defending the earth

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system. When they impacted a planet, lots of water, methane, and ammonia ... kilometer range is at least partially responsible for the fall of the dinosaurs and the ...
18 DEFENDING THE EARTH Nor the comet that came unannounced out of the north flaring in heaven, Nor the strange huge meteor-procession dazzling and clear shooting over our heads, (A moment, a moment long it sail'd its balls of unearthly light over our heads, Then departed, dropt in the night, and was gone;) Walt Whitman, from Year of Meteors

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EAR misses have happened many times in Earth's long history. A fragment of asteroid or an errant comet approaches our world closely, skips through the upper atmosphere and back into space leaving a noiseless contrail, or bathes the evening sky in the shimmer of light reflected from its tail. But sometimes, a near miss or deflection shot morphs into a direct hit. In the beginning, four and a half billion years ago, such impacts were a good thing. In those days, comet swarms criss-crossed the infant solar 203

Chapter 18

system. When they impacted a planet, lots of water, methane, and ammonia were released. Earth's oceans and atmospheric ingredients, necessary for the formation of life, are thought to have been deposited in this manner. It's not impossible that early life was actually transferred from Mars to Earth in the aftermath of an impact on the low-gravity Red Planet. As the solar system evolved, potential impactors of the inner planets became rarer. And instead of being beneficial for terrestrial life forms, the impacts that occurred were destroyers rather than creators. The fossil record reveals that many mass extinctions disrupted the terrestrial biosphere during the last billion years or so. The most famous of these, which occurred about 65 million years ago, ended the reign of the dinosaurs. Many believe that the impact of a celestial body in the 10kilometer range is at least partially responsible for the fall of the dinosaurs and the rise of the mammals. No humans existed on Earth at the time of the Great Fall, but we can imagine how the thunder lizards or our crude, early mammal ancestors might have reacted as the huge space mountain descended toward ground zero, in what is now the Yucatan. Perhaps they had been aware of subtle shifts in skylight for a few days prior to impact, if the object was a comet. Perhaps the normal cycles of sleep and the hunt had been disrupted by these strange changes in the sky. If the meteorite was instead of asteroidal origin, the first warning would have been flames in the sky and sonic booms. Perhaps some creatures craned their necks toward the intruder, perhaps others cowered in fear. It made no difference. The Cretaceous±Tertiary (K/T) meteorite struck the Earth with enormous kinetic energy. To get some idea of the effects on Earth's environment, consider that the largest hydrogen bombs in the Cold War arsenals of the super powers had about 1,000 times the explosive yield of the weapons that destroyed Hiroshima and Nagasaki in 1945. The energy released in the Yucatan by the K/T impact had something like half-amillion times the yield of the largest H-bombs. Such a large explosion, in one place and time, was certainly not a good thing for Earth's ecology. A huge fireball would have instantly extinguished all or most life within thousands of kilometers of ground zero. An enormous mushroom cloud would have risen to the stratosphere and ultimately dispersed to enshroud the entire planet. Firestorms would dart across the landscape; giant tsunamis would haunt the oceans and crash against coastal regions. It's not impossible that other seismic eventsÐ volcanoes and earthquakesÐwould be triggered globally by the impact.

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