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Journey to the centre of the earth

Deep down inside

(April 6, 2000, Gazette)

Dr. Michael Rochester

Photo by Chris Hammond

By Andris Petersons
SPARK student

The Earth is alive. Its heart is not just a rock. The Earth lives its life by spinning around the Solar system, the galaxy and the Universe.

“The Earth is the platform from which we observe not only the rest of the universe, but also the Earth itself – even what happens inside it,” said Dr. Michael Rochester, Earth Sciences, whose research involves a project in the theory of how the Earth behaves as an entire planet.

“It is only one stage in a series of projects that I have been engaged in for about 40 years. Since my graduate studies I have been interested in the ways in which things that we can observe at the surface of the Earth tell us something about the constitution of the Earth’s deep interior, and the processes that go on inside it.”

Over the last hundred years geophysical scientists have found convincing evidence that the planet Earth consists of three parts – solid mantle (extending down to a depth of 2,900 km), fluid outer core (some 2,300 km thick) and solid inner core (1,200 km radius). Dr. Rochester is very interested in mostly the fluid outer core but more recently also the solid inner core – how they affect the things that we can see on the surface of the Earth.

“My motivation is to understand the dynamics of the Earth,” he said. “I am particularly interested in the Earth’s fluid core. We can’t get a sample of it, we can’t drill that deep but we can infer some of the things it is doing.”

For Dr. Rochester, “the Earth is like a gigantic engine and heat is flowing out from the deep interior.”

Fluids in the outer core transport heat from the inner core to the bottom of the mantle where it is gradually carried off.

At the Earth’s surface the short-term consequences of this process are earthquakes and volcanoes, while the long-term consequences are mountain ranges rising and falling, and continents moving about.

One of the objects of Dr. Rochester’s research is to predict the enormously small changes in gravity associated with waves in the fluid core, or with movements of the inner core. Because it is surrounded by fluid, the solid inner core can move sideways, back and forth. In principle, an earthquake which is large enough and deep enough could jiggle the lower boundary of the mantle sufficiently that the jiggling could be passed on to the inner core.

To test how well the theory works, the resulting small periodic changes in gravity must be detected against the background of the much larger changes in gravity due to the tides. This detection may be possible if gravity is measured continuously and very precisely, for a decade or so, at a large number of places on the Earth’s surface, well separated from one another geographically.

The Global Geodynamics Program (started in 1998) aims at providing these measurements. It uses very sensitive gravity meters operating at the very low temperature of liquid helium. The essential part of such a gravity meter is a little metal ball suspended in a vacuum. Because the metal becomes a superconductor at these low temperatures, the ball can be levitated against gravity by a magnetic field. Any change in gravity can be measured by how much you have to change the electrical current producing that magnetic field, in order to offset the tendency of the ball to rise or fall slightly as gravity changes. There are about 20 superconducting gravity meters deployed around the world. One is in Canada, near Ottawa.

Because the iron fluid outer core is a very good metallic conductor of electricity, it produces and supports the magnetic field of the Earth. In the core of the Earth the magnetic field is quite strong. A weak magnetic field affects the North Pole moving it several metres every hundred years.

Because the outer core is fluid it becomes possible for the solid mantle and the solid inner core to each have its own axis of rotation, slightly offset in direction from one another. Because of their rotation, the solid mantle and the solid inner core are not spherical but slightly flattened at their rotation poles and slightly bulging at their equator, so as to have an ellipsoidal shape. Any changes in the relative orientation of the mantle and inner core can be accommodated by the fluid outer core moving so that it continues to fill the space between them.

It turns out that there are several ways in which the solid mantle, fluid outer core and solid inner core can be misaligned. With each such kind of misalignment the Earth shows a particular kind of wobbling motion in space, like a gyroscope which has been disturbed from equilibrium. These wobbles, with their characteristic periods, can be detected in the gravity records.

But they are more precisely detected by the associated disturbances in the locations of distant stellar radio sources relative to the Earth, as measured by radio telescopes. This technique, called very-long-baseline-radio-interferometry (VLBI for short), was actually developed in Canada over 30 years ago. Dr. Rochester is currently trying to track down the reasons why the theory of such wobbles predicts one of them to have a period significantly different from the value deduced from VLBI data.

From its density and from arguments based on chemical abundance, geophysicists conclude that the outer core is mostly iron. From seismology we know it is fluid. Because it is both fluid and a good electrical conductor, the outer core can produce and sustain the Earth’s magnetic field by working like a gigantic dynamo. Although the magnetic field that reaches the surface of the Earth is weaker than the field of an ordinary refrigerator magnet, it is nevertheless responsible for controlling things like a compass needle, and the Northern Lights.

Dr. Rochester is interested in how the presence of this magnetic field changes the rotation of the Earth. As the Earth rotates, its speed may change because of the changes in the magnetic field. These very small changes in the Earth’s rotation speed mean changes in the length of the day, amounting to a few thousandths of a second over a time span of a decade. Although so small, these changes too can be precisely detected using VLBI.

“One of the beautiful things about geophysics to me is that you are constantly having to bring together different bits of science in order to understand some phenomenon,” said Dr. Rochester. “Nothing stands alone. You can’t look at it and understand it without bringing in knowledge from many other areas. It’s a very interdisciplinary type of field. So many things in science are connected with one another.

“To my mind, in research you can be interested in something but you are never quite sure what it is you are going to find. That is why research is endlessly fascinating.”

SPARK, Students Promoting Awareness about Research Knowledge, is a NSERC - funded program designed to encourage writing about research.