Power Tools for Technical Communication:
Cause-Effect Formatting

In this lab, you add headings, lists, and citations to the unformatted text of a cause-effect discussion:
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Extraterrestrial impacts refer to asteroids and comets striking the Earth's surface intact. Historical evidence of impacts, such as the one that extinguished the dinosaurs 65 million years ago and drove 70% of all species into extinction or the catastrophe at the end of the Permian (245 million years ago) where over 90% of all species were snuffed out in a moment of time, clearly demonstrate their extraordinary worldwide destructive power [Verschuur source, page 7]. Their potential devastation to modern day civilizations is no less dangerous and merits a closer inspection of the effects of such an occurrence. For simplicity, the effects of a 1- to 2-kilometer impact is discussed because the danger from smaller impacts is considerably less and because the danger from larger impacts is total extinction.

Gases condensing out of the early Solar System not only created the planets but literally millions of much smaller bodies commonly referred to as planetesimals or minor planets, commonly known as asteroids and comets. Though fundamentally different in composition, both orbit the Sun in unstable elliptical orbits that are often perturbed by the gravitational interaction from the larger planets and by impacts with other planetesimals. As the Solar System evolved, the bulk of these planetesimals were either swept up by the planets in a barrage of impacts or ejected from the Solar System. Those that remain, however, pose a serious threat to the stability of the Earth and life as we know it.

Statistical analysis of historical records indicates a major impact of a 1-kilometer asteroid on Earth about once every 100,000 years [Steele source, page 125]. Depending on where it struck, an impact of this magnitude would easily devastate much of life as we know it. Anything larger, in the 2- to 10-kilometer range, would destroy civilization everywhere on the planet. Anything above 10 kilometers would eliminate all forms of life everywhere on the planet's surface, including life in the oceans—a scenario often referred to as "total extinction."

Computer simulations of a 1- to 2-kilometer asteroid striking the Earth at approximately 20 to 25 kilometers per second and examination of historical data, especially data from the extinction of the dinosaurs 65 million years ago by a 6-kilometer comet, yield the effects discussed in the following.

As the asteroid enters the upper atmosphere at supersonic speeds, the air, unable to move out of the way, begins to pile up in front of the object. As the asteroid penetrates into the deeper layers, the atmospheric density increases and begins to form a shockwave ahead of the asteroid. When the asteroid strikes the Earth, the inherent potential energy bound up in the shockwave front is transformed into kinetic energy as it plows into the surface [Melosh source, page 46]. The resulting physical impact of the incoming shock wave, the asteroid behind it, and the surface materials at point of impact—these forces combine to vaporize the asteroid and surrounding surface materials. The sudden transformation of solids into gases releases an explosion that combines with the expanding shockwave to produce a hot-air blast traveling outward and upward at high speed. The lateral portion of the air blast travels across the surface with a speed sufficient to destroy everything within a 1000-mile radius.

The effect of the combined velocity and mass of the incoming asteroid literally punches a hole in the atmosphere as the air is pushed aside. As a result, a column of reduced air pressure is created behind the asteroid and extends all the way up to the upper atmosphere [Gehrels source, pages 730-731]. On impact, a large portion of the vaporized material as well as some of the shockwave is sucked up into this rarefied wake and injected into the upper atmosphere where it is dispersed around the globe by high speed winds.

Two characteristics mark this ejected material. First, because it consists predominantly of vaporized material, it is extremely hot—around several thousand degrees. Second, as it encounters the thinner upper atmosphere, it becomes transparent as it spreads out, enhancing its radiation properties [Gehrels source, pages 732-733]. Think of it as a very large grill turned up to high encircling the earth. Since this ejected, vaporized material radiates downward, its effect on the surface of the Earth is to heat it to a very temperature, as much as 1000 centigrade. The result is fires igniting spontaneously around the Earth. The only portion to escape this scenario would be the area immediately surrounding the impact site where the air blast would blow out the fires.

Ahead of the vaporized portion of the impact site, the expanding shockwave compresses the underlying rock to a very high density. As the shockwave passes, the compressed rocks rebound with a violent force, ejecting as much as 100 times the total mass of the asteroid into the atmosphere [Melosh source, page 48].

The larger particles and boulders fall back to earth within 3 to 4 diameters of the crater while the finer particles reach the upper atmosphere, some actually escaping into space [Gehrels source, page 740]. The result is to blanket the Earth with a fine dust. This dust layer combined with the rising clouds of soot from the fires causes the atmosphere to become virtually translucent to the sun's rays. Darkness surrounds the globe and temperatures begin to fall.

Nitrous oxide is produced by shockwaves as they transform the chemical properties of the air ahead of the shockwave. With an impact of this magnitude, very large amounts of nitrous oxide are produced both at lower and upper levels of the atmosphere. In addition, the chemical composition of soot reacts with the atmosphere producing other acidic chemicals such as sulfuric acid. The combination of the two produces a very potent acid rain on virtually every part of the planet. The acid rain changes the pH value of the ocean waters which in turn affects all life living in the oceans. On land, the acid rain effectively kills much of the remaining plant life and thoroughly acidifies the soil [Gehrels source, page 730].

In addition to the life-threatening effects of acid rain, the nitrous oxides in the atmosphere have the unfortunate effect of destroying the ozone shield, the shield that protects the surface of the earth. What is left of life is easily destroyed by dangerous radiation from the Sun.

If the asteroid had the unfortunate luck of landing in the ocean (which is likely since the ocean covers 70% of the Earth's surface), the resulting shockwave would push the ocean water aside, creating a very large hole. As the shockwave passed, the waters would rush back in to fill the hole causing a large spike of water in the center as the water overruns the original diameter of the hole. A tsunami, which is a large undersea wave generated by shock or sudden movement, is would be caused by this initial shockwave and would be followed by reduced tsunamis resulting from the inundations in the water as it oscillates back into the hole. As the tsunami approaches land, the undersea wave begins to pile up (much like a regular wave) as the shallower continental shelf is approached. The wave builds to tremendous heights as it strikes land, devastating anything living along the coast. For a 1- to 2-kilometer asteroid impacting the ocean, the tsunami height might reach 2000 to 3000 meters as it reached land [Gehrels source, page 745]. That would wipe out the East Coast all the way to the Appalachian Mountains. In addition to tsunamis, the column of water vapor lifted into the atmosphere through the wake would significantly raise the water content of the upper atmosphere which is normally very low. The result would be a heightened greenhouse effect that would raise the average surface temperature of the entire planet uniformly by as much as 10 to 15 degrees centigrade [Steele source, page 50]. Even if the resulting increase were less than the predicted 10 to 15 degrees, the effect on both plant and animal life would be globally devastating over the long term.

Depending on whether the asteroid struck land or water, the death toll of a 1- to 2-kilometer asteroid would be in the billions, perhaps 1 to 1.5 billion people initially. Long-term effects from the greenhouse effect, acid rains, ozone depletion, and destruction of the photosynthesis cycle could easily wipe out another 1 to 1.5 billion [Steele source, page 49]. The devastation would be most severe in Third World countries where starvation and exposure would wipe out most of their populations.

These information sources were used in this document: Impact: The threat of Comets and Asteroids by Gerrit L. Verschuur (the book was published in New York, New York, by Oxford University Press in 1996.

Tom Gehrels was the editor for a bok entitled Hazards Due To Comets and Asteroids, published in Tucson, Arizona, by The University of Arizona Press in 1994.

Duncan Steele was the author of Rogue Asteroids and Doomsday Comets, a book published in 1995 by John Wiley & Sons in New York, New York.

Melosh, H.J., wrote the book Impact Cratering: A Geologic Process, published by Oxford University Press in 1989 in New York, New York.

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