Earth as egg: hard-boiled or raw?

Earth as egg: Hard-boiled or raw?

Most cooks know that a hard-boiled egg spins much faster than a raw or partially cooked egg. This difference often serves as a test for doneness. On a more global level, it may also illustrate one reason why Earth's magnetic field has sporadically switched direction in the past, according to a controversial theory proposed by Richard Muller and Donald Morris at the Lawrence Berkeley Laboratory in Berkeley, Calif.

Most geophysicists think the geomagnetic field arises from currents of liquid iron flowing in the planet's outer core. But this is only a general theory. Scientists disagree over many details, including why the magnetic field sometimes weakens and then switches direction. Some have suggested that meteorites hitting the Earth might turn the field around, but this theory has gained little support in recent years.

Now Muller and Morris are reviving it. They suggest that impacts or volcanic eruptions could loft dust into the atmosphere and suddenly cool the Earth to a point where great ice caps would develop over the polar regions. Like an ice-skater accelerating a spin by pulling her arms toward her body, the movement of water toward the poles would speed the Earth's rotation.

At this point, the difference between hard-boiled and raw eggs becomes important. Like the contents of a raw egg, the liquid portions of the inner Earth are not firmly attached to the solid parts, so the entire core will not accelerate at the same time. As the crust and mantle speed up, friction and electromagnetic forces accelerate the outermost portions of the liquid core, but deeper regions of the core should lag behind for several hundred years, says Muller.

It is this shearing between regions in the outer core that causes the reversals, he proposes. Spin-up "scrambles" the flow patterns and weakens the geomagnetic field. When the entire core finally reaches speed, the field rejuvenates itself. But starting from a weak state, the field has an equal chance of pointing north or south. Not all reversals would occur in this way, but sudden cooling could explain some, Muller says.

So far, the physicists have few data or calculations to prove their ideas, although they say the theory offers several predictions that scientists can test. Other researchers remain extremely skeptical. In particular, some say it will take the entire core only a few years to reach speed.