Magnetic Shield

Deep within the inner-core regions of the Earth, the temperature and pressures are sufficiently high that iron is rendered into its liquid form. Twisted and churned by convection and rotation, the molten iron acts just like a dynamo and a magnetic field is generated. While the magnetic poles are known to wander and drift across the Earth's surface, and have been known historically to flip orientations (north becoming south), the geomagnetic field provides an invisible reference system.

Darwinian selection has not failed to notice and utilize the Earth's magnetic field, and many birds, insects, and bacteria use it for navigation and orientation purposes. Indeed, the ancient mariners realized long ago that the mysterious loadstone always pointed along the north-south meridian, and this knowledge enabled them to steer (as safely as the seas would allow them) to their destinations (see Figure 4.9). Not only is the existence of the Earth's magnetic field betrayed by the motion (or, in fact, the lack of it) of the compass needle, it is also betrayed through the rhythmical dance of auroral curtains and streamers (Figure 4.10). The aurora signals, in fact, an interaction between the Sun and Earth, with Earth's magnetic field controlling the motion of charged particles (mainly protons, electrons, and helium nuclei) blasted out from the Sun by solar flares and coronal mass ejections.

Not only does the Earth's magnetic field protect life from the direct scouring of the solar wind (lunar inhabitants will have no such natural protection), but it also shields us from cosmic rays, which are charged particles produced by distant supernovae. These particles encounter the Earth at speeds close to that of light.

The tenuous atmosphere, that thin veneer of gases that sits on top of the Earth's surface, and separates us from the killing vacuum and cold of space, along with the invisible tendrils of the geomagnetic field, are both essential to our existence. Humanity does not need to maintain or make adjustments to them. Here is the lesson for the would-be terraforming engineer: the aim should be to emulate nature, as it is found here on the Earth, rather than work against it. What we need to do is understand the terrestrial cycles and interactions, the positive and negative feedback processes (to be

Figure 4.9. Frontispiece of William Gilbert's classic text De Magnete, published in 1628. A crude map showing the orientation (dip) of the Earth's magnetic field lines is seen in the upper-left-hand corner, while an illustration of the loadstone's application to navigation is shown at the bottom center.

Figure 4.9. Frontispiece of William Gilbert's classic text De Magnete, published in 1628. A crude map showing the orientation (dip) of the Earth's magnetic field lines is seen in the upper-left-hand corner, while an illustration of the loadstone's application to navigation is shown at the bottom center.

discussed in the next chapter) and re-create them as far as is possible on new worlds.

This will not be easy; some of the processes cannot be directly transported to other planets. But we need not invent the conditions necessary for life to thrive. We need only understand and emulate what 4.5 billion years of interaction and evolution on the Earth have already uncovered. This is not to say, of course, that the task is simply one of engineering on a massive scale. The complexities are manifold, and we still have a great deal to learn about the workings of the Earth.

Figure 4.10. The aurora Australis as observed from the space shuttle. Auroral displays occur at about 80-km altitudes, and are the result of light emission from nitrogen and oxygen atoms that have been excited by collisions with charged solar particles that have been channeled into the north and south polar regions by the Earth's magnetic field. Image courtesy of NASA.

Figure 4.10. The aurora Australis as observed from the space shuttle. Auroral displays occur at about 80-km altitudes, and are the result of light emission from nitrogen and oxygen atoms that have been excited by collisions with charged solar particles that have been channeled into the north and south polar regions by the Earth's magnetic field. Image courtesy of NASA.

Up to this point our discussion has been directed toward the understanding of what the Earth is, recognizing its boundaries, enumerating its characteristics, and determining its place within the Solar System. Our focus now shifts to the dominant and most technically advanced animal on Earth: us, Homo sapiens (from the Latin ''wise man'' or ''knowing man'').

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