Transport properties in the Earth's core
Lead Research Organisation:
University College London
Department Name: Earth Sciences
Abstract
The Earth's magnetic field is one of the key essentials that allow life on Earth in the form we known it, with its active protection against the radiation coming from the Sun as highly energetic charged particles. Indeed, the field itself can be affected by the Sun's activity, with effects extending from spectacular aurora borealis to problems on tele-communications, navigation systems, the operation of artificial satellites, and even extended black-outs. Variations of the Earth's magnetic field are common; since the first measurement by Gauss in 1835 it has shown a relative 5% intensity decay in the past 150 years. On longer time scales of 250,000 years on average the field shows large instabilities that lead to reversals of the North-South orientation of the axis of its main dipolar component. Geomagnetic excursions, characterised by significant changes in the field strength felt at the surface of the Earth by up to 20 % and only a partial temporary (with times scales of thousand or tens of thousand years) re-orientation of the dipolar axis, are also common in the Earth's history, and are thought to be due to transfer of magnetic energy form dipolar to higher order multipolar components of the field, which decay more rapidly with distance from the Earth's core. It was proposed that excursions are due to reversals in the liquid outer core, but not in the solid inner core, which acts as a stabiliser for the field preventing full reversal. Back in 1905 Albert Einstein hailed the origin of the Earth's magnetic field as being one of the greatest unsolved problems facing modern physicists. A century later this is probably still true, with the modelling of the geo-dynamo being one of the most challenging problems in the geo-sciences. Although originally thought to be due to permanently magnetised rocks in the Earth's crust, it is now widely accepted that the geo-magnetic field, present for most of the Earth's history, is actually generated dynamically in the Earth's liquid outer core, thanks to convective motions that couple to the Earth's rotation due to Coriolis forces, and arrange themselves mainly in North-South columns, aligned with the Earth rotation axis. The theory used to describe the behaviour of magnetic field generated by a rotating electrically conductive fluid is called dynamo theory or the geodynamo if referred to the Earth. A fundamental ingredient for a dynamo to work is electricity, or in other words the metallic nature of the core: spiralling metal in a magnetic field generates electric currents, which in turn generate another magnetic field. When this generated magnetic field reinforces the original one, a dynamo which sustains itself is created. In its general formulation, geodynamo theory involves the solution of at least five non-linear equations. These equations can be solved numerically, once the parameters entering their definitions are known. In these equations, and in particular the Ohm's law in a moving conductor and the induction equation, we find the electrical conductivity, while in the heat transport equation we find the thermal conductivity. The latter is particularly important to estimate the amount of heat carried along the core adiabat, as this heat is a base-line that needs to be supplied before anything else can be available to drive the geodynamo. These two conductivities also happen to be among the poorest understood of all the core parameters, being uncertain by factors of 2 or 3. Even a modest change of a factor of 2 over currently accepted values would have a dramatic effect on the heat conducted down the adiabatic gradient, changing many published scenarios for the thermal history of the Earth and its magnetic field. Here we plan to calculate the thermal and the electrical conductivity with accurate quantum mechanics methods, and assess the impact of the new estimates on the interpretation of existing and future geodynamo simulations and models for core evolution.
Organisations
People |
ORCID iD |
Dario Alfe (Principal Investigator) |
Publications
Pozzo M
(2011)
Electrical and thermal conductivity of liquid sodium from first-principles calculations
in Physical Review B
Miniussi E
(2011)
Thermal stability of corrugated epitaxial graphene grown on Re(0001).
in Physical review letters
Pozzo M
(2011)
Dehydrogenation of pure and Ti-doped Na3AlH6 surfaces from first principles calculations
in International Journal of Hydrogen Energy
Pozzo M
(2011)
Thermal expansion of supported and freestanding graphene: lattice constant versus interatomic distance.
in Physical review letters
Cavallin A
(2012)
Local electronic structure and density of edge and facet atoms at Rh nanoclusters self-assembled on a graphene template.
in ACS nano
Pozzo M
(2012)
Thermal and electrical conductivity of iron at Earth's core conditions.
in Nature
Gillan MJ
(2012)
Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters.
in The Journal of chemical physics
Alfè D
(2012)
Lattice electrical resistivity of magnetic bcc iron from first-principles calculations
in Physical Review B
Description | The Earth's core is a formed by a solid ball of almost pure iron, 2/3 the size of the Moon, surrounded by a shell of liquid iron alloyed with light elements like sulphur, silicon and oxygen. This shell extends roughly halfway towards the surface. The core is hot (~ 5500 degrees) because of primordial heat and because of radioactive elements that keep warming it up. Since the surface of the Earth is cold, heat is forced from the centre to the surface, and this heat transfer is responsible for all the dynamical processes inside our planet, including for example the generation of the Earth's magnetic field. Heat is transferred via conduction and convection, the latter involving movement of matter and therefore causing the generation of the magnetic field through a dynamo mechanism (the geo-dynamo). Energy available to convection is crucially determined by the thermal conductivity of the core material: a high conductivity means heat is efficiently carried by conduction and little is left to convection. The efficiency of the geo-dynamo depends on the quality of the conductor, and therefore on the electrical conductivity of the core material. We have computed these two crucial parameters, the thermal and electrical conductivity of the core, using first principles methods and the national service high performance computing service HECToR. In a series of publications, including an article in Nature in 2012, we reported initial results which showed large values for the both thermal and electrical conductivities, about 2-3 times higher than previous estimates (bases on extrapolations from conditions very far away from Earth'c core conditions). The implications of these high conductivities are broad, and include the need of a significant amount of radiogenic materials in the core and/or a young solid inner core. We are now in the process of extending our studies to the solid inner core and to several possible compositions of the Earth's core. |
Exploitation Route | New models for the Earth thermal structure are being developed. |
Sectors | Education Environment |
Description | Our new estimates of the transport properties of the Earth's core are being used to provide new models for the Earth's thermal structure and the generation of the Earth's magnetic field |
First Year Of Impact | 2012 |
Sector | Education,Environment |
Impact Types | Cultural |
Title | PHON - A program to calculate phonons using the small displacement method |
Description | I am not entirely sure if this is relevant, but here it is. The PHON code is a computer software that is used to compute vibrational frequencies of materials, and with them also compute their thermodynamic properties. The programme is freely available from my personal web-page, and also from Github and from the Computer Physics Communications website. |
Type Of Material | Improvements to research infrastructure |
Year Produced | 2009 |
Provided To Others? | Yes |
Impact | PHON is used by hundreds of groups worldwide, and the describing paper (Computer Physics Communication 180, 2622-2633 (2009)) has been already cited more than 400 times. I am associating this product with all my grants as I have been developing this code over the years, and so all my grants have contributed to sustain this development. |
URL | http://www.homepages.ucl.ac.uk/~ucfbdxa/phon/ |
Description | First principles calculations and the Earth's core |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Participants in your research or patient groups |
Results and Impact | Onsen II workshop "Transport properties in the Earth's core", Kawaguchiko Lake, Japan, |
Year(s) Of Engagement Activity | 2013 |
Description | Phase diagrams from ab-initio calculations |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Professional Practitioners |
Results and Impact | University seminar N/A |
Year(s) Of Engagement Activity | 2013 |
Description | The Earth's core from first principles |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Participants in your research or patient groups |
Results and Impact | University seminar |
Year(s) Of Engagement Activity | 2013 |
Description | The Enigma 1,800 Miles Below Us |
Form Of Engagement Activity | A press release, press conference or response to a media enquiry/interview |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Media (as a channel to the public) |
Results and Impact | News article in "The New York Times", published 28th of May 2012 |
Year(s) Of Engagement Activity | 2012 |
Description | Thermal and electrical conductivity of the Earth's core from first principles calculations |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Professional Practitioners |
Results and Impact | 16th Asian workshop on First-Principles electronic structure calculations (ASIAN-16), Beijing, China |
Year(s) Of Engagement Activity | 2013 |