Number of words: 545

Much of the evidence for the Alvarez idea involves an unusual layer of clay that separates two periods in geologic history: the Cretaceous and the tertiary. This boundary not only contains the last burial ground of the dinosaurs but an unusually high concentration of iridium, that is however much more abundant in this layer than in the earth’s crust. Walter Alwarez amazed everyone with his discovery that the bottom of this clay layer contains 300 times more iridium than the rest of the earth’s crust. This strongly pointed to an extra terrestrial event.

To explain this mystery, the Alwarez team of researchers proposed the following scenario. An object measuring about six miles across struck 65 million years ago. The material from the explosion enveloped the planet in a blanket of dust. This dust fell into the ground and into the ocean and produced this clay layer in the geologic record. The settling dust would also have cut out sunlight, halted photosynthesis, and caused much of life on the earth to die of starvation. The lack of sunlight might also have changed the earth causing yet more deaths. But perhaps the most important killing mechanism was due to the initial shock and heating of the atmosphere produced by the expanding fireball. This fireball would have caused unusual chemical reactions in the atmosphere and produced a deadly acid rain.

Geologists have now found three periods of iridium enhancement in the geologic record. This one 65 million years ago, another 38 million years ago and the last just 11 million years ago. These dates suggest that something deadly occurred with a surprising regularity every 26 million years or so. A similar periodicity appeared in the work of to University of Chicago paleontologists David Raup and Jack Sepkloski. Their test collection of observations of ancient marine life in rocks shows that 8 major episodes of extinction have occurred in the past 250 million years, each one about 26 million years apart. Additional evidence for the extra-terrestrial connection comes from the work of Michael Rampino and Richard Stothers of the NASA Goddard institute for space studies in New York city. They show that large terrestrial craters have been produced at nearly the same intervals. Evidently something from space has been killing off life on the earth with striking regularity.

The Alwarez idea that something crashed into the Earth 65 million years ago was a good one. But it did not fit all the facts. The recurrence of extinctions at other periods in time suggest the long-term effects of showers of objects such as comets rather than a single object. It is difficult to believe that they would get stuck by one large rock on a regular time schedule. Besides paleontologists examining the Fossil records have noted that the period of extinction did not occur suddenly but gradually; they took place over several billion years. And within these several million-year periods, they have identified separate periods of extinction,a phenomenon they call stepwise extinctions. Only a shower of large objects over time could produce such an effect. A shower of comets can occur only when a reservoir of comets is disturbed. The culprit can be either a star or a planet.

Excerpted from pages 169-170 of ‘The Big Splash’ by Louis Frank