The solar system abounds in comets and asteroids, some even bigger than the one that was nearing Earth on that day 65 million years ago. Most asteroids remain in a belt between Mars and Jupiter, and most comets orbit the sun far beyond distant Pluto. Occasionally, however, an asteroid has its orbit deflected by Jupiter's gravitational pull, or a comet orbit is altered by the gravitational tug of a passing star. A few of these asteroids and comets are diverted into orbits that cross the Earth's path. An impact occurs when such an object intersects the Earth's orbit just as Earth happens to be at the crossing point. This is what happens every time you see a shooting star flash across the night sky.

Large impacts can also happen, and they were frequent in the early history of the solar system, as witnessed by the ancient, crater-scarred face of the moon. But large impacts are rare nowadays, because the debris that was abundant in the early solar system has been swept up by the planets, large Earth-crossing comets and asteroids are now rare, and Earth is a very small target. To see how small, look at Venus just after sunset, when it is the evening star. Venus is the size of the Earth, and from our distance it is a tiny, although brilliant, dot in the sky — a very difficult target to hit.

Earth is protected, therefore, by the fact that large comets and asteroids rarely come into the inner solar system, and those that do are unlikely to hit something as small as our planet. So we can imagine the giant comet of 65 million years ago coming close to the Earth again and again, over a period of centuries or millennia, as it orbited the sun — sometimes far from Earth, sometimes close enough to put on a spectacular display in the night sky. A set of near misses like this must take place every now and then in Earth history, but usually the comet hits the sun or another planet, or is deflected out of the inner solar system. In this particular case, however, there came a time when the invader's orbit intersected that of Earth just as both were approaching the intersection point. This time there would be no escape. The comet was aimed toward the southern part of North America — toward the shallow seas and coastal plains that are now the Yucatán Peninsula of Mexico.

t is very difficult to appreciate the impact that was about to occur, because such an extreme event is far beyond our range of experience — for which we can be grateful. You can write down the measures of what happened — an object about 10 kilometers in diameter slammed into the Earth at a velocity of perhaps 30 km/sec.

But these measures only acquire meaning when you try to visualize them. How do you imagine a comet 10 kilometers in diameter? Its cross section matches the city of San Francisco. If it could be placed gently on the surface of the Earth it would stand higher than Mount Everest, which only reaches about nine kilometers above sea level. Its volume would be comparable to the volume of all the buildings in the entire United States. It was a big rock, or a big ice ball, but not of a scale beyond our comprehension.

What turned it into a cataclysmic weapon was its velocity. The estimated impact velocity of 30 km/sec is 1,000 times faster than the speed of a car on the highway and 150 times faster than a jet airliner. It is about six times faster than the speed of seismic waves in rock. When a collision takes place at velocities this high, our experience is not a useful guide, and rock materials do not behave in the ways we are used to.

Instead, a shock wave is produced — a kind of sonic boom in the rock. The shock wave from such an impact crushes and compresses the impactor and target rock so intensely that after the shock passes, the decompressing rock will fly apart, or melt, or even vaporize. The concept of rocks instantaneously boiling away to vapor conveys a gut feeling for the extraordinary and violent conditions during an impact.

The energy of motion of the comet just before impact was equivalent to the explosion of 100 million megatons of TNT, sufficient to vaporize the comet in about one second. To get a feeling for this quantity of energy, keep in mind that one large hydrogen bomb has a yield of about one megaton of TNT, and that the total nuclear arsenal of the world at the peak of the Cold War was about 10,000 such bombs. The 10-megaton impact of the comet that ended the Cretaceous was therefore equivalent to the explosion of 10,000 times the entire nuclear arsenal of the world (although the impact explosion was not nuclear).

t the instant of contact with the Earth's surface, where the Yucatán Peninsula now lies, two shock waves were triggered. One shock wave plowed forward into the bedrock, crushing shut all cracks and pore spaces and destroying much of the orderly crystal structure of minerals. Meanwhile, a second shock wave flashed backward into the onrushing comet. Reflecting off the back of the impactor, it tore apart the trailing edge of the comet. In the second or so it took for this to happen, the comet ceased to be recognizable as a spherical body. With its enormous momentum driving it forward, the comet penetrated deep into the Yucatán bedrock, forcing open a huge hole and molding itself into an incandescent coating on the inside of the growing hole, which was now opening out into an expanding crater.

As the rapidly vaporizing comet wreckage was carried forward into the growing center, the shock wave curved back up to the surface and spewed out ejecta — melted blobs and solid fragments of target rock — upward and outward on high, arching trajectories that flung them through the thin outer fringes of the atmosphere and beyond. Falling back to Earth within a few hundred kilometers of the rim of the crater, this debris built up a vast blanket of ejecta.

Even this did not exhaust the pyrotechnic potential of the impacting comet. The huge cloud of vaporized rock generated at ground zero was driven outward by its own heat and pressure in a colossal fireball. It overwhelmed the atmosphere, expanding and accelerating out into space and launching particles of rock into trajectories that carried them far around the Earth before they fell back to the ground.

n the zone where bedrock was melted or vaporized, no living thing could have survived. Even out to a few hundred kilometers from ground zero, the destruction of life must have been nearly total. Sterilized by the intense light from shock-compressed air and from the fireball of rock vapor, crushed when pores and cracks in rock were slammed shut by the passing shock wave, and bombarded by the falling debris of the ejecta blanket, little or nothing was left alive in this central area.

Animals living just over the horizon saw a flash of light, then a last moment of calm before the sky itself turned lethal. Illustration by Don Davis.

Out to a few thousand kilometers, into the area of modern Mexico and the United States, the Yucatán impact sent dramatic messengers of destruction. Animals living just over the horizon first witnessed a flash of light in the sky, then a last moment of calm. Then, as the ground began to shake uncontrollably from the passing seismic waves, the sky itself turned lethal. Beginning with a faint glow, the sky grew more and more intensely red, passing into incandescence, growing brighter and brighter, hotter and hotter. Soon the Earth's surface itself became an enormous broiler — cooking, charring, igniting, immolating all trees and all animals that were not sheltered under rocks or in holes. This fearsome phenomenon was produced by ballistic ejecta particles blasted into space by the impact, which were now falling back to Earth, reentering the atmosphere, heating up through friction with the air, and transmitting that heat to Earth as infrared light. Entire forests were ignited, and continent-sized wildfires swept across the lands.

Soon the Earth's surface itself became an enormous broiler — cooking, charring, igniting, immolating all trees and all animals that were not sheltered under rocks or in holes.

 

Even as the forests were set ablaze, another horror was approaching the coast of the Gulf of Mexico. The impact occurred in the shallow water and coastal plains that flanked the Gulf, but it produced a huge disturbance in the waters of the deep Gulf, through seismic shaking, submarine landslides triggered by the seismic waves, and by the splashdown of the ejecta blanket. The result was a gigantic tsunami, a massive wave perhaps a kilometer high, which spread outward across the Gulf of Mexico at terrific speed. Everyday waves do not disturb the bottom of deep seas like the Gulf, but the impact tsunami was so enormous that its keel swept across the bottom of the Gulf, digging channels into the fine sediments of the sea floor, and mixing them with the impact debris that had just fallen. As the tsunami front reached the shallow water of Florida and the Gulf Coast, it was pushed up higher and higher into a wall of water that towered above the shoreline.

Within hours of the impact, most of Mexico and the United States must have been reduced to a desolate wasteland. Where only the day before there had been fertile landscapes, full of animals and plants of all kinds, now there was a vast, smoldering netherworld, mercifully hidden from view by black clouds of roiling smoke.

Farther away from the Yucatán, the effects were less dramatic. The giant tsunami was largely confined to the enclosed Gulf of Mexico and could not reach Asia, Africa, or Europe. Ejecta particles rained down around the world, but fewer particles traveled to more remote areas, so the firestorms may not have been as intense as in North America. In contrast to the largely sterilized regions close to ground zero, distant continents may have escaped the direct effects of the Yucatán event. Tragedy would unfold more slowly in these remote areas, through the secondary effects of the impact.

H O R S E M E N   O F   O F   T H E   A P O C A L Y P S E

errible as the immediate, direct effects of the impact were in the surrounding region, they probably would not by themselves have caused the disappearance forever of whole families of plants and animals, because survivors in remote regions would have repopulated the devastated regions in the years to come. And yet an enormous mass extinction did follow the impact, and we now understand some of the longer-term global disasters that were secondary results.

Within days of the impact, the immediate effects had died down. The fires were going out, the tsunami had spent its main strength against the coast of the Gulf of Mexico, and violent winds were settling down. But the Earth was turning cold and dark. Vast quantities of fine dust had burst through the atmosphere in the fireball and the dust was now settling through the upper atmosphere around the world, blocking the sunlight. The land became so dark that you could not have seen your hand in front of your face, and this darkness and the accompanying cold probably lasted for a few months, until finally most of the dust had settled to the ground.

As the tsunami front reached the Gulf Coast, it was pushed   up into a wall of water that towered above the shoreline. Illustration by Ron Miller

But after the return of light, the climate went to the opposite extreme. Two greenhouse gases — water vapor and carbon dioxide — had been released in vast quantities from the site of the impact. The water vapor was probably removed quickly from the atmosphere as rain, which washed out the dust. Carbon dioxide can only be removed slowly from the air, and now it trapped the heat from the sun, raising temperatures to sweltering levels. It was probably thousands of years before the carbon dioxide was back to normal levels.

V I C T I M S ,   S U R V I V O R S ,   A N D   D E C E N D A N T S

y the time the physical devastation caused by the impact had faded, years or centuries after the event, Earth's biosphere was changed forever. Whole groups of plants and animals had disappeared, never to be seen again. By one estimate, half of the genera living at the time of the impact perished.

The best known victims, of course, are the dinosaurs. T. rex and other huge carnivores perished, as did the herbivorous dinosaurs and their relatives who swam, like the mosaurs, or flew, like the pterodactyls. Most paleontologists now consider that modern birds are very closely related to the dinosaurs, which, in this sense, did survive the end of the Cretaceous. Yet recently discovered fossils are revealing that birds were nearly wiped out as well.

Whole groups of plants and animals had disappeared, never to be seen again.

 

The loss of the dinosaurs is probably related to their position in the food chain, with herbivorous dinosaurs eating vegetation and carnivorous dinosaurs eating herbivores and perhaps small mammals. During the months of cold and darkness cast by the pall of dust in the atmosphere, plants would wither and the herbivores would starve, and so would the carnivores in their turn. Large animals are never abundant, especially top carnivores, so they would have been particularly vulnerable to extinction.

A dinosaur starved for sun during the last impact winter. Illustration by Ron Miller.

Many smaller land animals survived, including mammals, as well as reptiles such as crocodiles and turtles. No one really understands why these animals escaped extinction. Being smaller and thus more numerous would increase their chances of survival, and this may help explain the survival of birds as well.

The sudden loss of half the genera plants and animals on Earth is a catastrophe almost incomprehensible to us. It truly marked the end of a world. And yet, the darkness eventually faded, the heat died down, and the acids were neutralized. Survivors there were, and they found themselves in a new world, tragically changed, but with boundless opportunities for the future. For 150 million years dinosaurs were the large land animals of the planet, while mammals were confined to the role of small animals. With the disappearance of the dinosaurs, there were new opportunities for mammals, and evolution rapidly produced large ones. Our nostalgia for the lost world of the Cretaceous is tempered when we realize that it was a world that held no place for us — for large mammals. Our horror at the destruction caused by the impact that ended the Cretaceous is eased by the understanding that only because of this catastrophe did evolution embark on a course which, 65 million years later, has led to us. We are the beneficiaries of Armageddon.

Excerpted from T. Rex and the Crater of Doom, Princeton University Press © 1997. Reprinted with permission.