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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 The first few 107 years to 108 years
T0 = ± 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, Myr 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 Kleine et al., 2009 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 Spread over the planet’s surface increasing radius by ΔR
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 Gas has disappeared now
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.

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10 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

11 No samples means no constraints
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

12 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

13 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

14 Free parameters Late vs Early (mass of proto-Earth) Mass ratio
Early accretion poses compositional problem Mass ratio ~9:1 for late accretion ~Mars-sized impactor Impact parameter Controls angular momentum of final system Values 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

15 Canup, 2004

16 Canup, 2004

17 Isotopic ratios may have equilibrated through vapor cloud
Canup, 2004

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20 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

21 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

22 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

23 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

24 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?

25 Shares slope break with at ~1.5 basin radii with other basins
Borealis basin 208E, 67N 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

26 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 Marinova et al., 2008 Nimmo et al., 2008

27 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

28 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

29 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

30 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 Lunar Case

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

32 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

33 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 Ga Nittler et al., 2011

34 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

35 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

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

37 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 Marinova et al., 2008 Nimmo et al., 2008

38 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

39 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

40 Earth’s water Possible Sources Constraints
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

41 Asteroids match Earth’s D/H
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

42 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


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