Getting to the core of the problem

Using the equations of quantum mechanics, which normally govern the bizarre physics that occurs at tiny atomic scales, has enabled geophysicists to answer a much larger-scale question – what the Earth`s core is made from. At the Condensed Matter physics conference on Tuesday 9 April, part of the Institute of Physics Congress in Brighton, Prof Mike Gillan and Dr Dario Alfe from University College London will describe how their research has revealed not only the likely composition of the Earth`s core, but also its temperature.

“The Earth`s core is incredibly important because it is the heat flowing out of it that is driving things like continental drift and earthquakes,” says Gillan. “Knowing the temperature is a fundamental part of understanding these things,” adds Alfe. “The composition is also important because people want to understand how the Earth was formed, and how it came to have the layered structure that it has,” continues Gillan.

It has been known for some time, both from analysing the composition of meteorites and because the Earth has a magnetic field, that the solid inner core and liquid outer core of the Earth must contain iron. But the core cannot be made from iron alone because its density can be found from measuring the speed of seismic waves – which travel at a different pace depending on the density of the substance they are passing through – and the value obtained is too low to be coming from pure iron. This means there must be lighter elements in the core which are reducing its density, and these elements must be common in our solar system.

“You can rule out a whole lot of elements just because there`s not much of them in the solar system,” says Gillan. “When you`re talking about possible impurities in the Earth`s core, the most likely things are the most common things like silicon, sulphur and oxygen,” he explains.

It is almost impossible to replicate the conditions in the centre of the Earth in a laboratory – experiments often involve blowing up samples with explosives to generate the same sorts of pressures and temperatures experienced in the Earth`s core. So theoretical calculations provide a sensible alternative when trying to determine both which impurities are present, and what temperature the core might be.
The University College team are one of the leading groups using quantum mechanical calculations to try to solve these mysteries. The calculations are so demanding that only the largest computers are able to process them, so the researchers used the CSAR (Computer Services for Academic Research) supercomputers based at the University of Manchester for their work.

To determine the composition of the core, the first step is to assume what one of the impurities might be. For example if oxygen is chosen, the density is then calculated for different proportions of oxygen mixed with iron. If the density of one of these mixtures then matches up with the density obtained from the seismic data, this would mean if oxygen was the only impurity, the core must contain this particular percentage of oxygen along with the iron. However most people think there is more than one type of impurity in the core, and the situation is further complicated by the fact that the core has both a liquid outer part and a solid inner part.

Seismic measurements reveal a change in the density between the solid and the liquid part of the core, which means they must contain different amounts or types of impurities. These two parts of the core have also been sitting side by side for millions of years without changing each other. For this stable condition to exist, there has to be a set balance between the proportions of impurities in the different parts of the core. For example, a certain concentration of oxygen in the outer core must be accompanied by another particular concentration in the inner core.

“We`ve put all these things together and found that there`s got to be oxygen in the core because otherwise you can`t reproduce the seismic observations at all,” says Prof Gillan. “In fact it turns out that there`s got to be about eight percent of oxygen in the outer core that has to be mixed with a certain fraction of sulphur and silicon. We can`t tell how much of each of those there are, but we can tell that the total amount of sulphur and silicon together has also got to be about eight percent” he continues.

“We have also calculated the melting temperatures of iron in a whole range of pressures, from those at the edge of the outer core, all the way to the centre of the Earth,” explains Dr Alfe. The researchers believe the temperatures they are calculating are accurate to within 400°C of the actual temperatures inside the Earth. By contrast, temperatures obtained by other researchers using various experimental techniques differ by as much as 2000°C. The team hopes future calculations will provide them with even more accurate values for these temperatures