More than 99 percent of the species that have lived on the Earth are extinct. Extinction is a part of biological evolution, just as death is a part of an individual's life. Roughly speaking, for every new species that appears, one disappears from the world. However, as University of Chicago paleontologist David Raup has noted, naturalists have usually concentrated on the origin of species rather than on their extinction. Darwin's famous book is The Origin of Species, not The Origin and Extinction of Species. Dramatic changes displayed in the fossil record prove that the formation of new species and extinction of old ones are discontinuous processes. In this chapter we focus on the 1980 discovery that an impact triggered the mass extinction of 65 million years ago, in which three quarters of all species (including the dinosaurs) went extinct. This simple suggestion has brought about the biggest change in our view of evolution since Darwin, and we explore its many ramifications. We also turn to the conflicts these ideas generated among scientists, recounting in detail the scientific meeting (in 1981) in which biologists, geologists, astronomers, and physicists first confronted each other to debate the role of impacts in mass extinctions. Although most of the scientific community was soon converted to the idea of impact catastrophism, a few hold out even today against these revolutionary ideas.
CHAPTER 5: THE SMOKING GUN
Needs opening quote
The Spanish conquerors of Mexico arrived when the great Mayan civilization was in decline. In the Yucatan region, Mayans lived in small villages, and their great city, Chichén Itzá, was being reclaimed by the jungle. The fabled metropolis had been built in the 6th century A.D. near the only source of water in the region, two huge natural wells. "Chichén" in Mayan means "mouths of wells." From the same word is derived the term cenote, which refers to these and numerous other sink holes in the Yucatan formed by the collapse of underlying limestones.
The sixteenth-century Mayans regarded Chichén Itzá as a sacred place and told their conquerors of the cult of the cenote, in which sacrifices of human beings and valuables were made to the goddess of rain, thought to reside in the wells. Archaeologists have since dredged the cenotes and confirmed the legends. Yet only recently has Earth-orbiting satellite photography revealed a larger pattern to the cenotes. Many lie almost precisely along an immense, semi-circular arc, more than a hundred miles long. Most cenotes in the Yucatan, including those at Chichén Itzá, lie outside the ring. A map of the Yucatan shows that inside the cenote ring is a remarkably circular province, nearly 200 kilometers across, that is virtually devoid of cenotes. That is Chicxulub.
It is wondrous to realize that cenotes, these central features of Mayan life, are the only modern-day surface manifestation of one of the most stupendous events to happen on Earth in the last 600 million years. The ring of cenotes, we now realize, marks the outline of the crater that provided the proponents of the Alvarez hypothesis with their smoking gun. The crater is named Chicxulub for the Mayan village Puerto Chicxulub located near its center. Although Chicxulub is one of the largest impact scars on Earth, it was not easy to locate. Even satellite photos reveal nothing except the enigmatic ring of cenotes. Nor would such an impact, the equivalent of hundreds of millions of megatons of TNT, affect our planet as a whole. As a world, planet Earth would be left very much intact. There would be no change in its orbit, nor would its axis of rotation be tipped. The fossil changes at the K-T boundary arose not from any damage to the bulk of our planet, but from damage to that precious, thin, supersensitive layer of water and air that we call the ecosphere. After the publication of the Alvarez paper and the intense public interest in the "dinosaur-killing impact", several geologists began looking for the crater, and the scientific sleuthing began.
They could hardly be sure that the K-T crater still existed. A few large impact structures have been found in the older centers of continents, such as the Sudbury Basin and the Vredefordt Ring discussed in Chapter 3. However, eastern Canada and the Transvaal are among the most ancient and stable parts of the Earth's crust, like the Australian Outback. These regions have escaped much of the wrenching episodes of mountain-building, faulting, and volcanism that continually reshape the margins of the continents and other boundaries of Earth's ever shifting crustal plates. If the K-T boundary impact had struck near a continental boundary -- or, worse, on a part of the spreading ocean floor since consumed within the Earth -- there would be no chance of finding the impact scar. Some geologists were not deterred. When a murder has been committed, any good prosecutor knows that a jury may be hard to convince without a smoking gun.
By the mid-1980's, researchers were beginning to find evidence about where the K-T impact might have occurred. Some chemical clues from the boundary layer implicated the oceanic crust. Other evidence, such as impact-shocked grains of continental minerals, pointed to an impact on land. Some thought that the projectile hit on a continental margin, where continental and oceanic rocks were adjacent or overlapped each other. But such a location reduced the chances for finding a crater, since continental margins, more often than not, are geological war zones where the Earth's crustal plates grind into each other, changing topography even in our lifetime (as along the San Andreas fault in California).
One searcher was a University of Arizona graduate student, Alan Hildebrand. A Canadian, he had already spent years as a field geologist before he returned to academic studies in planetary science. Perhaps, as a geologist from a country where many of the world's largest and most ancient impact craters are preserved, he had a more open mind concerning impact as a geological process than geologists from elsewhere. In any case, Hildebrand began a hot pursuit for the K-T boundary crater.
Some scientists from the U.S. Geological Survey had focused on the southern part of North America (including Central America and the Caribbean) as a likely site for the K-T impact because of the prevalence of large shocked mineral grains in K-T boundary exposures in that general area. By 1988, Hildebrand had zeroed in on the Caribbean; he reported at the second Snowbird conference that he was intrigued by a basin northeast of Central America. The Geological Survey scientists shortly afterwards suggested western Cuba as a possible impact site, although few American geologists had visited Cuba during the preceding quarter century of hostility between the countries.
One of the most significant talks at the 1988 Snowbird conference was by Joanne Bourgeois, an expert on the geological effects of tsunamis. The Japanese word tsunami is the term scientists use for so-called tidal waves -- those shallow swells in the ocean surface that become towering, devastating waves when they run ashore. Elaborate alarm systems, including beachside sirens, warn residents of Hawaii and other coastal regions to seek higher ground when far-flung ocean detectors sense an onrushing tsunami. Most tsunamis are created when earthquakes joggle the ocean floor. But an occasional tidal wave, we are realizing, may result from an errant asteroid striking the ocean.
Bourgeois had studied ancient tsunami deposits in the geological record. She had also travelled to Chile and elsewhere to look at the effects of recent disasters. She had found evidence, from a K-T boundary region exposed at the Brazos River in Texas, of enormous tsunami deposits indicating waves 50 to 100 meters high. They meant that huge waves had sloshed back and forth in the Caribbean basin one day 65 million years ago. Tidal waves propagate poorly beyond the body of water in which they are generated, so it became clear that the K-T boundary impact must have occurred in the region we now call the Gulf of Mexico.
Alan Hildebrand was not the first K-T boundary researcher to focus on the Yucatan peninsula, but he was the first to promote it seriously as the K-T impact site. Hildebrand's determination was fueled by pursuit of his Ph.D. degree and by a race with his U.S. Geological Survey competition and other researchers who were also scouring the Caribbean for a crater. After several trips south of the border, gathering geological evidence about the impact, he finally found crushed rock samples in the Yucatan that contained shocked grains of quartz, proof of a nearby impact. It was Hildebrand who named the buried crater Chicxulub.
Hildebrand announced his conclusion that Chicxulub was the missing crater at the Geological Society of America annual meeting in 1990. It is an ancient crater at least 200 km across, buried beneath more than a kilometer of limestone sediment. Impact melt rocks, found in cores drilled through the sediments into Chicxulub, soon showed the crater to have the same age (65 million years) as the K-T boundary. Not only that, but the unusual chemistry of the rocks in which it is embedded matches that measured for the spherules and other ejecta deposits located in nearby Caribbean exposures of the K-T boundary, such as in Haiti. The fact that Chicxulub ejecta was found precisely at the K-T boundary clinched the relationship between the crater and the extinction: no age measurements by radioactive techniques was needed. Finally, melt rock from boreholes into the crater was found to be rich in iridium, the tell-tale signature of an extraterrestrial source.
A traveler to the Yucatan can see no hint of the buried crater, except for the cenotes. Today, the Yucatan coastline cuts across its center, but over its history it has sometimes been completely submerged or completely on dry land, depending on encroachments and recessions of the sea due to modest variations in sea level (the coastal margin is very shallow). To map out the crater, geologists had to probe beneath the sediments that had periodically been deposited on top of the crater.
Given the psychological importance of a "smoking gun," a skeptic might wonder why the world's largest crater was found only after the Alvarez hypothesis suggested that it might exist. Actually, Chicxulub was found three decades earlier -- found and then forgotten, in a strange tale of missed opportunities.
Much as we like to suppose that scientists work for the sake of pure knowledge, most research is funded for eminently practical reasons. The Mexican government-run petroleum company, Pemex, has long supported geological studies of Mexico to search for oil. Chicxulub was first discovered as a spin-off of oil-exploration research. About 1950 Mexican geoscientists, collaborating with the Houston geophysicist Glen Penfield, discovered an anomalous region on the northern coast of the Yucatan peninsula -- a region in which both the detailed direction of the local gravity and the strength of the magnetic field were different from surrounding areas. Though mentioned from time to time in the geological literature through the 1970's, the anomaly's significance lay dormant until 1981. In the mean time, Chicxulub began rewarding the company that funded its discovery. Subterranean rocks smashed by the great impact have held vast quantities of oil. Chicxulub may be responsible for more than a quarter of the oil reserves that have enabled Mexico to become one of the major petroleum producing countries in the world.
Glen Penfield remained intrigued by the possible crater in the Yucatan. When the Alvarez paper was published, he wondered if there was a connection. At the 51st annual international meeting of the Society of Exploration Geophysicists, Penfield collaborated with Mexican petroleum geologist Antonio Camargo to propose that the anomalies in the Earth's magnetic and gravity fields could be best explained as the subterranean signature of an enormous impact crater. The paper might have been ignored, because no one with a professional interest in impact craters or the K-T extinction heard Penfield's talk. Academic scientists involved in the K-T extinction debate, like Gene Shoemaker and Walter Alvarez, rarely attend meetings of their oil-exploring colleagues, who normally would not be studying impact craters. However, a Houston science writer who was familiar with the Alvarez work -- which had been regularly reported at scientific meetings on his beat, at NASA's Johnson Space Center in Houston -- wrote a news item about the purported Mexican crater. This story stimulated Sky & Telescope, a magazine widely read by astronomers, to publish an interview with Penfield. Soon news that there might be a large crater in the Yucatan began to circulate among the geologists studying the then-new Alvarez hypothesis.
But Chicxulub was prematurely discounted. Walter Alvarez himself considered the limited evidence about the buried crater, but he soon accepted the viewpoint of his geologist colleagues -- like Buck Sharpton, of the Lunar and Planetary Institute in Houston -- that the Yucatan structure was of volcanic origin. It is easy, with hindsight, to say that Alvarez, Sharpton, and others screwed up. But the reality of science is that one must pursue the most rewarding clues. There are too few geologists and too little time to study everything. Very little was known in 1981 about the buried Mexican crater, and more interesting K-T impact candidates were under study, including a region near the Bering Sea and another near Iceland. Besides, all of the drill-core samples from Chicxulub were thought to have been destroyed by a fire that gutted the building in which they were stored, precluding study of Chicxulub without a major new expedition.
Thus the Yucatan crater slipped through the fingers of the K-T geologists for seven more years, until other evidence shifted attention back toward the Gulf of Mexico as the most likely site for the K-T impact. But how to check the Penfield idea that there was a large crater in the Yucatan? If only the critical rocks had not been destroyed in that unfortunate fire! Finally, Alan Hildebrand found bore-hole samples that had survived the fire and were lying forgotten in a dusty desk drawer in New Orleans. These rocks contained impact-melted (and iridium-enriched) rock as well as fragments of shocked quartz, providing the critical evidence needed to clinch the identification of Chicxulub as an impact crater. In addition, the layering of the cratering ejecta confirmed that the impact was the source of the K-T boundary clay, and this temporal coincidence of the crater with the extinction clearly vindicated the impact extinction hypothesis.
How big is the Chicxulub crater? We would certainly like to know, because then we would know how large an impact must be to produce the devastating mass extinctions that ended the Cretaceous epoch. But we can't just go and measure the diameter of the cenote ring. Pictures of the Moon and other planets tell us that the largest impacts don't produce simple craters with unique rims. Instead, the rebounding waves produced by the impact freeze into a "multiring basin," with concentric rings inside the main crater rim and a succession of circular scarps outside, extending far beyond the crater rim, like a giant bulls-eye. While the primary rim is evident in pictures of an uneroded basin on the Moon's surface, it is far more difficult to decipher, from geophysical measurements of topography buried beneath hundreds of meters of subsequent rock deposits, just which circular outline marks the true rim.
Lately a debate has raged between Alan Hildebrand, who has his doctorate and is back in Ottawa working for the Canadian Geologic Survey, and Houston's Buck Sharpton. Sharpton belatedly agreed that Chicxulub is an impact structure, not a volcano. Not only was he convinced, he soon reported evidence in Science magazine indicating that Chicxulub was not just 200 km across, but about 300 km! If true, Chicxulub is so big that it could be the very biggest crater to have formed on the Earth in the last billion years, not merely the biggest in the last hundred million. That would make the K-T boundary event an exceptional event, indeed, in Earth's history. However, Hildebrand continues to argue for the 200-km crater size, and he appears to us to have a slight edge in interpreting the existing evidence. New results by a third team of geologists, reported in 1997, seem to indicate a diameter of 250 km, right between the values favored by Hildebrand and Sharpton.
* * * * * * *
In 1994, historian-of-science William Glen published a book on the mass extinction debates. He nicely summarized the evidence about Chicxulub, but he also included chapters by paleontologists who continue to argue that whatever may have happened in the Yucatan 65 million years ago had little to do with the K-T mass extinction. One contributor to Glen's book, Florida paleontologist John Briggs, even claims that there are no such things as "mass extinctions" at all. They are artifacts, he believes, of the lack of uniformity in the depositional history recorded in rock strata.
One might imagine that the multi-disciplinary detective work described in this chapter, identifying the Chicxulub crater and linking it unambiguously to the K-T boundary layer, would constitute irrefutable proof that an impact triggered the mass extinction 65 million years ago. We certainly consider the mystery solved, and the case closed. But science is not really the wholly logical endeavor it pretends to be. Instead, it is a thoroughly human enterprise. The training and beliefs of a geologist acquired throughout a lifetime career are hard to negate. So the news media are correct when they report that the K-T debate isn't wholly finished. Consensus has been achieved, but not unanimity; a few hold-out jurors remain. Meanwhile, the rest of the scientific community is moving on to ask just how -- not whether -- the Chicxulub impact wiped out species around the globe.
2750 words (5/5/97)
CHAPTER 5: THE SMOKING GUN SUMMARY
As the 1980s drew to a close, the biggest problem faced by the impact hypothesis for the extinction of the dinosaurs was the absence of a specific crater associated with this event of 65 million years ago. The scientific detective story of the successful search for this crater is the topic of Chapter 5. It is a convoluted tale, with several false starts, crucial data lost and then relocated, and ultimately a journalist who bridged the gap between different scientific communities to close in on the crater itself, called Chicxulub. Once located, this crater turned out to be the largest impact structure on Earth, and today there is no question of its association with the mass extinction that ended the Cretaceous period. There is still an ongoing scientific debate concerning the size of the crater, however, which is the subject of the concluding paragraphs of this chapter.
With Chapter 5 we end the first part of this book, devoted to the discovery that impacts have been important for the geological and biological history of our planet. Next we turn to the issue of what actually happens in a giant cosmic impact.
CHAPTER 6: FIRE AND ICE DM draft (2/15/97)
[In the K-T impact,] a world-immolating fire burned vegetation to a crisp all over the planet; a stratospheric dust cloud so darkened the sky that surviving plants had trouble making a living from photosynthesis; there were worldwide freezing temperatures, torrential rains of caustic acid, massive depletion of the ozone layer, and, to top it off, after the Earth healed itself from these assaults, a prolonged greenhouse warming ... It was not a single catastrophe, but a parade of them, a concatenation of terrors. Organisms weakened by one disaster were finished off by the next. It is quite uncertain whether our civilization would survive even a considerably less energetic collision. -- Carl Sagan, in Pale Blue Dot (1994)
The empirical evidence is that the Chicxulub impact killed off the dinosaurs and most other Cretaceous forms of life, but how, exactly, was the killing accomplished? What really happened, and why did it annihilate some species while others survived? These are important scientific issues. Unless we can provide answers, the connection between impacts and extinctions remains an empirical correlation without a sound basis in understanding. Our position would be analogous to a that of a public health official who had established a statistical link between smoking and lung cancer, for example, but did not know how smoke damaged the lungs or induced premature carcinoma.
There is another reason that we want to understand the environmental effects of impacts, related to self-interest. We know that big mass extinction events are rare, coming at average intervals of tens of millions of years. The chance of such an impact taking place in our lifetimes, or of our children or grandchildren, is very small. But there must be smaller and more frequent impacts that, while falling short of a mass extinction, would still make things very difficult for life on our planet. Suppose that an impact killed only 50% of each species. There would be no mass extinction, but the calamity would still be far greater than any other natural catastrophe we can imagine. If we are to estimate the frequency of such events we require a better understanding of the physical, chemical, and biological mechanisms by which an impact can disrupt the environment. Evaluating the environmental effects of impacts of various sizes is a prerequisite to rational public policy debate about what to do, if anything, about the impact hazard today.
We have never witnessed a large impact on Earth (but we did on Jupiter, as we will describe in later chapters), nor would we wish to carry out such an experiment, but there are relevant data. Direct evidence from the K-T extinction is preserved in the fossil record. In addition, theoretically-minded scientists can carry out calculations of the environmental effects of impacts using the same models and computational tools that are employed in estimating the effects of global warming or atmospheric ozone depletion, both issues of considerable contemporary interest. We will describe in this chapter how these lines of evidence are beginning to provide some answers, although much remains to be done.
The best direct evidence on the effects of impacts comes from the K-T boundary layer itself. This band of lithified clay consists of material blasted from the Chicxulub crater as well as finer dust that was suspended in the atmosphere and later diffused downward to fall to the surface. A careful examination of this layer tells us several things about events that happened 65 million years ago, in the immediate aftermath of the Chicxulub impact. The most significant layer is quite thin, about the width of a pencil. This has been called the fireball layer or the ejecta layer; it contains most of the iridium and appears to consist primarily of the mass blasted to great heights in the initial plume of debris ejected from the explosion site.
We have referred to some of the direct effects already, but it is useful now to summarize the whole sequence of events. In the Western Hemisphere, and particularly around the Caribbean Sea, some of these effects are quite dramatic. On Haiti there are deep layers of sediment at the K-T boundary that have been churned by powerful waves and currents, presumably in the immediate aftermath of the impact. Near the Gulf Coasts of Mexico and Texas, submarine gravels were carried to heights as great as 100 m, indicating that the coast was washed by huge tidal waves or tsunami. Wherever the K-T boundary layers have been preserved in the Americas, there are abundant mineral fragments that indicate the rock near the target area was shocked and shattered in an explosion of remarkable ferocity. These sedimentary layers bear mute witness to the tremendous explosion and massive tsunami that were associated with the impact itself.
Also in the North American sediments from 65 million years ago, scientists find abundant spherules of glass called tektites. This glass was formed when solid or molten fragments of ejected material plunged into the atmosphere and were melted by the heat of entry. The tektites provide evidence that some of the material was ejected far beyond the atmosphere and then fell back to Earth and re-entered at high speed -- presumably creating an awesome display of shooting stars that filled the entire sky.
A remarkable result has come from analysis of samples of the K-T boundary layer deposited on land sites all over the world. A team of researchers from the University of Chicago found that nearly all land samples contain bits of carbon that appear to be soot from massive fires. They have estimated the total amount of carbon in the boundary layer and hence the quantity of burned vegetation that was required to generate so much soot. The startling conclusion is that the greater part of the Earths biomass -- its forests and grasslands -- must have been consumed in a global conflagration.
Finally, there is the material in the boundary layer that is in the form of fine dust particles, which could have remained suspended in the stratosphere for months before falling to the surface. These are particles no more than one micrometer -- a millionth of a meter -- in size. To date there have not been many careful measurements of the fraction of the boundary layer that is made up of such particles, but indications are that it is a few percent. We can then perform a "thought experiment" and ask ourselves what would have been the consequences if all of these micrometer-size particles had been suspended in the atmosphere following the impact. The answer is that this much dust would have blocked sunlight completely from the surface, bringing on a darkness deeper than that of the darkest night. The presence of these fine particles together with an estimate of their residence time in the atmosphere leads to the idea, as suggested in the original Alvarez paper, that the impact induced a period of profound darkness and cold that lasted several months.
Armed with this information, several scientist have set out to understand how these effects came about and to estimate how they depend on the size of the impactor. In the following discussion we lean heavily on the recent work of two young atmospheric physicists at NASA Ames Research Center, Brian Toon and Kevin Zahnle. Toon is handsome, dresses soberly, and could pass as a banker as easily as a scientist. Zahnle is tall and gangling, prematurely balding, and prefers loud t-shirts and long hair; no one could mistake him for a banker. Both brought to their work great experience in using computers to model complex physical and chemical systems.
When a comet or asteroid smashes into the Earth, its kinetic energy -- the energy of its motion -- is instantly converted into shock waves, which pulverize the rock and generate massive earthquakes, and into heat, which vaporizes the projectile and a part of the target rock and blasts it upward in an immense explosion. Beyond the limits of the crater itself, the most immediate damage results from shaking of the ground and formation of an atmospheric blast wave moving at hurricane speeds. We have some experimental data on such effects from the testing of large surface or subsurface nuclear explosives.
The most important measure of the impact is simply its energy or, in military terms, its yield. Other considerations, such as the nature of the projectile or of the target area, are secondary. When you are hit by a hammer, the important thing is the strength of the blow, not the composition of the hammer. Energy is measured in megatons, or millions of tons of TNT. The Hiroshima nuclear bomb had an energy of 0.015 megatons (15 kilotons); standard-issue fusion bombs such as those carried on intercontinental missiles are about 1 megaton; and the largest nuclear explosion in history, set off by the Soviet Union in 1959 [check], had a yield of approximately 60 megatons. Among natural impact events, we have seen that the impact that produced Meteor Crater in Arizona had an energy of about 15 megatons.
Explosion of a 15-megaton nuclear weapon in the lower atmosphere (an airburst) or on the surface will knock down frame and brick buildings to a distance of about 30 km. This result is also consistent with the 1908 Tunguska impact (which we will discuss in Chapter 9), when a rocky asteroid about 60 m across plunged to within 8 km of the surface and exploded with an estimated yield of 15 megatons. This explosion flattened trees over an area of 2000 square kilometers. As we scale up to larger energies, the radius of devastation increases approximately in proportion to the cube root of the energy. As impacts become extremely large, however, they are less efficient at causing blast damage, as more of the energy is blasted upward into space. No impact can wipe out an area much larger than a country like France or Britain by direct blast effects.
A part of the energy of impact shakes the ground, causing damage even farther from the point of impact. For energies up to a few thousand megatons, the area in which structures are shaken down by these explosion-induced earthquakes is about the same as the area in which they are blown down by the blast wave, but for yields greater than a few thousand megatons, earthquakes dominate in causing surface damage.
More than half of all impacts strike in the oceans, and here we must also consider the damage done by tsunami waves, which can travel for thousands of kilometers across the ocean to devastate distant coastlines. The potential for destruction and casualties is very great because humans have historically settled on the coast or in river estuaries that can focus the incoming waves even more.
We can illustrate these effects of blast, earthquake, and tsunami waves with a hypothetical example. Imagine an explosion taking place in New York City, at the southern end of Central Park. If it is the size of the Hiroshima bomb, 15 kilotons, buildings will be destroyed to a distance of about 3 km, primarily by the blast, devastating most of Manhattan between the Empire State Building and Columbia University. An explosion of 15 megatons (equivalent to Tunguska or a large nuclear weapon) would extend the destruction to 30 km, taking out all five boroughs and much of industrial New Jersey across the Hudson River. At 15,000 megatons (the yield of an asteroid half a mile across), the blast damage would reach to Philadelphia and New Haven, and the earthquake would severely damage structures as distant as Boston or Baltimore. If this 15,000 megaton blast took place a few hundred kilometers to the east in the Atlantic Ocean, there would be little direct damage but the resulting tsunami would strike cities along the entire U.S. East Coast with waves more than 50 m high and would even endanger low-lying coastal areas of Europe.
A large blast blows a hole in the atmosphere and ejects pulverized and melted rock into space. The ejected material may skyrocket to altitudes of thousands of kilometers, but most does not escape entirely from the Earth, and within a few minutes it falls back into the atmosphere over a wide area. The tektites, which are found in many areas of the planet in association with recent impacts, are examples of ejected material that was heated as it re-entered the atmosphere.
In the case of the K-T impact, much of the material in the global boundary layer appears to consist of such ejecta, which was so widely distributed that it rained down over much of the planet. Calculations show that this material was accelerated to high speed as it fell back, so that it was heated to thousands of degrees in the atmosphere. Countless shooting stars or meteors could have been seen at any one time if the dinosaurs chose to look. The sky turned a brilliant red-orange as a tremendous pulse of heat struck the surface. This global heat pulse was so great that the forests and grasslands ignited, and presumably most of the exposed animals perished as well.
This meteoric heat pulse was first suggested in 1990 by planetary scientist Jay Melosh of the University of Arizona as one of the major causes of the dinosaur extinction. With three colleagues, Melosh submitted a scientific paper on this subject to the British journal Nature entitled "Broiled Alive! An incendiary approach to the Cretaceous/Tertiary extinction". However, the editors thought this title was too provocative, and when they published the paper they insisted on retitling it "Ignition of global wildfires at the Cretaceous/Tertiary boundary". Nevertheless, Melosh's original title accurately captures the implications of this event.
According to Zahnle, any impact with energy greater than about 10 million megatons is likely to generate a firestorm of at least continental dimensions. In the case of the K-T event, with an energy of more than 100 million megatons, the heat pulse and subsequent fires were probably the primary immediate killing agents on land. If these calculations are correct, we now have an answer to the question: how long was required for the extinction of the dinosaurs? For most species, the answer is probably about an hour -- just long enough to be cooked by the global broiler. For animals who survived this holocaust in caves or burrows, there was broiled dinosaur meat to eat for a while, but soon everything rotted and there was nothing but charred death across the land.
Since shock waves in air are known to generate nitric acid, we might expect global acid rain in the aftermath of a major impact. Sulfuric acid would also be produced in the atmosphere from the vaporization of a comet or asteroid, both of which are relatively rich in sulfur compounds. Brian Toon, Kevin Zahnle, and others have calculated how much and where such acids would have been produced following a major impact. For impacts even as large as the K-T event (roughly 100 million megatons), the atmospheric acid is expected to be confined largely to regions near the impact and to be washed out rather rapidly by rain. This acid rain could damage surviving land plants, but there is probably not enough acid produced to destroy the chemical balance of oceans or lakes, and the problem passes quickly. Compared with other environmental damage, acidification of the atmosphere and waters of the Earth may not be a dominant concern.
There is a more serious problem with nitric acid, however. This compound is capable of reacting with ozone and destroying the Earth's protective ozone layer if sufficient quantities are formed in an impact. According to calculations, as little as one part in 10 million of acid mixed into the stratosphere would be sufficient to destroy more than 75% of the ozone layer, rendering the ozone screen biologically ineffective against solar ultraviolet light. This much nitric oxide could be injected into the stratosphere by an impact of only a few million megatons, substantially smaller than the K-T impact. Following a catastrophe as large as the K-T impact, the temporary loss of ozone would make little difference since the sunlight (ultraviolet as well as visible) would be blocked anyway by stratospheric dust (as we describe in more detail below). For impacts in the millions of megatons, however, less dust is produced, and the ozone loss could be an important contributor to the extinction of land plants and animals.
While the larger fragments of rock ejected in an explosion fall back promptly to the ground, dust grains smaller than about a micrometer in size (like the particles in cigarette smoke) would remain suspended in the stratosphere for weeks or months. Unlike most of the impact effects discussed above, we have direct experience with injection of dust into the stratosphere. Large volcanic eruptions such as Mt. St. Helens (1980) or Pinatubo (1992) produced stratospheric dust clouds that quickly spread over the entire planet, producing beautiful red sunsets and blocking up to 1% of the incoming sunlight. Earth-observing satellites have tracked these dust clouds, and sensitive measurements of temperature have detected their effects on global climate. It is not too difficult to scale up from these volcanic examples to estimate the effects of injecting much larger quantities of dust from a large impact.
Brian Toon concludes from the K-T data that at least 100,000 tons of fine dust are generated for each megaton of energy in a large impact. For the K-T event at 100 million megatons, the quantity of stratospheric dust was thus about ten trillion tons. This is sufficient dust to turn off the sunlight completely, plunging the world into darkest night. According to Toon's calculations, an impact of about one million megatons would be sufficient to reduce the brightness at noon to the level of a moonlit night, and an impact of 100,000 megatons (equivalent to an impact by a 1-km asteroid) would reduce noon light levels equivalent to a heavy overcast. All of these examples represent considerably more dust than that produced in any historic volcanic eruption.
When sunlight is blocked, surface temperatures fall and photosynthesis ceases. The atmospheric circulation changes too, as the sunlight that would otherwise have heated the surface is absorbed instead by the stratospheric dust at altitudes of 20-30 km. Understanding how the atmospheric circulation responds to such changes is a difficult scientific challenge. Fortunately, however, a good deal of work on this topic had been done in the early 1980s in the context of nuclear winter.
In 1980, at about the same time the Alvarez team was deciphering the story of the K-T impact, another group of scientists was investigating the environmental effects of a large-scale nuclear war. Any nuclear exchange would be terrible to contemplate, of course, with the direct death of hundreds of millions of people in the targeted cities and military complexes. Military and civil defense planners were well aware of these effects. What they had not considered previously, however, was the possible effect of nuclear war on the global climate.
The scientists who explored the global consequences of nuclear war had been researching the effects of volcanic eruptions on the Earth and of the global dust storms that occasionally envelop the planet Mars. The late planetary scientists Carl Sagan and James Pollack were both members of the NASA Viking Team that studied the effects of martian dust storms in the late 1970s. They joined with atmospheric scientists Richard Turco, Thomas Ackermann, and Brian Toon to study the climatic effects of nuclear war. The initials of these five authors, when arranged in the proper order, form the memorable acronym TTAPS.
The TTAPS authors concluded that the atmospheric injection of smoke from burning cities could reduce temperatures and lead to widespread crop failures throughout the world, affecting non-combatants as well as the nations which were actually targets in a nuclear exchange. They called this phenomenon nuclear winter. Because of the potential importance of their result (which said, in effect, that all nuclear war was suicidal), many other scientists became involved, and a number of their studies were directed toward understanding the effects on the atmosphere of stratospheric dust and soot. When questions about impact-injected dust arose in the 1990s, the computer programs written to model nuclear winter were available to analyze the effect on the atmosphere and climate. By analogy with nuclear winter, the climatic consequences of a large impact are sometimes called impact winter. Nuclear winter was a controversial concept, but the application of these ideas to the impact scenario is more straightforward. Calculations of the global effects of a nuclear war depend on assumptions about how much smoke is produced by burning cities and how much of the soot reaches the stratosphere, neither of which is well known. In the case of impacts, however, there is little question about the production of pulverized rock and the injection of this dust into the stratosphere. As Comet Shoemaker-Levy's impact into Jupiter vividly demonstrated, ejecta really does rain down onto a planet's atmosphere following a large impact.
Even though the impact dust cloud would persist for only a few months, this would be sufficient following a multi-million megaton blast to lower continental temperatures by at least 10oC, leading to intermittent killing frosts at most latitudes. The damage would be greatest, of course, in the tropics and in the summer hemisphere of the planet at the time of the impact. Plants hit during the sensitive part of their growing season would be stunted or killed by the cold. In the oceans the temperature would remain close to normal, but photosynthesis would cease and the plankton that forms the base of the marine food chain would die, leading to the collapse of marine ecosystems. Only the bottom-feeders and scavengers from the deep ocean depths might be expected to survive undamaged.
As the dust settled and the atmosphere cleared, normal circulation patterns would be re-established. However, impact-induced changes in atmospheric chemistry might persist. The addition to the atmosphere of carbon dioxide and other greenhouse gases might lead to an enhanced greenhouse effect, causing the temperatures to rebound to unusually high values. Such global warming might persist for centuries before the planet fully recovered.
The many environmental effects enumerated above would combine to create a global catastrophe of unimaginable magnitude. We have described them in rather neutral scientific language, but we can imagine the horror that marked the end of the Cretaceous period on Earth.
For up to a thousand kilometers from ground-zero, no living thing is likely to have survived the initial blast, and the earthquake waves radiating from the impact site would have knocked down trees and shaken mountains over the entire Western Hemisphere. Within a few minutes walls of water bearing loads of gravel swept over the low-lying coasts and penetrated hundreds of kilometers inland. Approximately half an hour after the strike, the back-falling hot debris turned the skies of the entire planet as hot as fresh pig-iron from a blast furnace, causing the forests and prairies to burst into flame and broiling any exposed land animals. For days the conflagration contributed soot and smoke to the dust injected into the atmosphere by the blast. (Imagine the oil-field fires in Kuwait after the Gulf War expanded to global proportions). Within a week the entire world was shrouded in blackness, and the pulse of death begin to spread through the oceans. The marine food cycle collapsed, even as the scorched continents languished under a dismal blanket of ice and soot.
In such a world, it is surprising that any land species survived. And indeed, most did not. While every species of dinosaur went extinct, about half the species of mammals did also. The survival of life itself was in no danger, as sheltered environments in the deep oceans or terrestrial burrows were almost untouched, but clearly most individual living things succumbed, and the direction of evolution was profoundly altered.
When we look at the evidence for an impact catastrophe at the end of the Cretaceous, it is hard to understand how strongly this hypothesis was resisted by the majority of paleontologists. As we have already seen, desperate efforts were made to argue that volcanism or some other more familiar geological event was responsible for the death of the dinosaurs. If we judge the importance of an idea by how strongly it is resisted, then the Alvarez theory was truly revolutionary. It has forced the scientific community and the public as well to face the possibility of catastrophic change. And it has raised the interesting question of whether other mass extinctions are also due to impacts. Perhaps impacts have played a major role throughout the history of life on our planet.