39
PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

  • View
    218

  • Download
    0

Embed Size (px)

Citation preview

Page 1: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

PTYS 554

Evolution of Planetary Surfaces

Forming Planetary Crusts IForming Planetary Crusts I

Page 2: PTYS 554 Evolution of Planetary Surfaces Forming 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

Page 3: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 4: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

Earth’s Moon

A training ground for planetary science

Uniform highland crustOverlapping impact basinsVolcanic floods

Subsequent cratering

Page 5: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

Mercury

Page 6: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

MercuryCratered surface - like the MoonSupersized iron core – unlike the MoonSmooth plains interspaced with cratersLobate scarps indicating global compressionVery sulfur-rich crustPolar ice deposits

Page 7: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 8: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

MarsCrustal dichotomyCratered terrainsLarge Volcanic ProvincesWet Past

Valley NetworksLake DepositsLarge Flood channels

Modern dry/icy climatePolar ice sheets & groundice

Page 9: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

MarsCrustal dichotomyCratered terrainsLarge Volcanic ProvincesWet Past

Valley NetworksLake DepositsLarge Flood channels

Modern dry/icy climatePolar ice sheets & groundiceModern Mars is still active

Page 10: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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)

Page 11: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 12: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

Asteroids

Page 13: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

Vesta576 kmDawn 2012

Page 14: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

• 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

Page 15: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 16: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 17: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 18: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 19: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 20: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

Titan‘Earth-like’ processes with exotic materials

RiversLakesCratersDunesWeather

But also a subsurface ocean with high pressure ice phases below that

Page 21: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 22: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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.

Page 23: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 24: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 25: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

PYTS 554 – Forming Planetary Crusts I 25

Page 26: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 27: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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?

Page 28: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 29: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 30: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 31: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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…

Page 32: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 33: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 34: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 35: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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.

Page 36: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 37: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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)

Page 38: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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

Page 39: PTYS 554 Evolution of Planetary Surfaces Forming Planetary Crusts I

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