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The Earth's core

This page is about the physics of the Earth's core.

The core of the Earth accounts for about 30 % of the total mass of our planet, the inner solid core is crystallizing from the liquid outer core, and the heat released flows to the surface, driving all the living geological processes of the Earth, including plate tectonics, volcanism and earthquakes.  The cystallization of the inner core is also responsible for compositional convection in the liquid core, that is the engine which generates the Earth's magnetic field, shielding us from the lethal solar wind. So, a sound knowledge of the core is of fundamental importance, yet, it is one of the most difficult things to study, and its properties are poorly constrained. For example, we know that it is mainly made by iron, but it can't be pure iron, because its density is too low. So it must contain some light element, and the most likely candidates are sulphur, oxygen, carbon and silicon, but the real composition of the Earth's core remains one of the major unsolved problems. The density and the pressure are known quite accurately (within a percent), but the temperature is unknown, with estimates ranging from 4000 to 8000 K.

We have tackled some of these problems using first principles techniques, and a report on our recent work just published in the journal Nature was picked up by the media, and there were interviews on Channel 4 News and the Today programme. There was even a half-page item on our work in the
Daily Mirror, and on the BBC on-line web page.

More recently, we participated to the BBC Radio 4 science program "The Material World".

See a recent poster.

The aim of our work is to use ab initio techniques to investigate the properties of liquid and solid iron (both pure and alloyed with light elements) under the conditions of the Earth's deep interior.

Some of the work already done includes:

Composition and temperature of the Earth's core

We have recently used our free energy calculation techniques to put a constraint on the composition and temperature of the Earth's core. Among the possible light elements we consider Sulphur, Silicon and Oxygen.

At Inner Core Boundary solid and liquid are in equilibrium, therefore the chemical potential of all the elements must be equal in the two phases. This fixes the ratio of concentration of the elements in the liquid and in the solid, which in turn fixes the densities. A comparison with seismological data allows us to rule out all binary mixtures, i.e. the Core cannot be made of Fe/S, Fe/Si or Fe/O. The reason is that S and Si do not partition enough between solid and liquid, the concentration in the solid is almost equal to the concentration in the liquid, so the density jump at ICB cannot be reproduced. Oxygen instead partitions too much, very little of it goes into the solid and again the density jump cannot be reproduced.

The composition of the Earth's core must be at least a ternary mixture.

Assuming that the presence of one impurity does not affect the chemical potential of a different element we suggest the following composition for the Earth's core:

                                                                            Composition at ICB

         Solid         Liquid
 Sulphur/Silicon       8.5 +- 2.5 %    10.0  +- 2.5 %
Oxygen       0.2 +- 0.1 %     8.0  +- 2.5 %

The partitioning of the light elements implies a depression of the melting point with respect to that of pure Iron, and we estimate this depression to be about 600-700 K.

The first part of this work has been published in Nature and GRL recently.


1.  G. A. de Wijs, G. Kresse, L. Vocadlo, D. Dobson, D. Alfè, M. J. Gillan, G. D. Price, "The viscosity of liquid iron under Earth's core conditions", Nature, 392 , 805-807 (1998).

2.  D. Alfè and M. J. Gillan,  "First principles simulations of liquid Fe-S under Earth's core conditions", Physical Review B, 58, 8248-8256 (1998).

3.  D. Alfè  and M. J. Gillan, "The first principles calculation of transport coefficients", Physical Review Letter, 81, 5161-5164 (1998).

4. D. Alfè, G. D. Price, and M. J. Gillan,  "Oxygen in the Earth's core: a first principles study", Physics ofthe Earth and Planetary Interiors, 110, 191-210 (1999).

5. D. Alfè,  "Ab-initio molecular dynamics, a simple algorithm for charge extrapolation, Computer PhysicsCommunications, 118, 31-33 (1999).

6. L. Vocadlo, D. Alfè, J. Brodholt, M. J. Gillan, andG. D. Price, "The structure of Iron under the conditions of the Earth's Inner Core, Geophysical Research Letters,  26 , 1231-1235 (1999).

7. D. Alfè, G. D. Price, and M. J. Gillan, "Melting curve of Iron at Earth's core pressures from ab-initio calculations", Nature, 401, 462-464 (1999).
(News & Views).

8. L. Vocadlo, D. Alfè , J. Brodholt, M. J. Gillan, and G. D. Price, "Ab-initio free energy calculations on the polymorphs of iron at core conditions", Physics of the Earth and Planetary Interiors, 117, 123-137 (2000).

9. D.Alfè, G. A. de Wijs, G. Kresse and M. J. Gillan,  "Recent developments in ab-initio thermodynamics", International Journal of Quantum Chemistry, 77, 871-879 (2000).

10. D. Alfè, G. Kresse and M. J. Gillan,  "Structure and dynamics of liquid Iron under Earth's core conditions", Physical Review B, 61, 132-142 (2000)

11. D. Alfè, G. D. Price, and M. J. Gillan, "Thermodynamics of hexagonal close packed iron under Earth's core conditions",  Physical Review B, submitted, preprint.

12. D. Alfè, M. J. Gillan, and G. D. Price, "Constraints on the composition of the Earth's core from ab-initio calculations", Nature, 405, 172-175 (2000).

13. D. Alfè, G. D. Price, and M. J. Gillan, "Thermodynamic stability of Fe/O solid solution at inner-core conditions",  Geophysical Research Letters, 27, 2417-2420 (2000).

And if you read up to here you deserve to see a beautiful picture...

ast update 12 December 2000                                      Back to Homepage