When and how did the cores of terrestrial planets form?

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Presentation transcript:

When and how did the cores of terrestrial planets form? Ge/Ay133

Two end member hypotheses for core formation: Estimated core sizes of the terrestrial planets.

Two end member hypotheses for core formation: Q: Why is heterogeneous accretion unlikely? A: In a gas of solar composition, silicates and iron metal (it IS a reducing environment!) condense over similar ranges in T,P.

Two end member hypotheses for core formation: Estimated core sizes of the terrestrial planets. For homogeneous accretion, when does the onset of differentiation occur?

How might we distinguish these, and their timing? Step 1: Know your geochemical affinities!

Highly siderophile elements in the mantle: Late veneer…

…or high pressure chemistry? Mixing?

Step 2: Know how to measure isotopes very well!

Absolute Pb-Pb dating error bars getting down to ~1 Myr!

182Hf 182W 9

Once you have ages, can look for short-lived excesses: Hf/W ideal, since Hf is lithophilic, while W is a siderophile & t1/2 = 9 Myr.

Once you have ages, can look for short-lived excesses: Hf/Wdata from meteorites. 180Hf/184W

Ages highly model dependent! More in a bit… Kleine et al. (2002) Yin et al. (2002) Models here assume complete equilibration…

Size dependence? What about earth-moon?

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.

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…

Equilibrium Gas Abundances in Silicate Magmas SiO2 + 2Fe3O4 → 3Fe2SiO4 + 02 SiO2 + 2Fe + O2 → Fe2SiO4 The atmosphere is also strongly affected by core formation, and its timing.

Equilibrium Gas Abundances in Silicate Magmas SiO2 + 2Fe3O4 → 3Fe2SiO4 + 02 Quartz Fayalite Magnetite (QFM) SiO2 + 2Fe + O2 → Fe2SiO4 Quartz Iron Fayalite (QFI) Might the exogenous delivery of organics to the early Earth been important?

Giant impacts & the Earth-Moon System: R. Canup 2004, Icarus 168, 433 (and the slides that follow) 1. Temperature behavior

Where does material go, & what is its composition?

A `typical’ Moon forming event & simulation summary:

Temperature evolution:

Where does the iron/silicate go?