When presenting a theory far outside the mainstream, the first question is whether it is credible. The burden of establishing credibility properly rests with the proposers; if they are unable to do so convincingly, the theory is best let lie. A theory whose credibility has been weighed and found wanting may not have been proven false, but the finding does serve to direct research elsewhere. On the other hand, as we shall see, too often in geology a magisterial authority has made a pronouncement—that the earth can be no more than 20 million years old, that continents cannot move, that few or no terrestrial craters could have been formed by meteorite impact—and later been found to be dead wrong, costing decades of fruitful research. It is important not to pursue every offbeat idea but equally important not to draw conclusions too hastily. Yesterday's offbeat notion has often become today's paradigm. Arthur C. Clarke caught the proper spirit when he said that "if an elderly but distinguished scientist says that something is possible he is almost certainly right, but if he says that it is impossible he is very probably wrong."11
One way for the Alvarezes to test the credibility of their theory was to estimate the size of the alleged impactor. If it turned out to be as large as, say, Mars, or as small as the tiny meteorites that give rise to shooting stars, credibility would be undermined. A meteorite the size of a planet cannot have hit the earth 65 million years ago or all life at the surface would have been eradicated—-nothing would have made it through. At the other extreme, tiny shooting stars burn up in the atmosphere and thus have no effect. To be credible, the size of the putative impactor would have to be much smaller than a planet and much larger than a shooting star.
The size of the alleged K-T meteorite could no longer be measured directly—the impact explosion would have blasted it to pieces. But what of the residue it might have left behind, the iridium in the Gubbio clay layer? If the impact event had worldwide effects, approximately the same amount of iridium as found at Gubbio would have been deposited in a layer that extended all around the earth, coating its entire surface. Knowing both the amount of iridium in the Gubbio clays and the size of the surface area of the earth, the Alvarez team calculated that about 200,000 tons of iridium had been emplaced. Since they knew the average iridium content of meteorites, they were then able to figure out how large a meteorite would have been required to deliver that much iridium. Using reasonable assumptions as to density and shape, the answer was a meteorite about 6.6 km in diameter. Applying the same technique to the Danish clays gave about 14 km. That the two estimates agreed within about a factor of 2 was encouraging at this rough level of calculation. Averaging them gave 10 ± 4 km, neither as large as a planet nor as small as the pip-squeaks that produce shooting stars, and well within the credible range. The figure of 10 km has become accepted as the diameter of the Alvarez impactor. That happens to be about the elevation of Mt. Everest, the earth's highest mountain. Imagine that Everest, instead of standing above the already lofty Himalayan plateau, rose straight from the sea to its height of over 29,000 feet. Now imagine a mountain of that size approaching the earth at a speed of 100,000 miles per hour. No thanks!
The Alvarezes next compared a meteorite 10 km in diameter with three observational facts:
I . Asteroids (solid rocklike meteorites] and comets (balls of dirty ice), either of which could have produced the impact, in the range of 5 km to 10 km in diameter are relatively plentiful in space and are routinely observed through telescopes.
2. Estimates based only on astronomical observations show that an asteroid or a comet 10 km in diameter should strike the earth about every 100 million years, so having one hit 65 million years ago but none since would fit the observations (see Table 1).
3. Over 150 terrestrial impact craters are known; from their size and frequency, crater experts estimate that a 10-km object strikes about every '00 million years. This conclusion, based only on known craters, is completely independent of the one based on astronomy, yet it gives the identical result.
Thus the Alvarez impact theory described an event that is rare but that does occasionally take place and when it does, must produce large-scale effects. Even though in the early '980s geologists were still coming to understand the role of impact in the history of the solar system, the Alvarez theory was within the range of what was known and observable. It clearly passed the credibility test and needed to be taken seriously.
The theory itself consisted of two parts: first, that a meteorite struck the earth 65 million years ago, and second, that the effects thus produced were so severe that they led to the K-T mass extinction. Unfortunately for their theory, but fortunately for Homo sapiens, it is not easy to test the second part, for no large meteorite has struck in the minute fraction of geologic time recorded by human history.
One approach to the problem of verifying the theory's second claim is through computer modeling. In '983, influenced by the Alvarez theory, a group of scientists that included the late Carl Sagan used computer models to show how a nuclear war in which fewer than half of the combined number of warheads then available to the United States and the Soviet Union were exploded would throw enough dust, smoke, and soot into the atmosphere to block sunlight for several months, particularly in the Northern Hemisphere. This might set in motion the same sequence of events as predicted by the Alvarezes (lowering temperatures by tens of degrees, halting photosynthesis, destroying plant life, and disrupting the food chain). The ozone layer might also be affected, allowing the sun's ultraviolet radiation to penetrate and cause further damage. Their paper, which appeared in Science, concluded that nuclear war would have so few survivors, if any, that it would produce another great extinction—this time, possibly of Homo sapiens." The threat of nuclear winter caught the attention of the world and may have been influential in halting the growth of nuclear weapons and ending the Cold War.
Though it would not include deadly radioactive fallout, cosmic winter would be far worse than nuclear winter. The impact of a '0-km meteorite would release a vastly greater amount of energy than Krakatoa, which caused the death of 35,000 people. It would do far more damage than the atomic bomb that was dropped on Hiroshima, which had the energy equivalent of about '3 kilotons of TNT. Traveling at 25 km/sec or more, the mountain-sized meteorite would strike the earth with the force of 100 million megatons of TNT (1014 tons; 10 followed by 14 zeros), more than 7 billion times as much energy as the bomb dropped on Hiroshima—in fact, vastly more energy than the explosion of all of the 60,000 nuclear weapons that existed at the height of the Cold War. To comprehend the power of meteorite impact, try to imagine the simultaneous explosion of 7 billion bombs like the one dropped on Hiroshima— one for every person on earth and 10 for every square kilometer of the earth's surface. The terrorist bomb that destroyed the Alfred P. Murrah Federal Building in Oklahoma City in 1995 had an energy equivalent measured not in megatons, not in kilotons, but in tons— 2.5 tons. The K-T impact was 40 trillion times larger. The dinosaurs, concluded the Alvarezes, never had a chance.
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