It is now generally accepted among paleontologists and geologists that the most likely reason for the extinction of the dinosaurs 65 million years ago, during the so-called Cretaceous-Tertiary extinction event, was an asteroid impact. However, there is still some uncertainty over what precise impact might have caused the extinction. Currently, the of the strongest candidates for the asteroid impact that killed off the dinosaurs is the asteroid which slammed into the Yucatan Peninsula roughly 65 million years ago, creating the massive Chicxulub Crater.
- Asteroid Impact and Extinction Events -
So-called extinction-level events, catastrophes which destroy many or even most species then living on the Earth, historically occur every several tens of millions of years. The extinction of the dinosaurs during the Cretaceous-Tertiary extinction event, though it is the best known because of our fascination with dinosaurs, actually was not the most devastating such event. That dubious distinction belongs to the even earlier Permian-Triassic extinction event, which occurred 250 million years ago and may have eliminated as much as 90% of the species then alive, including the total elimination of several genera of marine life.
A wide variety of explanations have been put forward to account for these extinctions, many involving possible causes of rapid and catastrophic climate change. However, the dinosaur extinction event in particular is believed to have resulted from an asteroid impact. Because of the tremendous speeds at which space objects travel, impacts by even seemingly small objects can be devastating. An asteroid a hundred metres wide could produce devastation similar to that of a nuclear weapon. One a kilometre wide could devastate current human civilization. One more than ten kilometres wide would have permanent consequences for the biosphere more generally - and it is just such an asteroid which is believed to have come down at Chicxulub at the end of the Cretaceous.
- About the Chicxulub Crater -
The Chicxulub crater was discovered by a geologist searching for oil in Mexico's Yucatan Peninsula during the 1970s. The landmark was so massive that the man in question, Glen Penfield, was initially unable to confirm that it was even a crater: Chicxulub is horrifyingly massive, at 180 kilometres wide (about 110 miles). His company, Pemex, initially decided not to release the findings, but eventually the data was presented at a scientific conference in 1981, after which it found its way to American scientists Luis Walter Alvrez and Alan Hildebrand, who had already proposed the likelihood of a massive asteroid impact causing the extinction of the dinosaurs and were now searching for possible craters.
Chicxulub crater seemed to fit most of their predictions. For example, it was extraordinarily large, and contained materials like tektite glass, which requires so much extreme heat to form that it is only known to exist in the aftermath of nuclear explosions - and asteroid impacts. It also contained iridium, which exists on asteroids in much higher quantities than on Earth itself - and, moreover, a telltale concentration of iridium is dispersed in clay dated roughly to the period of the Chicxulub impact, all around the world (where it would have been flung after the impact). Eventually it was estimated that the asteroid which struck Chicxulub would have been about ten kilometres or six miles wide, yielding an explosion equivalent to millions of nuclear weapons combined.
- What Might Have Happened -
Piecing together the effects of the Chicxulub impact are difficult, but scientists agree on several of the likeliest immediate consequences, as they would have been experienced on the surface of the Earth. The asteroid crashed near the coastline, flinging up a tremendous amount of steam and dust into the atmosphere as well as causing a massive tsunami as much as several thousand feet high. The shock wave and the tsunami raced around the Earth, creating an initial wave of immediate devastation from fire and flooding. For the next several years, clouds of debris thrown into the atmosphere would also have resulted in marked global cooling, killing a substantial proportion of plant life and thus leading to mass starvation throughout the food chain.
All of this is, of course, speculative. Not all scientists are convinced that an asteroid impact accounts for all, or even most, of the extinctions which occurred at the end of the Cretaceous. Moreover, it is possible that the devastation in the fossil record is actually the cumulative result of several such impacts, not just the one at Chicxulub. The Silverpit Crater off Britain, and the Boltysh Crater in the Ukraine, are both dated to the same period, though they are much smaller. The undersea Shiva formation, near India, is even larger, at about 600 kilometres long, and formed at roughly the same time. However, it has not yet been confirmed that the Shiva formation is actually an asteroid crater. If Shiva was created by an asteroid impact, the consequences would have been even more devastating than the impact at Chicxulub, and could be most responsible for the extinction of the dinosaurs.
- Finding the Asteroid -
At a few telescopes and laboratories around the world, astronomers are working on one of the oldest "cold cases" in the history of investigations: trying to identify the origins of the asteroid which caused the Chicxulub crater. This is a complicated search which involves both estimating which families of asteroids might have been most likely to cross Earth orbit at the time of the impact, as well as which have chemical compositions very similar to the Chicxulub asteroid. (Members of a particular group, or family, of asteroids tend to share similar chemical compositions.)
Currently, the leading hypothesis is that the Chicxulub asteroid was a member of the so-called Baptistinia family, created 160 million years ago as a result of an enormous collision and the break-up of their much larger parent asteroid. Today, the Baptistina asteroids are identifiable by their unusually high amounts of carbonaceous chondrite and their level of chromium (which are similar to those at Chicxulub). It is far from clear whether this hypothesis will stand up to further testing; if not, another possibility may be the Flora asteroid family.