Physical constraints on the likelihood of accreting a non-chondritic Earth

Lead Research Organisation: University of Bristol
Department Name: Physics

Abstract

The Earth is a differentiated planet. During its evolution, molten iron sank to its centre to form the core and a crust was produced at its surface, leaving a residual layer of mantle in between. Trying to determine the composition of the Earth is a difficult problem, given this division into several different, inaccessible layers. Indeed a common approach has been to assume a composition for the Earth and use this to deduce information about the interior layers by mass balance. This then begs the question, how is it possible to estimate the bulk composition of the Earth?

The Earth, like the meteorites and all other planets in our solar system, formed from a disk of gas and dust that contained too much angular momentum to fall directly onto the young Sun. For some elements of interest this disk of gas and dust was sufficiently homogeneous that the Earth should be the same composition as other planetary bodies that grew from it. Some of the smaller asteroid bodies never grew large enough to undergo melting and differentiation, so samples from these homogenous undifferentiated planetesimals can provide a compositional estimate for bulk Earth. Such samples exist in the form of the chondritic meteorites. These precious rocks have therefore been critical in constraining the chemical composition of the Earth as a whole and also its constituent layers.

However, the chondrite model for the Earth has recently come under close scrutiny. Measurements of the isotope ratio 142Nd/144Nd, which ought to be the same on Earth and in chondrites are notably divergent. To account for this alarming observation, two contrasting models of the Earth have been proposed. The first invokes a hidden reservoir, comparable in magnitude and composition to the continental crust that has been trapped at the bottom of the mantle throughout Earth History. The alternative is that the process of accretion, by which planets grow, leads to the preferential loss of planetary crust and so results in a non-chondritic Earth. These two models have very different implications for the structure, behaviour and composition of the Earth. Determining which of these scenarios is correct is therefore of fundamental importance. Whilst others have (unsuccessfully) attempted to identify a signature of the hidden reservoir, here we propose to address the physical plausibility of accreting a non-chondritic Earth using a novel dynamical simulations.

In this study we will run a code that follows the growth of planets by accretion but also tracks material that is lost from impacting bodies during this process. This work has become possible due to the recent development of a detailed collision model developed by PI Leinhardt and collaborator Stewart. We will further determine what portions of the planetesimal are lost during this process. Finally, the composition of the different portions of the planetesimal will be calculated using simple melting/crystallization models following an amount of melting determined by the energy of collision. Such simulations, to chart the chemical evolution of accreting planets, have not previously been attempted.

Preferential loss of crust will result in the depletion of some key elements (e.g. the heat producing elements Th and U) that are enriched in this outer portion of the planet. Likewise, other impacts may result in removal of mantle but not core, resulting in a more iron rich planet. By running a large number of simulations, we can assess the likelihood that a body the size of the Earth has the correct amount of core relative to mantle and an overall chondritic composition. For example, we may find that it is almost inevitable that the Earth is non-chondritic, or indeed the opposite. In either case this work will shed important light of the bulk composition of the Earth and the process of planetary accretion.

Planned Impact

Pathways to Impact

Specific users this work might be of interest to and how they will benefit
The research proposed here will lead to a better understanding of the accretion of the Earth and the collisional processes that lead to it. This fundamental knowledge will primarily interest academics concerned with the evolution of the Earth and the formation of the solar system - petrologists, geochemists, and geophysicists - who would gain deeper insight into the collisional, physical, and chemical properties of Earth's accretion. Other academics that will benefit are planetary scientists and astrophysicists as the work proposed will provide more accurate timescales for the final phases of terrestrial planet formation which is of fundamental importance to understanding the formation of our solar system. Even though the end users of this type of knowledge are mostly academics, the results have great media appeal because of human curiosity concerning the formation of the Earth.

Techniques, methods or activities with which you will engage with this group

Academic Users
The academic beneficiaries are best communicated through the traditional channels of publication in leading journals and attending scientific conferences. We have asked for funding in the main proposal to attend topic specific academic conferences.

Training
The project will result in the training of a PDRA. The PDRA will gain expertise in data analysis and two specific numerical techniques. In addition, the PDRA will learn to use a large supercomputer to run simulations and process large amounts of numerical data. The PDRA will also have many opportunities to present the work both nationally and internationally as a result the will be in a position to gain employment in a variety of science-based occupations. As part of their training, the PDRA will attend a course at NERC focused on communicating science to the public.

Wider user interest
The collisional accretion of the Earth is of general interest to the public. The general public has an enduring interest in the formation of the Earth and the planets in our solar system, and the media are also keen to report results, which have broad public appeal. We intend to approach the non-academic communities through traditional means of Press releases and contact with popular media. PI Leinhardt has begun working with IOP Physics World delivering short videos on general topics surrounding the formation and evolution of planets.

Measurement of success
The success of the academic interaction can be measured by citations, and invitations to present our results at conferences. The impact of broader interaction could be measured by the number of public presentations or outreach events.

Summary of resources
We do not request any additional funds to support our Pathway to Impact goals.

Publications

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Carter P (2018) Collisional stripping of planetary crusts in Earth and Planetary Science Letters

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Carter P (2015) COMPOSITIONAL EVOLUTION DURING ROCKY PROTOPLANET ACCRETION in The Astrophysical Journal

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Veras D (2017) Explaining the variability of WD 1145+017 with simulations of asteroid tidal disruption in Monthly Notices of the Royal Astronomical Society

 
Description Using numerical experiments we have found that long term collisional evolution can produce planetary embryos with a distribution of chemical compositions that differ from the initial compositions of the building blocks. For example, some planetary embryos have larger cores and some have more mantle. This is important because the Earth does not match the composition of the meteorites. The collisional evolution hypothesis had not been quantitatively tested as an explanation. We have now shown that it is definitely a viable hypothesis and we are working on making our numerical models as realistic and accurate as possible.
Exploitation Route We are currently developing collision stripping laws for predicting the amount of material stripped from a differentiated body as a result of a particular impact. We anticipate others will use our stripping laws in astrophysical and geophysical problems involving collisions between gravity dominated objects. We also expect that the method we used to track compositional change during planet formation will be used by scientists studying extrasolar planets.
Sectors Other