For millions of years, giant, thundering reptiles roamed the lands, dominated the skies, and swam in the oceans of a prehistoric Earth. Dinosaurs reigned supreme. Then, quite suddenly, some 65 million years ago, they vanished. What happened to these giant creatures and to thousands of other ancient animal species?
From archaeological and geological records, scientists know that some tremendous catastrophe occurred about 65 million years ago on this planet. It affected life more extensively than any war, famine, or plague in human history, for in that cataclysm, about 70 percent of all species then living on Earth—including, of course, the dinosaurs—disappeared within a very short period. This mass extinction is also referred to Cretaceous-Tertiary Mass Extinction event, or simply the K-T event.
In 1980, the scientists Luis W. Alvarez (1911-88) and his son, Walter Alvarez (1940- ), along with their colleagues at the University of California at Berkeley, discovered that a pronounced increase in the amount of the element iridium in Earth's surface had occurred at precisely the time of the disappearance of the dinosaurs. First seen in a peculiar sedimentary-clay area found near Gubbio, Italy, the same iridium enhancement was soon discovered in other places around the world in the thin sedimentary layer that was laid down at the time of the mass extinction. Since iridium is quite rare in Earth's crust and is more abundant in the rest of the solar system, the Alvarez team postulated that a large asteroid (about 6 miles [10 km] or more in diameter) had struck the ancient Earth. This cosmic collision would have promoted an environmental catastrophe throughout the planet. The scientists reasoned that such an asteroid would largely vaporize while passing through the Earth's atmosphere, spreading a dense cloud of dust particles, including quantities of extraterrestrial iridium atoms, uniformly around the globe.
Stimulated by the Alvarez team's postulation, many subsequent geologic investigations have observed a global-level of enhanced iridium in this thin layer (about 0.4-inch [1-cm] thick) of the Earth's crust (lithosphere) that lies between the final geologic formations of the Cretaceous period (which are dinosaur-fossil rich) and the formations of the early Tertiary period (whose rocks are notably lacking in dinosaur fossils). The Alvarez hypothesis further speculated that following this asteroid impact, a dense cloud of dust covered Earth for many years, obscuring the Sun, blocking photosynthesis, and destroying the very food chains upon which many ancient life-forms depended.
Despite the numerous geophysical observations of enhanced iridium levels that reinforced the Alvarez team's impact hypothesis, many geologists and paleontologists still preferred other explanations concerning the mass extinction that occurred about 65 million years ago. To them, the impact theory of mass extinction was still a bit untidy. Where was the impact crater? This important question was answered in the early 1990s when a 112-mile- (180-km-) diameter ring structure called Chicxulub was identified from geophysical data collected in the Yucatán region of Mexico. The Chicxulub crater has been age-dated at 65 million years. Further studies have also helped confirm its impact origin. The impact of a 6-mile- (10-km-) diameter asteroid would have created this very large crater, as well as causing enormous tidal waves. Scientists have found evidence of tidal waves occurring about 65 million years ago around the Gulf of Mexico region.
Of course, there are still many other scientific opinions as to why the dinosaurs vanished. A popular one is that there was a gradual but relentless change in the Earth's climate to which these giant reptiles and many other prehistoric animals simply could not adapt. So, one can never absolutely prove that an asteroid impact "killed the dinosaurs." Many species of dinosaurs (and smaller flora and fauna) had, in fact, become extinct during the millions of years preceding the K-T event. However, the impact of a 6-mile- (10-km-) across asteroid would most certainly have been an immense insult to life on Earth. Locally, there would have been intense shock-wave heating and fires, tremendous earthquakes, hurricane winds,
An artist's rendering of a killer impactor striking a coastal region on Earth. All life near the impact point is instantly destroyed from the effects of high temperatures and pressures. Giant tidal waves would carry destruction well beyond the impact zone—up and down the doomed coastline, as well as far inland. Finally, huge quantities of dust from this impact would be hurled high into the upper atmosphere, blocking life-sustaining sunlight and creating a planetwide "nuclear winter" that causes the eventual extinction of most species. (NASA; artist, Don Davis [artwork created May 9, 1992])
and hundreds of billions of tons of debris thrown everywhere. This debris would have created months of darkness and cooler temperatures on a global scale. There would also have been concentrated nitric acid rains worldwide. Sulfuric acid aerosols may have cooled Earth for years after the impact. Life certainly would not have been easy for those species that did survive. Fortunately, such large, extinction-level events (ELEs) are thought to occur only about once every hundred million years. It is also interesting to observe, however, that as long as those enormous reptiles roamed and dominated the Earth, mammals, including humans themselves, would have had little chance of evolving. So if the large asteroid impact hypoth-
esis is correct, the ancient catastrophic event was clearly a case of bad luck for the dinosaurs. Yet, this K-T event was also a case of exceptionally good fortune for the eventual emergence of human beings—the intelligent creatures who would dominate the planet some 65 million years later and start to travel beyond Earth into the solar system.
The possibility that an asteroid or comet will strike Earth in the future is quite real. Just look at an image of Mars or the Moon, and ask how those large impact craters were formed. A comet, called Shoemaker-Levy 9, hit Jupiter in 1994. Fortunately, the probability of a really large asteroid or comet striking the Earth is quite low. For example, space scientists estimate that Earth will experience (on average) one collision with an Earth-crossing asteroid (ECA) of 0.6-mile- (1-km-) diameter size or greater every 300,000 years.
Yet, on May 22, 1989, a small ECA, called 1989FC, passed within 428,840 miles (690,000 km) of humans' home planet. This cosmic "near-miss" occurred with just 0.0046 astronomical unit to spare—a distance less than twice the distance to the Moon. Cosmic impact specialists have estimated that if this small asteroid, presumed to be about 660 feet (200 m) to 1,310 feet (400 m) in diameter, had experienced a straight-in collision with Earth at a relative velocity of some 10 miles per second (16 km/s), it would have impacted with an explosive force of between 400 to 2,000 megatons (MT). A megaton is the energy of an explosion that is the equivalent to one million tons of the chemical high explosive trinitrotoluene (TNT). If this small asteroid had hit a terrestrial landmass, it would have formed a crater some 2.5-miles (4-km) to 4.4-miles (7-km) across and produced a great deal of regional-scale (but probably not global-scale) destruction.
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