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Forming Planetary Crusts II

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  1. Forming Planetary Crusts II

  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

  3. Kleine et al., 2009 • 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

  4. Overview of a rocky planet • Starts as homogeneous mix of rock & iron • Molten state allows differentiation • Iron core cools and solidifies (not yet complete for the Earth) Millions of years Billions of years ~12,800 km

  5. Planets start hot • Gravitational potential energy of accreting mass • Minimum energy delivered as velocity might be more than the escape velocity • Integrate over the planets radius to get total energy delivered • Convert this energy to a temperature rise: • Ignore cooling for now • ΔT for the Earth is very large >>> melting temperature • ΔT ~ melting temperature means R~1000 km • Objects bigger than large asteroids melt during accretion • Differentiation also releases gravitational potential energy • Amount depends on core/mantle density contrast and size of core • Typically enough to melt the body • Hf/W isotopes show differentiation essentially contemporaneous with accretion Spread over the planet’s surface increasing radius by ΔR

  6. 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 • Determines what regions contribute the terrestrial planet material • Final number, masses and positions of terrestrial planets are essentially random.

  7. Three possible impacts giant impacts to consider… • Formation of an iron-rich Mercury • Formation of Earth’s Moon • Mars Crustal dichotomy

  8. Mercury’s Abnormal Interior • Mercury’s uncompressed density (5.3 g cm-3) is much higher than any other terrestrial planet. • For a fully differentiated core and mantle • Core radius ~75% of the planet • Core mass ~60% of the planet • Larger values are possible if the core is not pure iron • 3 possibilities • Differences in aerodynamic drag between metal and silicate particles in the solar nebula. • Differentiation and then boil-off of a silicate mantle from strong disk heating and vapor removal by the solar wind. • Differentiation followed by a giant impact which can strip away most of the mantle.

  9. Basic story • Mercury forms and differentiates • Proto mercury is 2.25 times the mass of the current planet • Impactor is ~1/6 of the mass • Fast, head-on, collision needed to strip off mantle material • In contrast to slow oblique collisions at Earth and Mars • Head on collisions are less likely

  10. Impact timescale • A few hours to reform the iron rich Mercury • Magma ocean certain • Mercury must avoid re-accreting debris • Half-life of debris is ~2 Myr • Poynting-Robertson drag • Dynamical models suggest Mercury can reaccumulate some small fraction of its old silicate material • No samples means no constraints Benz et al., 2007

  11. Formation of the Moon • Facts to consider • Moon depleted in iron & volatile substances • Bulk Earth 30% iron (mostly core) • Bulk Moon 8-10% iron (mostly in mantle FeO) • Oxygen isotope ratios similar to Earth • Moon doesn’t orbit in Earth’s equatorial plane • Orbital solutions show that original inclination was close to 10 degrees • Angular momentum of Earth-Moon system is anomalously high • Corresponds to spinning an isolated Earth in 4 hrs • Geochemical evidence for magma ocean • Floating anorthosite • Uniform age of highland material – more on this later

  12. Possible theories (that didn’t work) • Earth and Moon co-accreted • Explains oxygen isotopes • Doesn’t explain iron and volatile depletion • Earth split into two pieces • Spinning so fast that it broke apart (fission) • …but the Moon doesn’t orbit in Earth’s equatorial plane • …and present day angular momentum isn’t high enough • Capture of passing body • Earth captures an independently formed moon as it passes nearby • Pretty much a dynamical miracle (Very hard to dissipate enough energy to capture) • Doesn’t explain oxygen isotope similarity to Earth • Current paradigm is Giant impact • Earth close to final size • Mars-sized impactor • Both bodies already differentiated • Both bodies formed at ~1 AU

  13. Free parameters • Late vs Early (mass of proto-Earth) • Early accretion poses compositional problem • Mass ratio • ~9:1 for late accretion • ~Mars-sized impactor • Impact parameter • Controls angular momentum of final system • Values 0.7-0.8 Rearth work best • Most probable impact angle is 45° (b~0.707Rearth) • Approach velocity • Minimum is escape velocity • Best results for v/vesc ~ 1.1 Canup, 2004 b

  14. Canup, 2004

  15. Canup, 2004

  16. Isotopic ratios may have equilibrated through vapor cloud Canup, 2004

  17. Most material in the lunar disk comes from the impacting body • Yellows/greens • Isotopic ratios shouldn’t match without re-equilibration • Temperature of material that goes into the moon is coolest • Still several 1000K • Enough to remove volatile elements and water • Cores of bodies merge • In the Earth Canup, 2004

  18. Disks are 1.5-2 lunar masses • Formation of a lunar sized body is possible in months • Tidal forces > self-gravity when inside the Roche limit • ~2.9 Rearth for lunar density material • Optimum place to form moon is just outside this limit where disk is thickest • Conservation of angular momentum • Moon ~15x times closer • Earth’s rotation ~3.9x faster (~6 hours) • Tides have removed some of this angular momentum • Moon drifts outwards • From disk interaction • From terrestrial tides aR ~ 2.9 Rearth Tk ~ 7 hours Kokubo et al., 2000

  19. Timeline constraints? • Hf/W put the impact at >50Myr after CAIs • Anorthosite Sm/Nd 112 ± 40 Myr formation of lunar crust • Norman et al. 2003 • KREEP (Zircon Pb/Pb) 150 Myr • Nemchin et al. 2009 • Whole moon Rb/Sr 90 ± 20 Myr • Halliday 2008 • Earths magma ocean gone by 163Myr • Zircons again

  20. Mars: Crustal Dichotomy • Northern and southern hemispheres of Mars are very distinct: • North • Low elevation • Few Craters – Young • Smooth terrain • Thin Crust • No Magnetized rock • South • High elevation • Heavily cratered – Old • Rough terrain • Thick crust • Magnetized rock • Dichotomy boundary mostly follows a great circle, but is interrupted by Tharsis • No gravity signal associated with the dichotomy boundary - compensated • Theories on how to form a dichotomy: • Giant impact • Several large basins • Degree 1 convection cell • Early plate tectonics Zuber et al., 2000

  21. Despite all this the difference is only skin deep • Buried impact basins in the northern hemisphere have been mapped • Before this burial the northern and southern hemispheres were indistinguishable in age • Rules out Earth-style plate tectonics • Northern hemisphere is a thinly covered version of the southern hemisphere • Mantled by 1-2 km of material (sediments and volcanic flows) Frey et al., 200?

  22. Borealis basin • 208E, 67N • 4250-5300 km in radius • Shares slope break with at ~1.5 basin radii with other basins • Largest impact structure in the solar system Andrews-Hanna et al., 2008

  23. Marinova et al., 2008 Nimmo et al., 2008 • Hydrocode modeling of a vertical and oblique impacts • 3x1029 J impact, 6-10 kms-1 at 30-60° • No global melting – melt layer 10s of km thick within basin • Northern crust extracted from already depleted mantle • May correspond to Shergottites formed from a depleted reservoir 100Myr after most SNCs

  24. Giant Impacts make Magma Oceans • Lunar magma ocean was probably at least a few hundred km thick • Apollo 11 returned highland fragments, first suggestion of Magma ocean • Idea since extended to other terrestrial planets • A melt has a bulk chemical composition, but no crystals • Minerals are mechanically separable crystals with a distinct composition • Terrestrial planets are dominated by silicon-oxygen based minerals – silicates • Silicate rocks are built from SiO4 tetrahedra

  25. Depending on how Oxygen is shared • Olivine • Isolated tetrahedra joined by cations (Mg, Fe) • (Mg,Fe)2SiO4 (forsterite, fayalite) • Pyroxene • Chains of tetrahedra sharing 2 Oxygen atoms • (Mg, Fe)SiO3 (orthopyroxenes) • (Ca, Mg, Fe) SiO3 (clinopyroxenes) • Feldspars • Framework where all 4 oxygen atoms are shared

  26. What happens when you cool a melt? • Bowens reaction series • Minerals begin to condense out in a certain order • Dense minerals sink e.g. Olivine (Mg,Fe)2SiO4 • Buoyant minerals rise e.g. Anorthite Ca Al2Si2O8 • ‘Undesirable’ elements get more concentrated in remaining liquid • Potassium (K), Rare Earth Elements (REE), Phosphorus (P) • The reverse happens when you melt a solid • More on that in the volcanism lectures

  27. Lunar Case • Original planetary crusts from silicate differentiation • Calcium-rich plagioclase feldspar (anorthosite) • Floats in an anhydrous melt – moon, mercury? • Sinks in a hydrated melt – Earth, Mars, Venus • Unstable at high pressures – so sinking anorthosite is doomed • Olivine and Pyroxene • Sinks in shallow magma ocean • Undesirables form KREEP layer • Non-uniformly distributed • Earth/Venus/Mars • Olivine rains out • Remaining composition is called Basaltic • Basalt is a broad term (to be expounded upon in the volcanism lectures!) • Variations in water content • Variations in alkali metal content • Variations in silica content • These are initial crusts that will be heavily modified by: • Stripping by Giant Impacts • Plate Tectonics • Volcanism

  28. Ultrabasic Primative Acidic Evolved Basic Fe poor Light Less-dense Fe rich Dark Dense

  29. End result is a chemically distinct skin of rock called a crust • 10s of km thick • Density ~3000 kg m-3 • Two main consequences of crustal formation • Mantles depleted • Upper mantle is more Olivine rich • Crusts enriched in isotopes • The ‘undesirables’ are concentrated in the crust • Radiogenic isotopes (heat sources ) mostly in the crust Mantle rocks Average

  30. The Moon has the ‘predicted’ anorthosite crust • Some resurfaced by later basaltic flows • Unexplained: • crustal thickness variation • Non-uniform KREEP distribution • Mercury should have lost any original anorthosite crust in its giant impact • Messenger indicates lower Ca/Si and Al/Si than the lunar highlands • …but abundant volatile species are a problem to explain • Very low Fe and Ti abundances 3.8 Ga 3.1 Ga Nittler et al., 2011

  31. Venus rock composition • Sampled in only 7 locations by Soviet landers • Composition consistent with low-silica basalt • Exposed crust is <1 Gyr old though Venera 14

  32. Earth’s crust is continuously recycled by plate tectonics and so we don’t see any original crust • But we can see production of basaltic crust ongoing today • Characteristic stratigraphic sequence: • Gabbro • (large grained basalt) • Sheeted dikes • Each sheet was the wall of the inner ridge • Pillow basalts • Blobs of basalt that are quickly quenched • Ocean sediments • Fine-grained muds • Called an ophiolite sequence • Can be obducted onto a continental setting • Isua supracrustal belt – southern Greenland • 3.8 Ga

  33. Martian in-situ and orbital measurements • Crust dominated by basalt • With a thin weathered coating McSween et al., 2009

  34. Marinova et al., 2008 Nimmo et al., 2008 • Hydrocode modeling of a vertical and oblique impacts • 3x1029 J impact, 6-10 kms-1 at 30-60° • No global melting – melt layer 10s of km thick within basin • Northern crust extracted from already depleted mantle • May correspond to Shergottites formed from a depleted reservoir 100Myr after most SNCs

  35. In decreasing order of severity… • Mercury – head-on, high velocity, collision • Total planetary disruption • Earth – grazing, low velocity, collision • Forms very large Moon • Global magma oceans on both bodies • Mars – grazing, low velocity, collision • Forms hemispheric dichotomy • A baby magma ocean, no large moon • Vesta • Distorted shape of object • Ejected crustal and mantle samples to Earth • Giant impacts may have had other roles • Formation of Pluto’s moons • Rotation of Venus

  36. Explaining Earth’s water is a problem • Best done with Jupiter and Saturn on circular orbits • Explaining a small Mars is a problem • Best done with Jupiter and Saturn on eccentric orbits, e ~ 0.1 • Inconsistent with Nice model for later giant planet migration Raymond et al., 2009

  37. Earth’s water • 1 Earth ocean ~ 1.4 x 1021 Kg • Estimates of Earth’s water content of ~5 oceans, about 0.1% MEarth • Inner nebula was too hot to allow water or hydrous minerals • Possible Sources • Adsorbed on dust grains at 1 AU • Comets • Asteroids (either ice or as hydrated minerals) • Constraints • D/H of Earth’s water • Late veneer of highly siderophile elements • Moon is (mostly) dry • Surface water after moon-formation

  38. D/H rules out comets • But only 3 Oort cloud comets have been measured • Condensed near Jupiter’s current position • Bulk comet might be different than its coma • Jupiter family comets might have a different D/H • Condensed in Kuiper belt • Mars D/H matches comets • Lack of crustal recycling? • Asteroids match Earth’s D/H • Only Carbonaceous Chondrites have significant water • But addition of these asteroids would produce the wrong Os isotopes • Earth has a late veneer of highly siderophile elements (added post differentiation) • At ~0.003 of CI abundances (but in CI ratios) • Ordinary chondrites are an isotopic match • Requires a ~1% MEarth addition after the moon forms • But late veneer and water delivery could come from different sources Drake, 2005

  39. Adsorbed onto dust grains? • Simulated adsorption onto forsterite grains shows a few oceans can be stored in this way • …but, not all adsorption sites would contain water (e.g. competition from H2) • Ordinary chondrites are not hydrated… Muralidharan et al., 2008