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| THE KT-BOUNDARY
According to abundant geological evidence, an asteroid roughly 10 km (6 miles) across hit Earth about 65 million years ago. This impact made a huge explosion and a crater about 180 km (roughly 110 miles) across. Debris from the explosion was thrown into the atmosphere, severely altering the climate, and leading to the extinction of roughly 3/4 of species that existed at that time, including the dinosaurs. Many asteroids of this type are now known; their orbits pass through the inner solar system and cross Earth's orbit. Some of these could potentially hit Earth in the future. Most, but not all are smaller than the one that hit us 65 million years ago.
Fossils found in soil layers of different ages show a record of slow, gradual changes in species, with simple organisms gradually being replaced by more complex organisms, apparently by evolutionary processes driven by natural selection. For example, 1000 million years (abbreviated here as My) ago, the oceans held only simple organisms like algae, while the land was relatively lifeless. Fish fossils appear in strata after about 500-600 My ago; dinosaurs and giant reptiles were on the land by 200 My ago. Mammals were not common until after 65 My ago, and humanlike creatures appeared only in the last 4 My.
|Figure 1. Earth as it appeared at the time of the impact 65 My ago. Note that the Atlantic Ocean had just formed by the Americas breaking off from Europe and Africa, and that the Atlantic is narrower in this picture than it is today.|
Ever since the fossil sequence began being mapped around 1800, geologists noticed that striking "breaks" occurred in the sequence, when one group of fossilized species gave way to other groups during short intervals. Indeed, these breaks were the basis for dividing geologic time into different eras with different names. For example, the Paleozoic era gave way to the Mesozoic era of middle life forms, which in turn gave way to the present Cenozoic era of recent life. Prior to the 1980s, the causes of the breaks were unknown and were vaguely attributed to climate change or passed over in many geology textbooks.
The above description has been challenged for two centuries by religious fundamentalists. Fundamentalists are defined as people who believe that the primary way of learning about nature should not be the scientific method, or compilation of evidence tested in different labs in different countries, but rather interpretation of ancient manuscripts, such as the Koran, the Old Testament of the Bible, the New Testament, or other ancient writings. The scientific method was hammered out mainly in the 1600s, when naturalists of that period agreed that information about nature could best be determined by direct observations of nature, and experiments, which would be published openly, in international literature. The key to the method is that assertions published by one scientist can be tested by new, independent observations and measures made by other scientists. Daniel Boorstein's book, "The Discovers," gives an excellent overview of how the scientific method was established. This method of learning about nature continues to be under challenge in America from fundamentalist groups, for example in school boards and state legislatures. In 1998, the Arizona state school board recommended that the term "evolution" be dropped from public school curriculum in the state, only to reverse themselves some months later after a public outcry. In 1999 the Kansas state school board dropped evolution and the Big Bang theory of the universe's origin from the subjects tested in their school system. Several other state school boards subsequently followed in their footsteps. This battle is not new; it is four centuries old!
The breaks in the fossil record play a role in this ongoing story, because many fundamentalists used to argue that the breaks disproved the idea of slow, gradual evolution. Today, the evidence points in a different direction. Evolution typically takes a few million years to alter species dramatically. If an asteroid impact, massive volcanic eruptions, or other major events cause drastic climate changes on a very short time scale, such as 1000 years or less, evolution cannot keep up. Species ill-adapted to the new conditions may die out, and previously obscure species may fill the empty niche.
|Figure 2. One minute before the impact.|
|Figure 3. Two seconds before the impact.|
In the late 1970s, a team of geochemists headed by Lewis Alvarez of Berkeley was studying chemical changes in soil layers corresponding to breaks in the fossil record. In the soil layer that separates the Mesozoic Era from the Cenozoic Era, dating 65 My ago, they found an excess of the element iridium, which is common in meteorites. Meteorites are believed to be fragments of asteroids. Therefore, the Alvarez team theorized that an asteroid had hit Earth at this time, and that the debris ejected from the explosion were spread in the soil layer.
For about ten years, this theory was extremely controversial. However, compelling evidence has accumulated to support the theory.
There are now many lines of evidence to prove that a relatively large impact happened 65 My ago.
|Figure 4. One minute after the impact. This view is based on laboratory experiments and computer simulations of giant impacts. Some debris is being blown out of the atmosphere into space, only to fall back minutes later.|
|Figure 5. One month after the impact. Earth is covered by a hazy layer of dust blown out by the impact and smoke from forest fires. This blocked sunlight, killed much plant life and plankton, and disrupted the food chain.|
|Figure 6. One thousand years after the impact. The dust has cleared and a large crater is revealed. The crater form shown here is based on similar size craters discovered on Venus. Dinosaurs and many other species have mostly died out, and species such as small mammals are beginning to fill the empty environmental niche.|
Asteroids hit Earth typically at high speeds of 10 to 20 km/sec (6-12 miles/sec). During the impact, the kinetic energy in the asteroid (or energy of motion) is converted to explosive energy, blowing debris of dust, soil, and rocks not only into the atmosphere, but out into space, where it fell back into the top of the atmosphere. Early calculations in the 1980s (using in part ideas worked out by Carl Sagan and his colleagues) showed that so much dust entered the high atmosphere that the Earth was shrouded in a dust layer that blocked sunlight for several weeks or months. This would have killed some plants, disrupting the food chain.
Later calculations (especially by Jay Melosh at the University of Arizona) indicated that for the first few hours after the impact, rocky debris would have fallen back into the high atmosphere, creating a storm of glowing fireballs in the sky. The radiant energy from these would have heated the surface to boiling temperatures for some minutes. In the oceans, sea creatures would have been buffered from effects in the first hours. There was probably not enough heat input to cause lakes and oceans to boil (because of the subsurface cool waters), but plankton on the surface might have died out over the weeks of darkness, decreasing the food supply for small fish, which affected the bigger fish, and so on. On the land, the radiant heat pulse from the sky probably would have killed many animals and plants, even far from the impact. In regions of heavy rainstorms or snowstorms, however, these organisms would have survived the first few hours in pockets of almost undisturbed life, but would later find the surrounding food chain disrupted.
These examples show how hard it is to predict the exact effects of the impact. Many species who lived on the surface (such as dinosaurs) might have been decimated in hour or weeks. Species who lived in burrows, or hibernated (like some mammals) might have survived. This may explain why mammals replaced giant reptiles after the impact. Tiny primitive mammals may have emerged from their dens, to find that their giant reptile competitors were mostly gone.
|Figure 7. Some asteroids are double bodies - an asteroid and a satellite. Here such a pair is encountering Earth, and the distant satellite is already entering the atmosphere. Double craters have been found on Earth, caused by such pairs. An example is Clearwater Lakes in Canada. Such pairs of asteroids have also been photographed by space probes.|
Probably not. The biggest known break is the break between the Paleozoic Era and the Mesozoic Era, about 230 to 250 My ago, when something like 90% of then-existing species died out. This has been called the "Great Dying," but has not been explained. So far there is no evidence of an asteroid impact at that time. The second greatest break is the one that we have discussed, 65 My ago, caused by an asteroid impact.
Geologists have divided the eras into shorter intervals called Periods, such as the Jurassic Period, noted for its large dinosaurs. These Periods are also defined by breaks in the fossil record, smaller than the breaks between eras. Many species went extinct during these breaks, but not as many as in the breaks between the Eras. Evidence for impacts, smaller than the one 65 My ago, has been found at some of the breaks, but not at others.
Thus, most scientists now believe that some, but not all of the mass extinctions of species, and breaks in the fossil record, were caused by impacts of asteroids (or comets) of various sizes.
|Figure 8. An Earth approaching asteroid on its way toward our planet (upper center, background). W. Hartmann based this 1989 painting on knowledge that some asteroids have very irregular shapes, and protrayed an extremeley large crater on one side. Nine years later, in 1998, the NEAR spacecraft passed close to asteroid 433 Eros and discovered that it had almost exactly this shape!|
Yes. Asteroids are constantly being deflected from the asteroid belt onto orbits among the inner planets, including Earth. Earth gets hit by meteroids big enough to cause atom bomb sized explosions apparently once every 1 or 2 centuries, but 6 out of 7 of these events occur over oceans. The last one was the Siberian explosion of 1908 (see our Siberian impact web page). Impacts big enough to wipe out many species occur only every 100 My or so, so it is unlikely that we or even our great-great-grandchildren will see one. And by the year 2100, we may have the capability to deflect any approaching asteroid before it hits.
|Figure 9. The asteroid in figure 8 comes close to Earth. Will it hit our planet? It may either hit the Earth or narrowly miss it, passing on one side or the other and being deflected onto a new orbit.|
Astronomers are constantly searching for previously unknown asteroids. During the 20th century they found over 100 such bodies that approach close to Earth's orbit. These are called Earth-approaching asteroids. They range in size from around a kilometer to around 30 km. Probably all possible larger examples are now known, but 21st century astronomers continue to find new ones at smaller sizes below a few km, down to 100 meters or so.
These astronomers estimate that there are only about 20 objects larger than 5 km that could actually hit Earth. A hit by such an object would cause a global catastrophe, but the probability of any one of them hitting our planet within even 1 million years is very low. You'd probably have to wait 10 or even 100 million years to get a high probability of such an impact. This is why we say that the probability of a global catastrohic impact in any person's lifetime is quite small. However, it is not unlikely that a much smaller asteroid fragment could hit Earth and cause a localized, bomb-sized explosion may occur over an inhabited area in our lifetime, producing some localized destruction.