ScienceWise - Winter 2012

Journey to the centre of the Earth

Article Illustration
A section through the earth showing the liquid iron outer core and solid iron inner core

Exploring the planet’s inner core

As the Earth formed from the hot dust and gas that surrounded the infant Sun, the heavier elements such as iron and nickel fell towards the centre. The outer rocky layers then cooled and solidified forming an insulating blanket trapping in much of the heat of the core. So it makes perfect sense that our planet should have the hot iron core it does today. However at some point in the past, the iron and nickel at the centre of the Earth began to solidify into a solid ball and that requires a little more physics to understand.

Most materials can exist as gas, liquid or solid depending on the temperature and pressure. Water is the most familiar example of this. We cool water and it freezes, heat it and it boils. But the temperature it does these things at also depends on the pressure.

The same is true for iron. So although the iron in the very centre of the Earth is at least as hot if not hotter than that in the liquid outer core, it exists as a solid because the pressure is so high. You know how much an iron anvil weighs. Now imagine ten million of them stacked on top of each other - that’s the pressure on every part of the inner core.

Dr Hrvoje Tkalcic  leads a research group at the ANU Research School of Earth Sciences, (RSES) who are exploring this inner core.

“Actually one of the problems we face in modelling the Earth’s inner core is that it’s very difficult to replicate those pressures and temperatures in the laboratory so there’s a degree of uncertainty about way iron behaves in such an extreme environment. In addition, we don’t even know exactly what the core temperature is,” Dr Tkalcic  says, “We know it’s around 5500°C but it could be several hundred degrees higher or lower than that. And all these factors make it difficult to determine the rate at which the inner core is solidifying.”

The only effective way to probe the deep structure of the Earth is to use sound waves, much like a doctor might use an ultrasound scan to examine an unborn baby inside its mother. However to see deep into the Earth the sound energy has to be enormous. The only sources that regularly provide this kind of intensity are earthquakes.

It was just such observations that led Inge Lehmann to discover the inner core back in 1936. She noticed that earthquake shock waves travelling through the centre of the Earth were partially refracted by a region within the core, suggesting that there was a solid/liquid boundary present.

Modern computer power has allowed scientists to refine this technique by simultaneously monitoring the thousands of quakes that shake the globe every year. This data can be used to build up a tomography image of the Earth’s interior.

“It’s a lot like an X-ray CAT scan in a hospital,” Dr Tkalcic  says, “Except that we have no control over the wave sources.”

“Because the inner core is buried deep within many other layers, these tomographic imaging techniques are not nearly as effective as we’d like in this region,” Dr Tkalcic explains, “So instead we create mathematical models of wave transmission through all possible core structures and compare the various configurations to what we actually observe.”

Observations of the propagation of shock waves through the Earth also confirm that the outer core is liquid.“Compression waves, like sound, travel through both liquids and solids,” Tkalcic  says, ”But shear waves are lost in a liquid. We can see from the disappearance of such shear waves that the outer portion of the core must be liquid.”

Most of the earthquakes that provide scientists with data occur along tectonic plate boundaries and there are more of those in the equatorial regions than near the poles. The RSES team recently installed instruments in Antarctica and in remote areas of Australia to monitor the weaker and less frequent waves with polar sampling of the inner core.

“Some results have suggested that inner core is not perfectly homogeneous, so we’re trying to establish more data and investigate what’s going on.” Dr Tkalcic says, “And it’s proving to be a very interesting area of research.”

The scientists are hoping to answer questions such as whether the inner core rotates at a slightly different rate to the rest of the planet. If it is indeed asymmetric or inhomogeneous in some way. How it solidifies and what implications all this may have on things like the Earth’s magnetic field.

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