Earth Design Update:
One Amazing Dynamo

By Hugh Ross

Depending on your schooling in earth sciences, you may or may not know that a huge electrical generator, a dynamo, exists at Earth's core. If you've ever visited an electrical generating plant, such as the one at Hoover Dam, you have a clue, at least, how this dynamo, or power generator, operates. Instead of being whirled by rushing, gushing water, however, this one is whirled by Earth's rotation.

If the dynamos at a power plant fail, customers suffer. If Earth's dynamo fails, all but a few primitive life forms die. They (we) would get zapped by deadly solar (and a little cosmic) radiation. The stable, powerful dynamo at Earth's core produces the magnetic field surrounding Earth. This field could be called a magnetic shield, for it traps dangerous rays and dissipates their energy. You may know this shield by its proper name, the Van Allen Belt.

The existence of a dynamo is not completely surprising. A dynamo requires only three components: 1) a fluid capable of conducting electricity, 2) a means of circulating that fluid, and 3) adequate rotation to organize the fluid motions.(1) (Mercury, Jupiter, Io, and Gannymede all have detectable dynamos; Mars has an extremely weak one.) Evidence for deliberate design arises from the delicacy of the dynamo's power output and long-term stability. If Earth's dynamo were just a little weaker, the magnetic field it generates would be inadequate to shield life. (Too strong is less of a problem. If it were much, much stronger, it would cause deadly magnetic storms.) Additionally, the dynamo must have some means of minimizing the effects of energy degradation through time (energy loss cause by caused by electrical resistance, technically referred to as ohmic decay).

The crucial factors include 1) the fluid components, which determine conductivity, 2) their circulation method, which cannot be erratic, and 3) their viscosity, which must be very low. (Peanut butter has high viscosity, water has low viscosity, for example.) As it so happens, Earth's core has an deal set of components-iron, with a little cobalt and nickel-in an ideal volume and form-the inner part solid, the outer part liquid-and a remarkably small viscosity. In fact, a recent computer model produced by two Harvard University geophysicist suggests that "free slip" (virtually no resistance) conditions apparently exist at the place where the solid core meets the liquid core and where the liquid core meets the semi-solid layer, or "mantle," around it.(2)

This model is the first one produced that matches all the measured characteristics of the Earth's core and magnetic field. It even predicted that the inner core should over-rotate by a few degrees a year. Seismic measurements made just a few months ago reveal that indeed Earth's core rotates faster, by 1 to 3 degrees per year, than does the mantle.(3-4)

This just-right dynamo again brings us to the question, "Did Earth just get lucky (again!), or did Someone know what life needed and provide it?" What answer makes sense to you?

References:

1. Peter Olson, "Probing Earth's Dynamo," Nature, 389 (1997), p. 337.
2. Weiji Juang and Jeremy Bloxham, "An Earth-Like Numerical Dynamo Model," Nature, 389 (1997), pp. 371-174.
3. Xiaodong Song and Paul G. Richards, "Seismological Evidence for Differential Rotation of the Earth's Inner Core," Nature, 382 (1997), pp. 221-224.
4. Wei-jia Su, Adam M. Dziewonski, and Raymond Jeanloz, "Planet Within a Planet: Rotation of the Inner core of the Earth," Science, 274 (1996), pp. 1883-1887.


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