SUPPOSE the Earth鈥檚 magnetic field gave up and died on us. It would be quite a show: compasses going haywire, migratory birds getting lost, the surface of the planet bombarded with cancer-causing cosmic rays. If you want a taster, try watching Jon Amiel鈥檚 recent movie The Core, in which scientists discover that the Earth鈥檚 inner core has stopped rotating 鈥� though this is Hollywood, so best to take it with a pinch of salt.
Should we be worried? Will the future of the Earth follow the plot of a Hollywood disaster movie? It鈥檚 unlikely, but we can鈥檛 be absolutely sure. It鈥檚 true that the terrestrial magnetic field, generated by electric currents in the swirling molten outer core, is diminishing at an extraordinary rate. Most geologists believe this is just a temporary storm in the core鈥檚 鈥渨eather system鈥�. Let鈥檚 hope so. If the field carries on declining at the same pace, it will have collapsed completely in 2000 years.
What is going on down there? We know surprisingly little about the Earth鈥檚 core. The conventional theory is that it consists of a huge mass of iron and nickel which collected as a hot, molten glob when the planet formed and has been cooling ever since, creating a hard inner ball that grows as the molten layer around it solidifies. So far the inner core is about three-quarters the size of our moon, and we have about 4 billion years left before the entire core freezes over.
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How do we know this? Well, we don鈥檛 for sure. Unlike the heroes in the film version, we cannot go down and sneak a look 鈥� although someone recently proposed that we try, by using a nuclear bomb to blast a crack in the Earth鈥檚 surface and sending down a probe (New 杏吧原创, 17 May, p 20). The trouble with the conventional theory is that it should take just a billion years for a glob of iron and nickel the size of the inner core to cool. Yet the planet has been around for 4.5 billion years.
Any suggestions? Many geologists think radioactive potassium bound up in the rock is heating it up. Then there鈥檚 the more radical idea that the core is a massive nuclear reactor packed full of uranium, generating the Earth鈥檚 magnetic field and helping to power volcanoes. It is astounding that no one has been able to disprove this theory.
Where does all this leave the future of migratory birds and compasses? We know that every 250,000 years or so, the Earth鈥檚 magnetic field declines over a few thousand years, sprouts a handful of poles (usually around the equator) and then springs back into a two-pole state with 鈥渘orth鈥� in either the Arctic or Antarctic.
But no one knows whether the current decline is part of the next flip-around 鈥� or something far less dramatic. Screenwriters, watch this space.