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To try to understand whether and how the Alvarez theory might help to explain the selectivity of the K-T extinction, we need to know what would happen when a 10-km to 15-km meteorite strikes the earth. The two halves of the Alvarez theory—that impact occurred and that it caused the mass extinction—are linked by the assumption that the resulting effects would be sufficiently lethal to cause the death of 70 percent of all species. The Alvarez team had precious little evidence for this assumption; indeed, to gain some idea of the effect of a global dust cloud, Luis had to rely on the century-old Krakatoa report of the Royal Society. But over the last couple of decades, the science of computer modeling of impact explosions has made great strides, such that it is now possible to say more confidently what the actual effects would be, though not how they would all interact with each other and with living organisms. (The discussion in the next dozen paragraphs is taken largely from the work of modeler Brian Toon and his colleagues.62) To take the subject from theory to practice, in July 1994 the entire world saw an actual planetary impact when the fragments of Comet Shoemaker-Levy 9, some estimated to be 2 km in diameter, collided with Jupiter. To the delight of Gene Shoemaker, the effects were even more spectacular than the impact modelers had predicted.

According to the Alvarez theory, 65 million years ago a comet or asteroid 10 km to 15 km in diameter approached the earth (we do not know which it was, but either would have had the effects I am about to describe). It was traveling at cosmic speeds somewhere between 20 km and 70 km per second and for that reason carried with it an energy on the order of 1031 ergs, or 100 million megatons of TNT (100,000,000,000,000 tons of TNT), far more energy than contained in all the world's nuclear weapons at the height of the Cold War. Once the object struck, that amount of energy had to be dissipated. An almost irresistible force was about to meet an immovable object.

As we saw when Shoemaker-Levy 9 struck Jupiter, a meteorite entering a planetary atmosphere at cosmic velocities generates a giant shock wave—a kind of cosmic backfire—that sends a 20,000-degree jet of flame thousands of kilometers back up the incoming trajectory. In the largest impacts, the entire atmosphere in the vicinity of the entry point is blasted into space.

The midair explosion of a meteorite at Tunguska in Siberia in 1908 and the eruption of Mount St. Helens in 1980 were strong enough to level trees for miles around. The K-T impact event released an amount of energy millions of times greater than these relative pip-squeaks. The resulting shock wave leveled everything standing within thousands of kilometers of ground zero, providing fuel for the subsequent fires.

Sixty-five million years ago, the Yucatan Peninsula was an area of shallow sea, so that the meteorite probably landed in less than 100 m of seawater. Modeling indicates that the resulting earthquake caused submarine landslides that displaced huge volumes of seawater and generated a tidal wave that dwarfed even the most devastating in human history. Traveling outward at about 0.5 km/sec, like the ripples from a stone hurled by a giant, this ancient tsunami rose to a height of 100 m and rolled inexorably across the oceans. Hardly slowing as it went ashore, it traveled inland for 20 km, inundating the coastal plains on half the globe.

As the meteorite penetrated deeper into the earth, a huge shock wave converted it and the rock underneath into vapor and ejected them outward at ballistic velocities. Some 100 km3 of excavated rock and 1014 tons of vaporized comet or asteroid rose to altitudes as high as 100 km. Much of this debris quickly fell back to earth, but 10 percent to 20 percent of it remained at high altitudes for months. The temperature at ground zero rose to hundreds of thousands of degrees, causing everything within a radius of several hundred kilometers to burst into flame. The expanding fireball rose quickly and within only a few hours had distributed itself around the earth. Meanwhile, the shock wave had excavated a crater 15 km to 20 km deep and at least 170 km in diameter. The impact generated an earthquake of magnitude 12 to 13, a temblor at least 1,000 times larger than any humans have ever experienced. Even 1,000 km from ground zero, the earth's surface heaved in waves hundreds of meters high.

A few minutes later, the mixture of vaporized meteorite and rock, still traveling at ballistic velocities of 5 km/sec to 10 km/sec, began to reenter the atmosphere. The individual globules were traveling so fast that they ignited, producing a literal rain of fire. Over the entire globe, successively later the greater the distance from the target, the lower atmosphere burst into a wall of flame, igniting everything below. The effect was like "a domestic oven set at 'broil'."63 Everything that could burn did.

Smoke and soot rose to mingle with the huge number of fine particles that the explosion had carried into the stratosphere. Together they darkened the earth enough to cause the average global temperature to fall to the freezing point. Darkness came at noon, and remained for months. Photosynthesis halted and the food chain that depended upon it ceased to function.

The blast wave acted as a chemical catalyst, causing atoms of oxygen and nitrogen to combine to form various noxious compounds, many found in today's smog. Sulfur oxides joined them, for in a coincidence unfortunate for life at the end of the Cretaceous, the Yucatan rocks at ground zero included sulfate deposits. As happened in the modern eruptions of Pinatubo and El Chichon, sulfur dioxide formed tiny droplets that further obscured the sun and lowered visibility even more. Kevin Pope, Kevin Baines, and Adriana Ocampo have calculated that the impact into the sulfur-rich deposits of the Yucatan would have produced over 200 billion tons of both sulfur dioxide and of water, leading to a decade-long impact winter.64

As precipitation washed out the nitrogen and sulfur compounds, it generated acid rain that may have destroyed the remaining susceptible plants. Gregory Retallack of the University of Oregon65 has found evidence in the boundary clay in Montana of severe acid leaching, possibly enough to have dispersed the iridium and dissolved the shocked minerals and spherules. Thus, Retallack says, some impacts might be "self-cleaning," eliminating traces of their own existence. Because some soils naturally buffer acids and others do not, acid rain might also explain some of the K-T extinction selectivity. For example, the floodplains of ancient Montana would have remained above a pH of 4, which according to Retallack would spare small mammals, amphibians, and fish, but harm plants, nonmarine mollusks, and dinosaurs. Acid-vulnerable plants such as the broadleaf evergreens would have suffered, whereas the acid-tolerating plants would have done better, more or less consistent with the evidence.

The rain may have acidified the surface layers of the oceans sufficiently to kill the surface-dwelling plankton and phytoplankton, which would have caused a breakdown of the oceanic food chain that was based upon them. The reactions that formed nitrogen oxides also absorbed ozone, reducing the earth's protective ozone layer and allowing ultraviolet radiation to penetrate to the surface, causing further loss of life.

Some of the vast amount of water vapor that was blasted into the atmosphere froze; the rest formed a vapor cloud that lasted for years. There it was joined by the most insidious long-term effect of the impact—a worldwide cover of carbon dioxide, generated by impact into the thick limestones (calcium carbonate) that also were present in the Yucatan Peninsula of that day. Just when the dust, smoke, and soot had dissipated and conditions might have returned to near normal, this gas cloud produced a greenhouse effect that lasted for a thousand years or more. Those creatures that had miraculously survived all that came before, now faced a millennium of greenhouse temperatures. (Recent modeling by Pope and Ocampo, however, indicates that the greenhouse effect might not have been this strong.)

Obliterating shock waves, stupendous earthquakes, enormous tsunami, a rain of fire, smoke, soot, darkness, a global deep freeze, worldwide acid rain, ozone loss, greenhouse warming—it seems a miracle that anything could have survived, and yet, remember our thought experiment on just how difficult it is to exterminate an entire species. Over 99.99% of individuals can die and enough breeding pairs might be left alive to allow the species to survive. But certainly no one can claim that the impact of a 10-km meteorite in the Yucatan Peninsula 65 million years ago lacked the power to cause the K-T mass extinction.

We know that a 100-million-megaton impact happened at K-T time; we know that it must have had some combination of the effects just described. What we do not know is just how the many lethal possibilities would have interacted with each other and with living organisms. These questions will occupy impact modelers, geo-chemists, paleontologists, and others, for years. Meanwhile, some paleontologists, though now objectively required to admit that impact happened, remain unwilling to grant that it had anything to do with extinction until "precise biological/ecological mechanisms are proposed that uniquely account for observed taxic patterns and the stratigraphic timing of K-T extinction and survivorship."" The clear implication is that the burden of proof still rests entirely with the pro-impactors: They must explain how the impact effects killed certain species and spared others. But the existence of the Chicxulub crater shifts the burden. Since we know that impact occurred, those who deny that it caused the mass extinction have just as much of an obligation to explain how species escaped as those who support the link between impact and extinction do to explain how they did not.

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