SEISMICALLY CONSTRAINED ACCRETION SCENARIO FOR THE EARTH'S CORE

The seismic wave speed and density profile in the Earth's outer core is constrained by observations of eigenperiods of the free oscillation of Earth and by seismic wave travel times. These sources yield wavespeed and density profiles through the outer core whose averages are known to 0.05-0.5%. The profiles suggest that the bulk of the core is compositionally uniform and wavespeed and density changes due to self-compression.

Recent high-resolution studies of wavespeeds at the top of the core indicate that the outermost ~350 km has a different composition and/or temperature structure than the core's bulk. If the wavespeed profile is viewed as the outcome of a diffusion process homogenizing a compositionally different layer started when the Earth accreted, the layer was initially 186±20 km thick and represents about 14% by mass of the present core.

One intriguing interpretation of these observations is the possibility that the layer represents a compositionally different, late-accreting core material. The mass of the conjectured Moon-forming impactor is about Mars's mass, and Mars' core has a similar mass to the layer (~14% of the Earth's core). If the layer is a relic of an impactor, compositionally different to the bulk of the early accreting Earth's core, it could provide compositional constraints on the impactor's core and thus accretion histories of the Earth.

I explore this by modelling the composition of core liquids that match the seismic wavespeed and density in the outer core and examine perturbations to them that yield the observed deviations in the top of the core. The bulk core appears to be low in sulfur based on meteorite trends, and it is possible to model core liquids subject to this constraint and those on refractory siderophile element abundances. In order to agree with experimental Mo and W partitioning between silicate and metal, Wade et al. (2012) invoke a late-stage accretion of more sulfur rich material to the core. The observed wavespeed reduction at the top of the outer core can be achieved with added sulfur, up to about 12 wt% in the late-stage core liquid.