noted in Chapte

r 1, is that by observing


and asteroids

astronomers can

calculate how often one of a


size is apt to

hit the earth. Note from Table 1, from the work of Shoemaker and his colleagues, that the larger the crater, the more likely that it was formed by a comet. Astronomers and crater scientists usually express the frequency of crater formation as the number of craters larger than 20 km that form during a 100-million-year period on each 10 million km2 of earth surface. Based on his astronomical observations, Shoemaker estimates the rate (for asteroids and comets combined) at 4.9 ± 2.9. In other words, Shoemaker reckons that during the last 100 million years, for each 10 million km2 of earth surface (the earth has a total surface area of 500 km2), 4.9 ± 2.9 (or roughly between 2 and 8) craters larger than 20 km have formed. From his observations of terrestrial craters, Richard Grieve of the Geological Survey of Canada obtains a rate of 5.5 ± 2.7, basically the same number as Shoemaker's. In recent years, Shoemaker and his wife Carolyn, when not searching for comets, were apt to be found camped in the Australian outback, mapping ancient impact craters. It was on such a field trip in July 1997 that he lost his life in a tragic automobile accident. Based on his most recent mapping and age dating, Shoemaker's last estimate of the frequency of impact for the old Australian craters, presented at a conference at the Geological Society of London in February 1997, was 3.8 ± 1.9. For impacts on the moon, including the lunar farside, it is 3.7 ± 0.4. It is simply astounding that all these rates, measured from the back of the moon to the outback, give the same answer.

Reason requires that we acknowledge that meteorite impact has been an inexorable fact of life in our solar system and on our planet. But even if we admit that, it does not tell us why meteorites are so terribly destructive. How can a ball of ice—even one much larger than the snowballs we hurled during our childhood with little effect—create a crater as large as those excavated at sites of nuclear explosions? The answer comes from a fundamental law of physics: Kinetic energy is equal to one-half the mass of an object times the square of its velocity. An object of little mass, if traveling fast enough, can contain a vast amount of energy. Comets and asteroids move at speeds far beyond our common experience—at cosmic velocities in the range of '5 km/sec to 70 km/sec, or 33,500 mph to '57,000 mph. On the average, comets travel at two to three times the speed of asteroids. Even though ice is less dense than rock or metal, because energy varies with the square of velocity and because of their greater speed, comets can do just as much damage as asteroids.

When a comet or an asteroid strikes the solid earth, the energy inherent in its great speed is converted into an enormous shock wave. An impact that creates a crater '0 km wide releases about ' 0 2 5 ergs of energy (' 0 2 5 = '0 followed by 25 zeros; the erg is a standard unit of energy; to lift a pound weight one foot requires '.35 x '07 ergs). The impact that produces a 50-km crater releases about '028 ergs. For comparison, the '980 eruption of Mount St. Helens released 6 x ' 0 2 3 ergs, the '906 San Francisco earthquake about ' 0 24 ergs. The energy budget of the entire earth for one year—from internal heat flow, volcanic activity, and earthquakes— is about '028 ergs. The asteroid envisioned by the Alvarezes would have released almost ' 03' ergs. Because of their great speeds and the inexorable laws of physics, the impact of comets and asteroids releases more energy than any earthly process, placing impact in a destructive class by itself.

During the '960s and '970s, impact cratering came to be seen as a ubiquitous process in our solar system, one in which every solid object has been struck countless times by impactors of all sizes. Some of the projectiles were themselves the size of planets. As knowledge of the scale, frequency, and ubiquity of impact began to spread through the community of geologists, the notion that one might have occurred at the end of the Cretaceous, and even that it might have caused a mass extinction, no longer seemed quite so heretical. The saving grace of the Alvarez theory, and the reason it has proven so useful is that, in contrast to many other explanations of mass extinction, it can be tested. If the Alvarezes are wrong and the theory is false, the evidence would show it.

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