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Isotopic outcomes of N-body accretion simulations: Constraints on equilibration processes during

Summary: Isotopic outcomes of N-body accretion simulations:
Constraints on equilibration processes during
large impacts from Hf/W observations
F. Nimmo , C.B. Agnor
Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, United States
Received 12 August 2005; received in revised form 3 December 2005; accepted 12 December 2005
Available online 7 February 2006
Editor: S. King
Most estimates of planetary core formation timescales using hafnium­tungsten (Hf­W) isotopes employ analytical expressions
assuming either continuous planetary growth or instantaneous core formation. In contrast, dynamical modelling of planetary
accretion suggests that the final stage of terrestrial planet formation is punctuated by multiple large and stochastic impacts. Such
giant impacts have significant thermal and isotopic consequences. We present a framework for calculating the Hf­W isotope
evolution of individual bodies based on the results of an N-body accretion simulation and assuming constant partition coefficients.
The results show that smaller bodies exhibit a larger range in isotopic values than larger bodies, because the latter have suffered
more impacts. The analytical core formation timescales calculated using these isotopic values can differ very significantly from the
timing of the final giant impact each planet actually experiences. Simulations in which 1) even the largest impactors undergo re-
equilibration with the target's mantle, rather than the cores merging directly, and 2) the original planetary embryos possessed
radially variable iron:silicate ratios, produce results which are consistent with the observed physical and isotopic characteristics of
inner solar system bodies. Varying W partition coefficients (due to changing mantle oxidation state) or initial planetesimal Hf/W


Source: Agnor, Craig B. - Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London
Nimmo, Francis - Department of Earth Sciences, University of California at Santa Cruz


Collections: Geosciences; Physics