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Core Outer Layer Stratified by a Sunken Impactor

Evidence has been found that there is a stratified layer at the top of the Earth’s core, though the origin of this stratified layer is not well understood. It is suggested by laboratory experiments that the stratified layer could be the remnants of a giant impact that formed the Moon.

The core of the Earth is comprised of a solid inner core and a liquid outer core. A stratified layer at the top of the outer core is suggested by the magnetic field of the Earth on short timescales (Buffet, 2014) and the thermal properties of an iron alloy (Gomi et al., 2014). The layer is indicated by recent seismic observations to possibly be less dense than the remainder of the liquid core, and about 300 km thick (Helffrich & Kaneshima, 2010). This seismic model could have implications for the formation and evolution of the Earth, though controversial, but there is no general consensus as to how a stratified layer could be formed. Previously such a layer has been attributed to the transport of light elements from the mantle to the core by chemical interactions (Buffet & Seagle, 2010), or to light element accumulations at the top of the core associated with the crystallisation of the solid inner core (Fearn & Looper, 1981). According to Nakajima both models struggle to generate a layer that was thick enough. It has been hypothesised that the stratified layer is a remnant of the impactor that is believed to have caused the formation of the Moon when it struck the early Earth about 4.5 Ga (Landeau et al., 2016).

Light elements such as silicon, oxygen, carbon, sulphur, nitrogen and hydrogen, which are present in the core that had been delivered by impacts during the formation of the Earth. After a collision between a large rocky body and the young Earth, the core material of the impactor would have sunk through the mantle of the Earth that was partially molten, and exchanged elements with the mantle through the process of silicate equilibration. Some light elements would have been partitioned favourably into the sinking iron liquid and eventually delivered to the core of the Earth. The process that occurred repeatedly during the formation of the Earth is responsible for the first order bulk chemical compositions of both the mantle and core

It is proposed by Landeau et al., based on their laboratory experiments, that the giant impact that formed the Moon could have done more than just bring light elements to the core of the Earth: the core of the impactor could have mixed turbulently with the core of the Earth which produced a thick stratified layer. The researchers performed laboratory experiments and then scaled them up to planetary size. These experiments involved releasing a liquid into a tank that held 2 liquid layers: a dense lower layer, that represented the proto-core of the Earth, and a layer that was less dense to represent the molten mantle of the Earth. The liquid that was released, that represented the core of the impactor, and the denser upper liquid was miscible only with the lower liquid and not with the upper liquid, to represent the way iron is more dense than silicate and is not miscible with it.

It was found by Landeau et al. the liquid that was released was mixed turbulently with, and entrained liquids from the upper layer while sinking, and generating a turbulent cloud in the process. Subsequently this cloud collapse to float transiently at the interface between the lower and upper liquids, as the liquid that was released was temporarily less dense than the lower liquid, because if the liquid of the upper layer that was entrained. The entrained liquids segregated slowly back to the upper layer over time. There were 2 types of outcome that were observed by Landeau et al., which depended on the starting conditions. The liquid material that was released eventually sank to form a structure that was compositionally stratified throughout the entire lower liquid, if the initial density of the released liquid exceeded that of the lower liquid. Alternatively, the liquid materials that were released remained floating at the top of the lower liquid as a stratified layer, if the liquid that was released was initially less dense than the lower liquid.

The experimental results of Landeau et al. were then extrapolated to impacts on a planetary scale, and it was argued that the stratified layer in the core of the Earth at its inferred thickness of 300 km could be explained as a remnant of the impactor that formed the Moon if the following 2 conditions were met:

1)      The core mass of the impactor was smaller than that of Mars, and

2)      The core of the impactor was less dense than the proto-outer core of the Earth.

The first condition is consistent with some earlier models of the impact that formed the Moon which suggested a core mass of an impactor of about 5-10 % of the core mass of the Earth (Ćuk & Stewart, 2012; Canup & Asphaug, 2001). It has also been proposed there was a larger impactor with a larger core (Canup, 2012), though the stratified layer produced by such a large impactor would be thicker than has been indicated by observations.

The low density of the core of the impactor that is required is more difficult to explain, though if the impactor had formed in a reduced environment, which would have resulted in the partitioning of more light elements into the core of the impactor at the time the body differentiated (Rubie et al., 2011). Alternatively, the iron in the core of the impactor could have become enriched in light elements during, as well as after, the impact by the metal-silicate equilibration of the deep magma ocean of the Earth under high pressures and temperatures.

It is suggested by Landeau et al. that this could have occurred at parts of the Earth that were even deeper. They mentioned that in their experiments a portion of the upper liquid layer is initially trapped within the released liquid that is sinking. The possibility is raised by this that some materials of the mantle may have undergone metal-silicate equilibration near the core-mantle boundary, or possibly deeper parts of the core. A certain quantity of light elements might have been delivered by this process to the core of the Earth. Though an intriguing result, but to constrain the amounts of light elements that are delivered to the core by this process needs further investigation.

That the thickness of the stratified layer has not changed significantly over time is one of the key assumptions underlying the scenario proposed by Landeau et al. It is argued by Landeau et al. that influence on the thickness of the layer would have been limited by the post-formation processes. The stratified layer could have thickened at its top by 10-100 km, according to the estimate by Landeau et al., as a result of the diffusion elements, or thinned by 50 m at the bottom due to erosion by convecting liquid iron. But further dynamical studies are required, as these estimates were based on scaling arguments. It is possible that the giant impact itself could have mixed, at least partially, the core of the Earth as well. to constrain the stratified layer thickness and overall structure of the core of the Earth, seismic studies are also needed.

Also, it is assumed by the experiments of Landeau et al. there was a head-on collision between the proto-Earth and the impactor in which the projectile sinks vertically. If the impact was oblique the outcome may have been different, which is more likely to have formed the moon. If the core of the impactor was injected obliquely the impactor’s core might have been stretched and partially disrupted to form smaller blobs prior to entering the magma ocean (Kendall & Melsoh, 2016), thereby leading to less mixing with the core of the proto-Earth and a stratified layer that was thinner.

It was found by Landeau et al. (Landeau et al., 2016) that the stratification of the outer core may have occurred consequent to core merging in the aftermath of a giant impact between the Earth and an impactor, depending on the densities and pressures of the 2 cores. Their findings suggest that the enigmatic structure of the core may hold clues to the big Moon-forming event, though based on experiments that were carried out on a much smaller scale than the actual impact.

Sources & Further reading

  1. Nakajima, M. (2016). "Core science: Stratified by a sunken impactor." Nature Geosci 9(10): 734-735.


Author: M. H. Monroe
Last Updated 03/11/2016
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