Spotlight

Journey to the Centre
of the Earth

David Stevenson has daydreams about drilling into the centre of the Earth. He's even been a consultant to a Hollywood company that was planning a film version of Jules Verne's Journey to the Centre of the Earth. But he has yet to come to close grips with his area of scientific interest -- the bowels of the planet beneath our feet.

As one of the world's leading planetary physicists, Professor Stevenson has had some interesting ideas on possible methods for getting "down under". He's even done the maths to see whether they'd work. Alas, they do not stack up.

"It's just a science fiction story," Stevenson admits. He will have to continue his research by indirect means, such as simulations on some of the world's fastest computers.

Introduced as "one of Victoria's most distinguished graduates" before his public lecture "How the Earth Works", Stevenson is a Fellow of the Royal Society and professor of planetary physics at Caltech, visiting his old university on sabbatical leave.

One idea for finding out at first-hand what really happens below the Earth's crust was a nuclear-powered vehicle that would heat the rock and melt its way throughbut it would take thousands of years.

Or a nuclear explosion could make a crack in the surface, which could be kept open by molten ironexcept that the energy required would be totally prohibitive.

As for the film, the producers wanted to know whether there could really be tunnels in the Earth's crust, as Verne had suggested. This turns out to be another example of Verne's remarkable foresight.

"I believe that there actually are such tunnels, probably going deep down into the Earth, but they are filled with molten rock," Stevenson says. He sees the internal workings of the Earth governed by convection, the same process seen in convection heating where currents of hotter and cooler air circulate around a space.

According to seismological and magnetic evidence, the core of the Earth is molten iron surrounded by a thick mantle of rock with a relatively thin crust on top. Differences of temperature set up patterns of convection movement within this spherical system, with the hotter materials tending to rise towards the surface and cooler (and therefore denser) materials falling towards the centre. Some of this movement, Stevenson believes, takes place through the tunnels that interested the film-makers. These constant movements within the Earth relate to volcanoes, earthquakes and the movement of land masses, although as yet the convection theory does not fully tie in with plate tectonics.

Stevenson developed his convection model using what was, at the time, the world's fastest computer, the Intel Touchstone Delta. It was the first time that three-dimensional modelling had been carried out with the inclusion of phase transitions, taking account of the changes in crystal structures with increasing depths and pressures.

The models produced a vision of the Earth cut into slices, enabling hot and cold zones in the mantle and on the surface to be seen. They also produced a similar view of magnetic fields, and revealed some striking similarities between the two. The movements of the molten iron core which cause the Earth's magnetic fields seem to be related to the convection movements in the mantle.

"So it brings together two fields that were thought to be separate," Stevenson says.

Stevenson believes that the Earth is constantly recycling itself.

"Almost everything on the surface of the Earth is intimately coupled with what happens deep down below -- thousands of kilometres below the surface," he says. "There are molecules in the oceans now that came from thousands of kilometres within Earth. There is at least as much water within the Earth as there is in the oceans on its surface."

There is abundant evidence that the whole Earth system is chaotic, like weather.

"That doesn't mean we don't know the physical processes involved, it means we can't predict the form of the motions some time hence. There is an unpredictable character to the evolution of the Earth, both on the geological time scale of hundreds of millions of years and on the human time scale of years and decades, and individual earthquakes and volcanic eruptions.

"It's like taking a piece of glass and subjecting it to stress until it breaks. You know the properties of the glass and the magnitude of the stress, but you can't predict exactly how each piece of glass will fragment and splinter, because pieces of glass are not identical; they all have slightly different imperfections and the prediction is virtually impossible."

Stevenson sees the Earth as still "cooking" -- long-term chemical processes are taking place which affect the whole planet. Investigating these processes is a major challenge for the future.

Stevenson is hoping to integrate his investigations of the evolution of the Earth itself with studies on the origin of life.

"Study of the origin of life has to take account of the physical and chemical state of the Earth at the time," he said. "This is perhaps becoming closer to being a real scientific question, one that can be meaningfully addressed."