Asteriods, Dinosaurs, and Geology: Catastrophic Events and the Theory of Mass Extinction

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Asteriods, Dinosaurs, and Geology: Catastrophic Events and the Theory of Mass Extinction

Overview

Fossils were first determined to be the remains of extinct animals in the nineteenth century. This realization carried with it the concept of extinction, something that did not play a major part in the religion-dominated science of the day. As the science of geology developed, so did the understanding that, every so often, a very large percentage of species on the Earth vanished for no apparent reason. There have been at least 10 mass extinctions recorded in the fossil record in the past 600 million years. While many reasons for this have been proposed, there is only one such event for which the cause is known, that which precipitated the extinction of the dinosaurs. This catastrophic event, caused by the impact of a large body such as a comet or small asteroid on the Earth's surface, posed a completely novel approach to understanding our geologic past. It also helped spur awareness of events ranging from nuclear disarmament to the effects of potential asteroid impacts on the modern world.

Background

Dinosaurs were first brought to the public's attention in the nineteenth century when it was realized that their fossils represented the remains of huge reptilian creatures that no longer existed. For over 100 years they have captured the public's attention, both for what they might have been like as well as how they might have died out. Over this time, speculation abounded with suggestions that they fell to evolutionary degeneracy, overspecialization, egg predation by mammals, climate change, disease, or other factors.

In the late 1970s, Nobel laureate Luis Alvarez (1911-1988), his son Walter, Frank Asaro, and Helen Michel were conducting investigations of rock layers in Italy, looking for iridium that might have come from cosmic dust settling to the surface. They felt that measuring the accumulation of this dust could help to correlate sections of rock over the planet, showing which sediments were deposited at the same time. They decided to look for iridium because it is more common in meteorites than on the surface of the Earth (most terrestrial iridium now resides in the Earth's core) and because iridium is more easily detected than are other likely metals.

What they found was a layer of clay that marked the boundary between the Cretaceous period (the latest geologic period in which dinosaurs were known to have lived) and the subsequent Tertiary period. This "K-T" boundary (so called because of the geologic shorthand for these two periods) contained a layer of very distinctive clay with iridium levels that were significantly higher than those in neighboring sediments. After eliminating other possible reasons for this geologic anomaly, they concluded that a large object had struck the Earth, causing the dinosaurs' extinction. This finding was published by Alvarez in the premier scientific journal Science in June 1980. Science fiction writers had for years speculated about such impacts, and Eugene Shoemaker had convincingly demonstrated that the Earth still bore impact craters. This was, however, the first time that a major geologic event was directly tied to an impact.

Initially viewed with a high degree of skepticism, further discoveries, combined with a lack of counterevidence, led scientists to realize that such a large impact must have happened. The final piece of evidence was the discovery of a large impact crater just off the Yucatan Peninsula. The date and physical characteristics of this crater were such a close match to that predicted by Alvarez that virtually no doubt remains about this event, although the role of impacts in other mass extinctions is still hotly debated.

Impact

Alvarez's paper presented results that seemed more like science fiction than scientific theory. His findings affected science and society in several ways:

Scientists had to consider and, later, accept the role that catastrophe can play in geologic and biologic events. At first, this seemed to herald a retreat from uniformitarianism, the doctrine that the forces of geological change that shaped the prehistoric world are the same processes at work today. In addition, scientists such as Stephen Jay Gould (1941- ) began to think increasingly about the role of contingency, or independent phenomena, in evolutionary theory.
Further research showed that nuclear war could mimic some of the effects of large impacts, particularly a "nuclear winter" induced by clouds of sun-obscuring dust raised after a massive explosion. This vivid picture of the potential outcome of nuclear weapons use may have changed the way nuclear weapons were viewed toward by end of the Cold War.
The public, always fascinated with dinosaurs and their extinction, started thinking about the chance that such events could happen during their own lifetime. This led to several programs to search for near-earth asteroids and ways to divert asteroids or comets that might hit the Earth.

Charles Lyell (1797-1875) led one of the first great revolutions in geology by advocating the theory of uniformitarianism, the view that geologic change occurs at a steady, slow rate over millions of years, an idea first proposed by James Hutton (1726-1797). Though Georges Cuvier (1769-1832) proposed that catastrophic events and extinctions had occurred in the earth's past, the notion that they might have played a major role in the history of life was largely discounted. Recent geological evidence of a massive asteroid impact, however, has prompted modern scientists to revisit their views.

Largely as a result of Alvarez's paper, scientists increasingly accept the view that catastrophic events do indeed play a major role on Earth. Whether that event is a massive volcanic eruption, a nearby supernova, or a meteor impact, random catastrophes can occur. Uniformitarianism has been recast somewhat in the wake of the K-T impact; scientists now feel that catastrophes happen, but the causes for them and the mechanisms by which they wreak havoc can be understood; furthermore, these causes and mechanisms remain the same over time. For example, meteors fall to Earth—they are not thrown by angry deities. Further, the damage that a meteor will do can be calculated, and is the same as an identical meteor would have done 100 million years ago. In other words, the natural and physical laws that govern our world and universe are unchanging, even if they sometimes manifest themselves through random catastrophe.

Another victim of large impacts is the concept of steady, gradual survival of the fittest. Replacing it is a more dynamic theory of evolution that recognizes long periods of evolutionary quiet, punctuated with brief intervals of rapid change. Among the events that can usher in this change is a mass extinction event such as the K-T impact, which opened up a tremendous number of evolutionary niches. Many organisms moved in to fill them, evolving at accelerated rates. Once the niches were filled, a quieter, steady-state condition resumed until the next major event. Survival of the fittest in this scheme is tempered by the concept of contingency. Some organisms died out, not because they were less fit, but because they were unlucky enough to live underneath a future asteroid impact site. Other organisms lived because they happened to be on the other side of the world when the asteroid fell, regardless of their degree of evolutionary fitness.

In addition to the scientific impact of this discovery, there were social and political effects as well. People began to realize that such events could happen again and that, even if they didn't, nuclear weapons could produce the same results. This realization, coupled with the wide coverage of Comet Shoemaker-Levy 9's massive 1994 impact on Jupiter, made people aware that "it could happen here." One result of this was a spate of movies and novels about comets hitting the Earth. News stories about near-Earth asteroids gained more attention, and there was some discussion (still continuing in a haphazard fashion) about deploying systems to detect and either divert or destroy potentially deadly comets and asteroids. Largely as a result of Alvarez's paper, people increasingly view the solar system as a giant shooting gallery in which the Earth is bound to be hit again at some point.

Many assume that, because the K-T extinction was caused by an asteroidal impact, then all mass extinctions must have been caused in the same way. Other explanations for mass extinctions include continental drift, massive volcanic eruptions, nearby supernovae, and nearby gamma ray bursts. Interestingly, virtually every paper suggesting a mechanism by which a mass extinction may have occurred tends to suggest that all extinctions occurred for the same reason. This is known as conceptual simplification. The K-T extinction has taught us only that this single extinction was caused by an asteroid impact; others might have been caused in the same way. More importantly, it has shown us that large, catastrophic events, of which impacts are one, can profoundly affect life on Earth. Other, similarly catastrophic events may have caused other mass extinctions. However, there are at least as many potential causes for mass extinctions as there are extinctions recorded in the fossil record. It is not realistic to think that a single type of event caused them all.

P. ANDREW KARAM

METEORITES ON ICE: HUNTING FOR METEORITES IN ANTARCTICA

Shooting stars have captured man's attention for millennia. Most meteorites vaporize high in the atmosphere because they are generally specks of dust or grains of sand, and the high temperatures caused by friction with the air heats them to the point of disintegration. Some, however, reach the ground, giving our only direct evidence of what lies beyond the Moon. Until recently, meteors were found more or less by luck. The bottoms of impact craters were mined, some found their way to museums, and some were bought by researchers. Meteors helped to establish the age of the Solar System, Earth's age, and the chemical composition of asteroids and comets. However, this wealth of information was doled out only sparingly because confirmed meteorites were rare and hard to find. In the late 1980s researchers realized that the ice caps might hold many meteorites, and expeditions were sent to Antarctica to look for them. Meteorites were found almost immediately. In particular, some areas were discovered where, due to the ice flow and lack of native rocks, virtually every rock seen seemed to be a meteorite.

Most meteorites are thought to originate from comets and asteroids. However, several Antarctic meteorites seem to share key chemical signatures with Mars, leading to the supposition that they were blasted from Mars by a giant impact at some time in the past. One in particular was examined and thought to contain signs of ancient Martian microbes, the first signs of life elsewhere in the universe. Unfortunately, this was shown to be an unlikely interpretation of the data, and the search for extraterrestrial life continues.

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