.52 AU
Asteroid belt (2-4 AU) Hundreds of members Several groups Sizes from dust to ~950 km
(Ceres)
Giant planets Jupiter 5.2 AU Saturn 9.6AU Uranus 19.2 AU Neptune 30.1 AU
Kuiper Belt (30-50 AU) Contains Pluto Several groups Sizes from dust to >2400 km
(Eris)
Oort cloud Long period comet reservoir Affected by passing stars
Earth’s Moon
A training ground for planetary science
Uniform highland crustOverlapping impact basinsVolcanic floods
Subsequent cratering
Mercury
MercuryCratered surface - like the MoonSupersized iron core – unlike the MoonSmooth plains interspaced with cratersLobate scarps indicating global compressionVery sulfur-rich crustPolar ice deposits
VenusVolcanic PlanetRunaway greenhouse leads to high surface Temps.Topography similar to the Earth but no plate tectonicsResurfacing in the past billion years – probably more continuous than catastrophic
MarsCrustal dichotomyCratered terrainsLarge Volcanic ProvincesWet Past
Valley NetworksLake DepositsLarge Flood channels
Modern dry/icy climatePolar ice sheets & groundice
MarsCrustal dichotomyCratered terrainsLarge Volcanic ProvincesWet Past
Valley NetworksLake DepositsLarge Flood channels
Modern dry/icy climatePolar ice sheets & groundiceModern Mars is still active
EarthA complicated place!Plate Tectonics
Two crustal types separated by composition and elevationAbundant ongoing volcanism
Abundant liquid water leads to high erosion ratesEolian and glacial process also commonLife produces an oxygen-rich environment – high rates of rock breakdown (and us)
PYTS 554 – Forming Planetary Crusts I 11
Inner planets Differentiated into iron cores and silicate mantles All very different – fewer bodies than free variables Surface processes have commonalities
Mercury Venus Earth Moon Mars Asteroids
Craters X X X X X X
Volcanism X X X X X X
Tectonics X X X X
Fluvial X X
Aeolian X X X
Asteroids
Vesta576 kmDawn 2012
• Huge size diversity – 3 orders of magnitude between Itokawa and Vesta
Generalize with two broad classes ‘Small’ asteroids - Irregular in shape Collisional fragments or rubble piles Show evidence for impacts and impact gardening, space weathering
‘Large’ asteroids - Quasi-spherical in shape Internally differentiated mini-planets All the above processes + Volcanism
Galilean SatellitesActivity controlled by tidal effectsLaplace resonance provides a way to keep tidal heating activeIo:Strong heating Continuous Volcanism – low silica basalts that mobilize surface sulfur depositsScattered mountains up to 17km high from crustal compression
Galilean SatellitesActivity controlled by tidal effectsLaplace resonance provides a way to keep tidal heating activeEuropa:Silicate body with thick liquid water oceanThin cover of ice flexed tidally Melt-through occurs in placesVery young surface
Galilean SatellitesActivity controlled by tidal effectsLaplace resonance provides a way to keep tidal heating activeGanymede:Fully differentiated with iron core, silicate mantle and thick water oceanHigh-pressure ice phasesSurface volatile mobilityTwo surface units – younger was created ~2 Gyr ago
Galilean SatellitesActivity controlled by tidal effectsLaplace resonance provides a way to keep tidal heating activeCallisto:4Gyr old cratered surfaceViscous relaxation of cratersSublimation of exposed ice
Outer Solar System MoonsOld tectonics on mid-sized bodied
Tidally driven water jets on EnceladusViscous ices and seasonal frosts on Triton
High-pressure ice phases in many of these bodies
Titan‘Earth-like’ processes with exotic materials
RiversLakesCratersDunesWeather
But also a subsurface ocean with high pressure ice phases below that
PYTS 554 – Forming Planetary Crusts I 21
Small extra-solar planets Several ‘super-Earths’ known
First Earth-sized planets being discovered now
Kepler has produced thousands of planetary candidates – most of which are probably real.
Planets are common! ~ 1 in 6 stars have them at least in
close-in orbitsFresen et al. ApJ 2013
PYTS 554 – Forming Planetary Crusts I 22
Giant molecular clouds exist throughout our galaxy
Tenuous and cold Densities of a few 1000 molecules cm-3
Temperatures of 10-30 K
Composition dominated by primordial H and He Molecules like NH3, HCN, CS, H2CO
Other elements provided by previous generations of stars
Supported by gas pressure and magnetic field lines
Confined by surrounding high T (104 K), low pressure gas.
PYTS 554 – Forming Planetary Crusts I 23
Clouds collapse to disks Usually gas pressure balances self-gravitation The minimum mass of such a cloud is called the Jean’s
mass (MJ)
Clouds on the brink of stability can be destabilized Passage through a spiral arm Nearby supernova or expanding H II region Strong stellar winds
Clouds collapse from the inside out Free fall timescale given by:
Cloud centers are denser so clouds cave-in from the inside out Typically several 100 Kyr
Temperature goes up during the collapse – Kelvin-Helmholtz contraction
Energy radiated away though transparent cloud Increasing pressure turns cloud opaque – temperature rises Center of cloud comes into hydrostatic equilibrium Deuterium burning starts at 106K, hydrogen burning starts at 107K
12
3
G
kTM J
G
t ff 32
3
PYTS 554 – Forming Planetary Crusts I 24
Our disk… Minimum mass of a few % Msun
Only 0.5% of the disk mass at 1AU were solids i.e. terrestrial planet formation is mostly just a
side-show
Solar composition approximated by CI Chondrites
Earth is depleted in volatile elements Depletion depends on solar distance
DePaolo, UC Berkeley
Taylor and McLennan, 2009
PYTS 554 – Forming Planetary Crusts I 25
PYTS 554 – Forming Planetary Crusts I 26
Gas rotates slower than a solid body due to pressure support
Dust settles to the disk mid-plane In ~10 Kyr (with no turbulence) Mid-plane starts to orbit at Keplerian speed Mid-plane gas dragged along with the dust Thickness: Turbulence (from shear with gas
layers) vs. gravitational settling
Grain-grain stickiness Enhanced by charge exchange Grains coupled to gas so low relative velocities
Chambers, 2004
PYTS 554 – Forming Planetary Crusts I 27
The meter-scale barrier Meter scale objects are decoupled from the gas
Feel headwind of ~50m/s
Spiral inward 1AU in ~100-500yrs ‘sandblasting’ stops their growth
Options?
PYTS 554 – Forming Planetary Crusts I 28
Calcium-Aluminum-Rich inclusions (CAIs)
mm-cm in size Found in Carbonaceous
chondrite meteorites Oldest solar system solids 4568.2 ± 0.6 Myr from Pb/Pb
dating
This is the T0 that everything is measured from
Chondrules Formed by a high-T event
followed by fast cooling Formed ~2-3 Myr after CAIs Thought to be building blocks
of planetary material
PYTS 554 – Forming Planetary Crusts I 29
Important in the early solar system Produced in supernovae or stars >20 MO
Decays to Mg26
Very energetic reaction!
Half life of 0.7 Myr Only affects the fastest forming bodies Initial abundances Al26/Al27 = 6x10-5 (CAIs)
Heat affects larger bodies more Heat produced α R3
Heat lost to space α R2
Easier to melt larger bodies
Role of Al26
McCord and Sotin, 2005 Al26 dominates heating of asteroids in the
early solar system Al26/Al27 difference between CAI’s and
chondrites 6x10-5 –> 8x10-6
Formation time ~ 2.1 Myr Heat from 26Al ~ 100 times other sources
PYTS 554 – Forming Planetary Crusts I 30
Compositional zoning in the asteroid belt Objects with a < 2.7 AU – anhydrous silicates
Dominated by S-type asteroids Differentiation, metallic cores – implies melting of silicates
Objects with 2.7 AU < a < 3.4 AU – hydrated silicates
Dominated by C-type asteroids Clay minerals – implies melting of ice
Objects with a > 3.4 AU – no hydration (ice never melted)
Primitive (unprocessed) objects
Objects too small for magma oceans Radioactive heating Induction and shock heating
Grimm and McSween, 1993
PYTS 554 – Forming Planetary Crusts I 31
Hafnium-Tungsten 182Hf decays to 182W with a half-life of 8.9 Myr
They’re both refractory and get incorporated with chondritic ratios into forming bodies
Hafnium is lithophile Tungsten is siderophile
So, after differentiation… Hf/W in the core is ~0 Hf/W in the mantle is very high
Early accretion allows for a lot of W production in the mantle
Lowers Hf/W
Late accretion allows for less
Knowing Hf/W allows us to date differentiation Complication of knowing the partition coefficient of W
Pressure, T, fO2 dependant…
PYTS 554 – Forming Planetary Crusts I 32
Parent bodies of iron meteorites… Differentiated within 1 Myr of T0… before chondrules
Willamete meteorite - iron Stony iron, Pallasite Stony
PYTS 554 – Forming Planetary Crusts I 33
Back to the dynamical models Somehow 1km planetesimals appear…
Gravitational focusing increases the collisional cross-section
If v >> vesp then effect is small Doubles cross-section when v~vesp
Dynamical friction Near-misses slow large bodies and speed up
the smaller ones Gas drag also reduces relative velocities
Largest planetesimals enter runaway growth phase
Become planetary embryos Stops when Membryo > 100 Mplanetesimal
Embryos start to perturb each other
22 1 v
vr e
Chambers, 2004
PYTS 554 – Forming Planetary Crusts I 34
Oligarchic growth Neighboring embryos grow at comparable rates Bigger embryos increase relative velocities of the planetesimals
Slows their growth rates Allows neighboring embryos to catch up
Embryos still have small eccentricities Distinct feeding zones Regularly spaced ~0.01 AU apart
Lasts 0.1-1Myr Planetesimals have been accreted onto embryos Dynamical friction ends Embryos start to stray out of their feeding zones and interact with each other
Stage ends with several dozen moon-mars sized embryos
Gas in disk dissipates about now
PYTS 554 – Forming Planetary Crusts I 35
Final phase High relative velocities
Low gravitational focusing An inefficient process Takes ~ 100Myr
Gas has disappeared now Jupiter and Saturn are fully formed Heavily affects outcome in the
asteroid belt
Final number, masses and positions of terrestrial planets are essentially random.
PYTS 554 – Forming Planetary Crusts I 36
Material that make up planets depends on orbits of Jupiter and Saturn
Chemical differences in embryos are blurred
O’Brien et al., 2006
PYTS 554 – Forming Planetary Crusts I 37
More geochemical constraints? We have samples of Earth, Mars, Moon,
Vesta
Vesta Howardite-Eucrite-Diogenite (HED)
meteorites Eucrites are near-surface basalts
Hf/W data show Vesta differentiated in 1-4 Myr
Flows occurred shortly after that
Other meteorites have been identified with volcanic flows from other parent bodies
Non-vesta basalt within 3Myr of T0 (Wadhwa et al., 2009)
PYTS 554 – Forming Planetary Crusts I 38
Earth and Mars are more complicated Result of many embryos added at random times Cores tend to merge, but some W gets mixed with the mantle
Earth With exponentially declining accretion 63% of the Earth within 11Myr Other studies give single-stage dates of 30Myr to 100Myr
Moon Has no 182W excess Moon-forming impact occurred >50 Myr after T0
Mars Core formation at 11Myr after T0
Uncertainties mean core could form anytime within the first 10 Myr
Silicate differentiation (crust) at ~40 Myr
PYTS 554 – Forming Planetary Crusts I 39
The first few 107 years to 108 years T0 = 4568.2 ± 0.6 Myr formation of the CAIs Rapid formation of planetesimals < 1Myr
Intense Al26 heating Melting and differentiation into iron meteorite parent
bodies
Formation of Chondrules and Chrondrites a few Myr later
No differentiation due to lower 26Al levels
Vesta-like bodies formed with volcanic activity in progress
Gas disk dissipates ~10Myr Mars in ~10 Myr
Silicate differentiation ~40 Myr
Earth in ~30-100Myr Ends with the moon-forming impact, 50-150Myr At 163Myr Earth has a solid surface (zircons)
Next phase (~50 Myr) involves giant impacts – the leading theory for…
Stripping of Mercury’s silicate mantle Formation of Earth’s moon Formation of Mars topographic dichotomy
Chambers et al., 2009
Kleine et al., 2009