ge/ay133 when and how did the cores of terrestrial planets form?
Post on 22-Dec-2015
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Ge/Ay133
When and how did the cores of terrestrial planets form?
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Estimated core sizes of the terrestrial planets.
Two end member hypotheses for core formation:
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Q: Why is heterogeneous accretion unlikely?
Two end member hypotheses for core formation:
A: In a gas of solar composition, silicates and iron metal (it IS a reducing environment!) condense over similar ranges in T,P.
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Estimated core sizes of the terrestrial planets.
Two end member hypotheses for core formation:
For homogeneous accretion, when does the onset of differentiation occur?
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How might we distinguish these, and their timing?
Step 1: Know your geochemical affinities!
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Highly siderophile elements in the mantle: Late veneer…
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…or high pressure chemistry? Mixing?
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Step 2: Know how to measure isotopes very well!
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Absolute Pb-Pb dating error bars getting down to ~1 Myr!
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182Hf 182W 9
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Once you have ages, can look for short-lived excesses:
Hf/W ideal, since Hf is lithophilic, while W is a siderophile & 1/2 = 9 Myr.
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Once you have ages, can look for short-lived excesses:
Hf/Wdata from meteorites.
180Hf/184W
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Yin et al. (2002)Kleine et al. (2002)
Ages highly model dependent! More in a bit…
Models here assume complete equilibration…
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Size dependence? What about earth-moon?
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Geochemical partitioning is sensitive to the depth at which silicates last “see” Fe/Ni metal.
The atmosphere is also strongly affected by core formation, and its timing.
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Need to know how large an incoming differentiated planetesimal can be and still equilibrate…
Answer seems to be pretty small! Means that full equilibration is unlikely…
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The atmosphere is also strongly affected by core formation, and its timing.
Equilibrium Gas Abundances in Silicate MagmasSiO2 + 2Fe3O4 → 3Fe2SiO4 + 02
SiO2 + 2Fe + O2 → Fe2SiO4
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Might the exogenous delivery of organics to the early Earth been important?
Equilibrium Gas Abundances in Silicate Magmas
SiO2 + 2Fe3O4 → 3Fe2SiO4 + 02 Quartz Fayalite Magnetite (QFM)
SiO2 + 2Fe + O2 → Fe2SiO4 Quartz Iron Fayalite (QFI)
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Giant impacts & the Earth-Moon System:
1. Temperature behavior
R. Canup 2004, Icarus 168, 433 (and the slides that follow)
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Where does material go, & what is its composition?
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A `typical’ Moon forming event & simulation summary:
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Temperature evolution:
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Where does the iron/silicate go?