The Comets Did It

Does the wobbling of the earth on its axis cause reversals of the earth's magnetic field? Have these periodic reversals of the field temporarily allowed more cosmic radiation to strike the earth, killing off whole species of plants and animals? Were tektites--small, glassy globules strewn in distinctive patterns on the earth's surface--formed on the moon or on the earth? Impatient with a piecemeal approach to the solution of these major scientific mysteries, two physicists at the University of Birmingham in England have proposed a single phenomenon that could account for them all: the impact of giant comets that have occasionally collided with the earth.

Patiently examining dozens of tiny tektites taken from the ocean floor off the Ivory Coast, Physicists Saeed A. Durrani and Hameed A. Khan found a striking correlation: large numbers of tektites had drifted to the sea bottom at about the same time that the earth's magnetic field is known to have had one of its reversals, some 900,000 years ago. Checking with other research, they found that another major concentration of these microtektites had settled around Australasia during another reversal that took place 700,000 years ago.

Such reversals had already been found to have coincided with the extinction of many species of plants and animals (TIME, Nov. 30). This led Durrani and Khan to speculate about what kind of event could cause the reversals and wreak the other damage at the same time. They concluded that the earth's magnetic field may be so precariously balanced (171 reversals in the past 76 million years) that even a small jolt would be enough to upset it. Such a jolt, they argue in a recent issue of Nature, could easily have been caused by a comet.

Letting their imaginations roam, the authors provide a vivid description of what might follow the catastrophic impact. As the comet's head, or nucleus, struck the earth's surface, tons of molten rock would be hurled thousands of miles, falling and hardening in tektite-like patterns far from the point of collision. At the same time, another spectacular event may well have occurred. The gases contained in the comet nucleus--particularly frozen ammonia and methane--would have spread through the atmosphere and the water, drastically changing the environment for primitive life. Then, in a swirling finale of son et lumiere, atmospheric lightning would have ignited the mixtures of methane and air, filling the primordial skies with vast, spreading explosions.

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