**2. Disaster**

Except for the confusing new data supplied by Raup and Sepkoski, our
understanding of what happened *65 *million years ago had seemed nearly
complete. The Alvarez team and teams of other scientists around the
world had painted a fairly complete picture of the disaster. There were
still some details that weren't fully understood, but the broad picture
can be summarized as follows.

At the end of the Cretaceous period, the golden age of dinosaurs, an
asteroid or comet about *5 *miles in diameter (about the size of Mt.
Everest) headed directly toward the Earth with a velocity of about 20
miles per second, more than 10 times faster than our fastest bullets.
Many such large objects may have come close to the Earth, but this was
the one that finally hit. It hardly noticed the air as it plunged
through the atmo­sphere in a fraction of a second, momentarily leaving a
trail of vacuum behind it. It hit the Earth with such force that it and
the rock near it were suddenly heated to a temperature of over a million
degrees Celsius, several hundred times hotter than the surface of the
sun. Asteroid, rock, and water (if it hit in the ocean) were instantly
vaporized. The energy released was greater than that of 100 million
megatons of TNT, 100 teratons, more than 10,000 times greater than the
total U.S. and Soviet nuclear arsenals.

Before a minute had passed, the expanding crater was 60 miles across and
20 miles deep. (It would soon grow even *larger.) *Hot vaporized
material from the impact had already blasted its way out through most of
the atmosphere to an altitude of 15 miles. Material that a moment
earlier had been glowing plasma was beginning to cool and condense into
dust and rock that would be spread worldwide. The entire Earth recoiled
from the impact, but only a few hundred feet. The length of the year
changed by a few hundredths of a second.

The deep crater may have reached through the crust of the Earth to **the
**mantle. The rock at this depth is very hot due to the natural
radioactivity of trace amounts of potassium, uranium, and thorium. The
hot rock had turned to liquid as soon as the weight of the rock above it
was removed. Great pressure from the Earth's interior quickly filled in
most of the crater with melted rock from below. It is possible that
molten rock continued to flow in a great outpouring. This is not known
for certain, because the impact site has not been definitely identified.
One suggested location is at the Deccan traps in India, one of the
greatest outpourings of deep basaltic rock that has taken place in the
last billion years of the Earth's history. Two hundred thousand square
miles of the Earth were covered when the Deccan traps were formed, and
this is known to have taken place at about the right time, approximately
65 million years ago.

Shock waves from the impact rattled the Earth with energy much greater
than that of the largest earthquakes humans have experienced, probably a
million times more energetic than the one that devastated San Francisco
in 1906. The shock may have triggered other earthquakes, causing
aftershocks lasting for months or years. This is speculation, for we
don't know enough about the crust of the Earth to say with any certainty
Weak points in the Earth's crust may have opened and sprouted new
volcanoes. It is impossible to guess all the effects, or how long they
lasted.

Our failure to find the crater suggests either that it was obscured by
the subsequent outflow of magma or that the impact occurred in the two
thirds of the Earth that is covered by ocean. According to the latter
scenario, the asteroid quickly punched through the water layer much as a
rock would through a puddle, since in most places the depth of the ocean
is less than the hypothesized 5-mile diameter of the asteroid. The
splash created a great tsunami, or tidal wave, which grew hundreds of
feet high as it swept toward shore. The giant wave circled the Earth
many times, inundating the coastal regions. Only the interiors of the
continents were spared.

Rock from the crater, mixed with vaporized comet material, cooled to
several thousand degrees Celsius as it rose in a fireball up through the
atmosphere. It was then about as hot and bright as the surface of the
sun, the greatest fireball ever seen by living creatures. But they
didn't watch it for long, because the intense heat radiated from the
glowing cloud burned everything within sight. The heat of the fireball
also fused nitrogen and oxygen in the air to make nitrous oxides, a
constituent of modern smog. Some of this gas later combined with water
in the atmosphere to make nitric acid. Likewise, sulfur dioxide from
burning plant material formed sulfuric acid, which together with the
nitric acid eventually fell to Earth. This acid rain may have been
strong enough to dissolve the shells of creatures that lived in the
surface waters of the oceans.

Some of the material ejected from the crater flew out of the atmosphere
in ballistic trajectories and, like ICBMs (intercontinental ballistic
mis­siles), rained havoc on distant continents. Some escaped into space,
perhaps to hit the Earth on an anniversary of the impact when both the
Earth and the ejecta returned to the same part of their orbits around
the sun. As the fireball reached the top of the atmosphere, it bobbed
like a cork, floating on the cooler air beneath it, but it had nothing
to hold it together and it began to spread out over the entire globe. As
it spread, its color cooled from a glowing red to an impenetrable black.
The surviving creatures below probably thought that night had come
early, but it was a night without a moon, without stars. The dinosaurs
could not see their own claws in front of their faces. Morning would not
come for several months.

A few animals had avoided the initial destruction, and at first they
seemed to manage surprisingly well. Most of the plant eaters could still
find food, although the settling dust added a gritty texture to it all.
Some of the carnivores were accustomed to hunting in the dark, although
they had never experienced blackness such as this. But the ultimate
source of all food, the sun, had been effectively blocked out. Without
sunlight, there was no photosynthesis, no creation of sugar and starch
from carbon dioxide and water. Unseen by the animals, the plants were
turning from green to yellow, and then to brown. If not for the
darkness, it would have been a beautiful autumn scene. The larger
herbivores began to starve, followed soon afterward by the large
carnivores. Similar death occurred in the oceans. Phytoplankton, the
first link in the food chain for the oceans, died from the acid rain and
lack of sunlight, and the higher organisms quickly followed.

Without sunlight, the temperature of much of the Earth began to drop. On
much of the land the temperature soon fell below freezing. Only those
fortunate few animals that had already begun to hibernate didn't notice.
Warm-blooded creatures had an advantage in their ability to endure the
cold, but they also required more food. The coastal regions had
tempera­tures moderated by the oceans, but these were the areas that had
been devastated by the tsunami. Tremendous storms were generated by the
great temperature differences between the oceans and the land masses.
The storms rained the nitrous and sulfuric acids onto land and sea.

The tiny particles of dust high in the atmosphere began to stick to each
other, agglomerating into larger particles, which fell to Earth more
quickly. All over the Earth the dust settled to form a layer about a
half-inch thick. (One day Walter Alvarez would puzzle over a section of
this layer in an outcropping near Gubbio, Italy, where it had been
brought up to the surface as the crust of the Earth folded during the
creation of the Apennine mountains. He would cut a piece of it out to
give as a gift to his father.)

As the dust settled, sunlight began to filter through to the Earth's
surface. Virtually every animal and plant had died. The plants were
probably the first to revive. Many spores and seeds had been left
behind. Life is incredibly robust. For some plants, the three-month
period of darkness and cold was no worse than a severe winter; these
resprouted from seeds and roots. It was a miracle that any of the higher
life forms made it through. Indeed, almost every land animal that
weighed more than about 50 pounds had been extinguished, probably
because they were most vulnerable in their high position on the food
chain. The reptilian dinosaurs, both on land and in the sea, had
vanished forever. The nearest relatives of the dinosaurs to survive were
the birds. We don't really know why. Perhaps it was their mobility,
their ability to search for warmth and food. Perhaps they were better
able than other dinosaurs to feed off decaying matter and seeds. Perhaps
it was their warm blood. We can't be sure because we don't even know if
the other dinosaurs were warm-blooded or cold-blooded. There was no
obvious pattern that explained why certain creatures had survived, and
others had not. Life is very robust, and yet very fragile.

With plant life suddenly sprouting all around, the few creatures that
had made it through the catastrophe found themselves in a virtual Garden
of Eden. Their natural enemies were gone, and food from plants was
abundant. However, their species would not continue unless they could
find mates. Those that did quickly spread out over the Earth, like (much
more recently) the rabbits let loose in Australia. They filled the
ecological niches that had been denied them by their previously "fitter"
enemies. The slate of evolution had been wiped nearly clean. Now there
was plenty of room for Nature to try new inventions. In fact, the great
catastrophe was marked in the paleontological record not only by the
disappearance of species, but also by the great proliferation of new
species that followed. Like a forest fire clearing the brush, the
destruction may have been necessary to clear the world of weak creatures
that had nevertheless held on to a niche simply by virtue of filling it
before any other animal. In evolution, possession really is nine tenths
of the law. Survival of the fittest had been hindered by an equally
powerful principle: survival of the first. The catastrophe had cleared
out whatever stagnation there had been in the process of evolution. Once
again there was room for free experimenta­tion.

In this picture, evolution wasn't exactly what it was once thought to
be, creatures fighting other creatures to determine which were the
fittest. To survive, creatures also had to be adapted to endure
catastrophe. Maybe that's why the mammals made it through. They had put
up with trauma for 100 million years, the trauma of attempting to live
with the dinosaurs.

When Luis Alvarez's team first analyzed the climatic effects of the
impact, it could not decide whether the dust surrounding the Earth would
cause the surface temperature to drop or rise. The answer depended on
which effects were most important. Blocking the sunlight should cause
the Earth to chill, they thought at first. But high dust is better at
absorbing sunlight than clouds, which reflect most of the light that
hits them. If the dust became hot enough, it could radiate the heat to
the surface of the Earth, warming it up. Geologist Eugene Shoemaker had
pointed out that if the asteroid hit in the ocean, the injection of
water vapor into the atmosphere could increase the "greenhouse effect"
and warm the cli­mate. It was hard to know which effect would dominate,
whether the temperature would rise or drop. This question had interested
Brian Toon, a physicist who worked at NASA'S Ames Research Laboratory in
Sunnyvale, California. Toon and his colleagues were eventually led to an
amazing conclusion, one that had immediate practical consequences far
afield from the mystery of the dinosaurs. (A few years later this work
was under extensive study by the U.S. Department of Defense.)

Toon and his colleagues recognized that the dust cloud would cause both
the absorption and reemission of the solar radiation to take place at

higher altitudes than usual. Normally the surface of the Earth receives
light radiation from the sun and roughly an equal amount of infrared
radiation from the upper atmosphere. With the dust layer stopping the
direct sunlight, the land would receive only half as much energy.
Nor­mally the temperature of an object bathed in radiation is
proportional to the fourth root (i.e., the square root of the square
root) of the energy hitting it. Since the surface temperature of the
dust-free Earth is normally about 28 degrees Celsius (300 degrees above
absolute zero), a drop could be expected, on an absolute scale, of the
fourth root of 2, a factor of 1.18, down to 252 degrees above absolute
zero, about 20 degrees below freez­ing. More detailed computer
simulations that took into account the fact that the atmosphere has
several effective layers essentially verified the simple calculation,
although the temperature drop was not quite as severe.

If snow covers a large fraction of the Earth, then even the return of
sunlight may not warm up the Earth. Although it can begin to reach the
ground once the dust settles, the sunlight will be reflected by the
white snow before it can turn into infrared heat. It may take something
external, such as another impact or a volcanic eruption that would
spread dust over the snow, to make the Earth absorb sunlight again. Some
paleontologists had believed that it was climate change that had killed
the dinosaurs. But it now seemed likely that one was not a consequence
of the other, but that in fact both had been brought about by the same
cause, the asteroid impact. Perhaps a climate change *had *been the
responsible agent, but it was a climate change brought about by an
impact.

The impact-caused winter was a major contribution, an important new
idea. But then, in a great conceptual leap, Toon and his collaborators
realized that a similar effect could occur today as a result of an
all-out nuclear war, if sufficient dust were lofted into the high
atmosphere. It was the birth of the idea of the "nuclear winter." Toon
discussed these ideas with Carl Sagan, his former thesis adviser, who
had analyzed the climate effects of dust clouds on Mars and had reached
similar conclusions. They realized that the major contribution to the
darkening of the atmosphere would not be dust lofted in the fireball,
but soot from the extensive burning of cities that would very likely
accompany an all-out nuclear war. This analysis was published in
*Science *magazine, in a paper that soon became known as the TTAPS
report, from the initials of the authors:

Turco, Toon, Ackerman, Pollack, and Sagan.

The TTAPS report predicting a nuclear winter hit the entire defense
community like a shock wave. A few years earlier, a committee of the
National Academy of Sciences, the most prestigious scientific
organiza­tion in the United States, had analyzed the long-term effects of
nuclear war and concluded that they might be relatively minor.
Populations had sprung back from past disasters, such as the bubonic
plague of 1350, with amazing resiliency and speed. But the Academy
committee had never thought of the nuclear winter, which could be far
more devastating than immediate blast or long-term radiation effects.
Depending on the duration of the chill, it seemed possible that all
higher forms of life could again be destroyed, this time in action
brought on by ourselves.

Some critics of the TTAPS report said that the calculations had been
overly simplified. The existence of the oceans, of storms, and of cloud
formation had been neglected. The nuclear winter threat might not exist,
or might be much less severe than the TTAPS report suggested. However,
many sober-minded scientists felt that this criticism, although
accurate, was irrelevant. The TTAPS group had shown that the best
scientists in the world, in their previous thinking about nuclear war,
might possibly have missed the most important, the most damaging effect.
The TTAPS group might be wrong in their detailed climate calculations,
but they were absolutely right in their demonstration that nuclear
warfare was too large a departure from prior experience for us to have
any confidence that we could predict its consequences. Nuclear winter
might turn out to be right or wrong, but we had all learned a nuclear
humility.

Soon after the Alvarez group published its discovery of the primordial
impact, NASA commissioned a special study to figure out what to do if we
discovered that an asteroid or comet was heading directly toward the
Earth. One could not simply blow the asteroid to smithereens, since even
a small smithereen can destroy a city. In 1908, a small piece of a
comet, probably less than 100 feet across, hit the atmosphere above the
Tunguska region of Siberia. It vaporized and never reached the ground,
but the blast from the hot gases was equivalent to that of a 50-megaton
nuclear explo­sion. Tens of thousands of square miles of trees were
flattened. Observers flying over Tunguska a decade later reported that
from the air the broken trees looked like matchsticks, with their bases
all pointing back toward the place where the fragment would have hit.

The NASA study, which included Luis Alvarez as a participant, con­cluded
that the best thing to do was to land on the asteroid, dig a hole in it,
and set off a small nuclear explosion in the hole. In effect, the entire
asteroid would become a rocket, with the blast of the explosion giving a
thrust that would push the asteroid out of its old orbit. If you caught
the asteroid early, a relatively gentle blast could deflect it enough so
that it would miss the Earth Physicist Roderick Hyde figured out that it
would be even easier to deflect a comet, which is largely ice and dust.
Detonate the nuclear weapon a few miles above the surface of the comet
core, he suggested. The blast of heat would vaporize water on the
surface of the comet, and this vapor would push the comet away. If it
were sufficiently spread out, the explosion would not tear the comet
apart.

We are the first creatures on the Earth capable of putting up this kind
of "comet umbrella." It would be wise to practice on a few distant
asteroids and comets, however, to make sure that the method works before
we try it on a comet or asteroid that is headed directly toward the
Earth.