Chemical Interactions in the Earth's core

The Earth is a differentiated planet with a heavy, metallic core at its centre, formed by a solid inner core surrounded by a liquid outer core. It is generally accepted that it formed ~ 4.5 billion years (Gy) ago from the condensation of the solar nebula, and it may have gone through a period in which most of the planet was completely molten. Melting was necessary for the Earth to differentiate, and initially the required energy was provided by the primordial heat of accretion, which came from the kinetic energy of the impacts from which the Earth formed. This kinetic energy was progressively boosted by the growing planet, increasing the amount of heat deposited towards the later stages of the accretion process.
The core makes up ~ 32 % of the Earth's mass and ~ 15 % of the Earth's volume. It is the seat of major global processes, and the outer core is the place where the Earth's magnetic field is generated. The paleomagnetic record clearly indicates that the Earth has had a magnetic field for more than 3 Gy, that this field is predominantly dipolar at the Earth's surface (due to Earth's rotation), and that it suffered several reversals of polarity throughout its history. The Earth's magnetic field has been instrumental to the appearance of life on the planet, and to its evolution to the richness of forms we know today. It protects the atmosphere from being stripped away by the constant stream of highly energetic charged particles beamed by the sun, and shields the DNA of all life forms from damage and mutations that they would suffer from such a bombardment of ionising radiation. From measurements accumulated systematically since 1840, we know that the field has dropped in intensity by at least 10 %. Modelling the field with the goal of being able one day to predict its evolution is one of the most exciting problems in the earth sciences. To do that properly, we need to understand how and where the field is generated. We know that the physical mechanism must be motion of the fluid in the outer core, but we do not know the exact composition of this fluid, nor how much of the core is actually involved in the generation of the field. The fluid moves because of convection (the same principle why hot air raises in the atmosphere and cold air sinks), which is driven by several energy sources, including secular cooling (i.e. escaping of the primordial heat of accretion from the centre of the Earth) and compositional convection (release of light elements at the bottom of the outer core as the inner core freezes). Today, the release of light impurities at the bottom of the core is probably the largest energy source to stir the outer core. However, it is possible that these light impurities accumulate at the top of the core, forming a stratified layer. In particular, oxygen is believed to be one of the main light elements in the core, and it is also a major constituent of the Earth's mantle, which is in contact with the outer core. It is possible, therefore, that the mantle dissolves oxygen in the core, and in doing so creates an oxygen rich layer that is buoyant and does not participate to convection. If this is the case, then the magnetic field must be generated only by a portion of the outer core, excluding this stable layer at the top.
Indeed, the seismic record shows anomalous behaviour at the top of the core.
In this project we will put strong constraints on the exact composition of the outer core, and we will study equilibrium between the core and the mantle to find out if the mantle is continuously pumping oxygen in the core. The research will be used to provide better constrains to model the geodynamo, and it will help the interpretation of the seismic record. We will use quantum mechanics methods and computer simulations to study thermodynamic equilibrium in the core. We will develop methods that are completely general, which may find applications also in other fields, including material design and alloy characterisation.

The main beneficiary of knowledge arising from this proposal is the academic community, and in particular the geodynamo community which will be able to use the new constraints on the composition of the core, and on the size of the outer core convective region, to refine models for the generation of the Earth's magnetic field.
New simulations and predictions for the evolution of the Earth's magnetic field will be possible.

The condensed matter and material science community will be provided with new tools for the computation of chemical potentials in mixtures with an arbitrary number of solutes, and will be able to exploit these tools for instance in materials design.

Our involvement with industrial companies like the AWE will provide a point of contact with the real economy. Although the proposal is about calculations on Earth's core properties, the techniques to be employed are completely general. Methods for high pressure research are of interest to the AWE, and this research may stimulate more research funding, for example in the form of sponsorship of industrial studentships.

Our involvement with the Numerical Algorithm Group (NAG) Ltd. for the optimisation of computer codes on the national facility HECToR will benefit the computer user community at large, with the developments arising from this project made available to them.

We will give the general public a better description of what the interior of our planet looks like. More generally, I believe that our work has the potential to inspire the younger generations, and attract them to science and the solution of scientific problems using high performance computing, an area where the U.K. is at the leading edge.