|
Yucatan, early June, sixty-five million years ago
The summer day begins as usual on the shallow reef. Fish and ammonites
forage among the vegetation as they try to avoid becoming shark
bait. Pterodactyls soar in the trade winds, waiting for tidbits. Suddenly,
the sky to the south brightens like a second sunrise. Within seconds, the
| How do we know when this impact took place?
We know the time of year from clues frozen in ponds and preserved in
the geological record. The time of day is artistic license, because more
goes on during the day than at night. |
entire sky glows at white heat. Every mobile organism instinctively dives
for cover. But there is no hope for them. One second later, coming in fast
and at a low angle, an asteroid fifteen kilometers in diameter crashes to
earth at twenty kilometers per second, vaporizing itself and the upper few
kilometers of its center of impact. The shock produces a crater as wide as
the Mediterranean Sea. A thick blanket of hot rock fragments fries every
living thing within a few hundred kilometers of the crater rim.
The pent-up rock vapor expands, carried rapidly northward by the momentum
of the projectile. Within minutes, the vapor cloud sears the surface of
western North America, igniting any exposed plant or animal. This is not an
ordinary forest fire; the massive heat wave destroys even seeds buried
underground.
Several minutes later, across the ocean in Europe and Australia, dawn comes
early as ejected sand-sized fragments return to Earth at cosmic
velocities. They glow like meteors, filling the sky for hours. Their heat
ignites exposed vegetation, leaving surviving animals with nothing to
eat. Soot from the fires fills the lower atmosphere, quickly bringing
darkness.
The calamity is just beginning. The meteor fragments vaporize into a fine
dust that circulates in the upper atmosphere, blotting out
sunlight. Sulfate, vaporized from anhydrite beds beneath the Yucatan,
contributes to the opaqueness of the stratosphere. It remains in the air
longer than the dust. Darkness brings cold after the heat. Lilies freeze in
Wyoming ponds. Photosynthesis stops in the open ocean for more than a
year. Plankton species perish, along with the creatures in the food chain
above them. Animals not dependent on photosynthesis eke out a
living. Survivors include crocodiles in ponds, our insect-eating ancestors
in logs, and a species of shore bird-the only remaining dinosaur.
Long before the great asteroid forever changed life on Earth, even worse
disasters occurred that make this more recent apocalypse pale, a mere
pebble in a pond. Four billion years ago, objects hundreds of kilometers in
diameter hurtled from the sky, reshaping the planets. These gigantic
impacts produced the enormous basins still visible on the Moon and
Mars. The heat from the kinetic energy of the projectiles partly vaporized
the whole terrestrial ocean. Amazingly, life was able to weather these
storms in the "Goldilocks Zone," which is located in rocks over a kilometer
deep, the only safe place for thousands of years after an impact. The
surface and shallow subsurface alternately teemed with life, and became a
death trap when a large asteroid hit every few ten million years. Both the
surface and the deep subsurface were too hot to sustain life. Only
heat-loving (thermophilic) organisms in the Goldilocks Zone, living at 100
degrees Centigrade, survived these tribulations to root the tree of life.
Evolution does not directly prepare organisms for conditions they do not
regularly experience, like a year of darkness in the tropical ocean. So
most organisms are unable to cope with the disastrous results of such freak
events. Some adaptations that arose to cope with other environmental
conditions, however, provided salvation for a species. For example, in
pre-human times the pike evolved sharp teeth for catching and eating its
usual diet of smaller fish. Today, these teeth are advantageous for biting
through fishing line. Other coincidental adaptations help during rare
events. For example, sixty-five million years ago, when most creatures
boiled to death during the great cataclysm, animals and plants that were
low on the food chain and nestled deep in swamps managed to survive.
Chicken Little was Right: The Risk Is Real!
|
Your chance of being killed by an asteroid is about the same as in a
passenger plane crash: one in a million per year. But with an asteroid,
billions of humans will be killed at the same time. How do we avoid the
indignity of a mass extinction, with no one left to bury us? Our species
possesses one helpful adaptation: our intelligence, which has evolved for
hunting, gathering, and social interaction. Unlike dinosaurs, we can
observe, predict, and act decisively. Can our smarts save us from an
otherwise inevitable tragedy?
Asteroid orbits are predictable in the short run, but over a geological
length of time the orbits change. The Earth is a tiny target in the
vastness of the solar system. Moreover, earth-crossing orbits are chaotic-a
series of minute changes in an asteroid's actual position and velocity
build up to huge changes over time. Thus, in the long term, asteroid orbits
are effectively random-more so than even a fair roulette table. We know an
asteroid will hit us, sooner or later, but we can't say which one, or when.
| Even a fair roulette wheel gives nonrandom
results due to mechanical variations. By collecting statistics and betting
appropriately, Joseph Jaggers broke the bank at Monte Carlo in 1873, and
mathematician Claude Shannon built a wearable computer to outwit the
roulette wheel in 1961. Nowadays, casinos regularly rebalance their wheels
to keep the spin results as random as possible. |
NASA monitors near-Earth asteroids. Before this program began a few years
ago, we did not know whether an impact was more likely next year or a
million years from now. Now, visual tracking and heavy mathematics predict
the orbits of large objects five hundred years in the future. Early results
are in: no ten-kilometer-diameter object has Earth's name on it. And five
centuries from now, society should be better equipped to deal with the
hazard of an impending collision. However, the more numerous one-kilometer
objects also present a danger of global catastrophe. We do not yet have a
complete manifest of these vermin of the sky.
What do we do if we find an asteroid in our path? Civilization will face
this calamity sooner or later, certainly within a million years. If we have
been diligent, we will have hundreds of years to prepare for an impact. We
will be able to soft-land a probe to track the object's course and confirm
the danger of collision. We will probably not choose to blow it up, as that
would turn one dangerous object into several. More likely we will change
its orbit by detonating a nuclear explosive nearby, to spall off some
material. Newton's law of the conservation of energy predicts that the
equal and opposite force would change the asteroid's orbital velocity by a
few centimeters per second. At that time, there may even be advanced rocket
motors with enough power to do the job.
The take-home message is that, thanks to the development of human
intelligence and our continually increasing knowledge base, the next time
an asteroid threatens to destroy the planet, there's a good chance that
life on Earth will be saved. And even if most life is destroyed, take
heart. After another sixty-five million years, the cycle of evolution may
lead to another civilization with the ability to protect Earth against
these behemoths from outer space.
|
|
