Accretion of terrestrial planets

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Accretion of terrestrial planets

• Ryuji Morishima (ETH Zurich)
• Collaborators Joachim Stadel, Ben Moore, Max
  Schmidt

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Contents

• Runaway growth phase
• Giant impact phase
• Questions and Strategy
• Toward the connection with geochemistry

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Basic scenario

• Formation of planetesimals
• Runaway growth of protoplanets (1Myr)
• Formation of Jupiter, dissipation of solar
  nebular, and beginning of giant impacts (10Myr)
• Moon-forming impact and formation of solar system
  ( 50Myr)

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Runaway growth phase

• Simulation with PKDGRAV1
• Velocity dispersion ltlt Escape
  velocity of runaway bodies
• Larger bodies grow faster than smaller bodies
• T_grow 1Myr(a/1AU)c, c 2-3 (Kokubo
  and Ida 2002)

Eccentricities and inclinations
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Runaway growth in the Asteroid belt in the
presence of Jupiter
Eccentricities and inclinations
Mass of the largest body
Without Jupiter
With Jupiter
Growth rate becomes about half owing to enhanced
velocity dispersion
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Giant impact phase

• A few tens of mars-size protoplanets form after
  the runaway phase
• Their orbits become unstable after a certain time
  and giant impacts occur
• A few planets remain in the end of accretion
  (50-100Myr), but

(Chambers and Wetherill 1998)
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Enhanced eccentricities of resulting planets
N 150

• Resulting eccentricities are larger than those of
  the present terrestrial planets
• If small planetesimals are left in the beginning
  of the giant impact phase, resulting
  eccentricities become smaller
• Remnant gas has the same effect

N 50
(Chambers 2001)
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Questions

• Were eccentricities damped by remnant
  planetesimals or by gas?
• How much small planetesimals were left in the
  beginning of the giant impact phase, especially
  in the asteroid belt? (most of planetesimals near
  1AU were accreted by protoplanets)
• When did Jupiter form? (Core accretion model or
  Disk instability model?)

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Strategy

• Simulations which directly ties the runaway
  phase (PKDGRAV2 code Ngt10000) and the giant
  impact phase (Mercury code N lt 1000)
• Generalize orbital and mass distribution of
  resulting systems for various initial mass
  distribution, timing of Jupiter formation, and
  dissipation time scale of solar nebular

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Toward the connection with geochemistry

• Fraction of iron (core) Fe/(SiFe) 0.7, 0.3,
  0.325, 0.2 for Mercury, Venus, Earth, and Mars
  -gt Stripping of mantle by giant impacts or
  volatility of Si?
• Partitioning of elements between Fe and Si (e.g.
  W/Hf is one of the most useful system)
  Partitioning coefficients are functions of
  Pressure, Temperature, and Redox state

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Partitioning of elements at impacts
Classical picture
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What happens in giant impacts?
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Strategy

• Degree of equilibrium in partitioning might be
  represented as a function of total mass, mass
  ratio, impact velocity, and impact parameter
  (Nimmo and Agnor 2006).
• Effect of imperfect accretion (bouncing,
  ejecting) is a challenging topic.
• Other materials such as water, noble gasses are
  also useful and should be considered.