Thursday, January 7, 2010

True polar wander

True polar wander is a phenomenon in which a planet or moon undergoes a solid-body rotation with respect to its spin axis. As this occurs, the geographic locations of the North and South Poles change, or "wander". This can happen when the two larger moments of inertia are near equal.

Description in the context of Earth
The Earth is not a true sphere, and therefore has three orthogonal axes of inertia. The axis around which the moment of inertia is greatest is closely aligned with the rotation axis (the axis going through the North and South Poles). The other two axes are near the equator. This is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when the brick is lying flat). But if the moment of inertia around one of the two axes close to the equator becomes nearly equal to that around the polar axis, then the constraint on the orientation of the object (the Earth) is relaxed.
This situation is like an American football (or a Rugby ball) spinning around an axis running through its "equator". (Note that the "equator" of the ball does not correspond to the equator of the Earth.) Small perturbations can move the football so that it spins around another axis through this same "equator". In the same way, when the conditions are right, the Earth (both the crust and the mantle) can slowly reorient so that a new geographic point moves to the North Pole, while keeping the axis of low moment of inertia quite near the equator.
Such a reorientation changes the latitudes of most points on the Earth, by amounts that depend on how far they are from the axis near the equator that does not move.

Claimed examples
Cases of true polar wander have occurred several times in the course of the Earth's history.
The speed of rotation (around the axis of lowest inertia) is limited to about 1° per million years. Mars, Europa, and Enceladus are also believed to have undergone true pole wander, in the case of Europa by 80°, flipping over almost completely. The crust of Titan has also shifted, though pole wander has not been detected.

Distinctions and delimitations
Polar wander should not be confused with precession, which is where the axis of rotation moves, in other words the North Pole points toward a different star. Precession is caused by the gravitational attraction of the Moon and Sun, and occurs all the time and at a much faster rate than polar wander. It does not result in changes of latitude.
True polar wander has to be distinguished from continental drift, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle.

Geomagnetic reversal
A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established. These events occur on a scale of tens of thousands of years or longer.
More generally, the term may refer to a reversal of the polarity of any magnetosphere.
Scientific opinion is divided on what causes geomagnetic reversals. One theory holds that they are due to events internal to the system that generates the Earth's magnetic field. The other holds that they are due to external events.

Internal events
Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core.
In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.
Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.

External events
Others, such as Richard A. Muller, believe that geomagnetic reversals are not spontaneous processes but rather are triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces resulting from very large impact events. Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that a recovery is seen as a reversal in about half of all cases.
Brief disruptions which do not result in reversal are also known and are called geomagnetic excursions.
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