Centuries ago, the Western vision of the past saw an Earth that had been created a few thousand years ago, and had been shaped since that time by a number of global cataclysms (see catastrophism). This view gradually gave way to the consensus that the Earth was several billion years old, and that its features reflected the slow processes of gradual change.
Since 1970, this view has gradually expanded to accommodate the fact that the Earth has in fact gone through periods of abrupt and catastrophic change due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extermination of large numbers of plants and animals.
The fact that this modified view of the Earth's history did not emerge until recently seems surprising. Based on crater formation rates determined from our closest celestial partner, Luna, astronomers have determined that during the last 600 million years the Earth has been struck by 60 objects larger than 5 kilometers or more across. The smallest size of these impactors would release the equivalent of 10 million megatons of TNT and leave a crater 95 kilometers across.
In the past 600 million years there have been 5 major mass extinctions that on average extinguished half of all species. The last such mass extinction led to the demise of the dinosaurs and has been found to have coincided with a large asteroid impact; this is the Cretaceous-Tertiary extinction event.
Evidence is also mounting that other mass extinctions may also have been caused or at least assisted by large impacts. The largest mass extinction to have affected life on Earth was the Permo-Triasic one that ended the Permian Period 250 million years ago and killed-off 90% of all species.
In 1980 Luis Alvarez[?] and his son Walter[?] led a team from the University of California at Berkeley that discovered unusually high concentrations of iridium, an element that is rare in he Earth's crust but relatively abundant in many meteorites From the amount and distribution of iridium present in the 65 million year old "iridium layer" , the Alvarez team later estimated that an asteroid of 10-14 kilometers must have collided with the earth. This iridium layer at the C-T boundary has been found worldwide at 100 different sites. Multidirectionaly shocked quartz, which is only known to form as the result of large impacts, has also been found in the same layer at more than 30 sites. Soot[?] and ash at levels tens of thousands times normal levels were found with the above.
Anomalies in chromium isotopic ratios found within the C-T boundary layer strongly support the impact theory. Chromium isotopic ratios are homogeneous within the earth, therefore this isotopic anomalies exclude a volcanic origin which was also proposed as a cause for the iridium enrichment. Furthermore the chromium isotopic ratios determined in the C-T boundary are similar to the chromium isotopic ratios found in carbonaceus chondrites[?]. Thus a probable candidate for the impactor is a carbonaceous asteroid but also a comet is possible because comets are assumed to consist of material similar to carbonaceous chondrites[?].
Probably the most convincing evidence for a worldwide catastrophe was the discovery of the crater which has since been named Chicxulub Crater. This so-called smoking gun is centered on the Yucatan Peninsula of Mexico and was discovered by Tony Camargo and Glen Pentfield while working as geophysicists for the Mexican oil company PEMEX[?]. What they reported as a circular feature later turned out to be a crater estimated to be 180 kilometers in diameter. Other researchers would later find that the end-Cretaceous extinction event that wiped out the dinosaurs had lasted for thousands of years instead of millions of years as had previously been thought. This would be the final piece of evidence that convinced the vast majority of scientists that this extinction resulted from a point event that is most probably an extra-terrestrial impact and not from increased volcanism and climate change (which would spread its main effect over a much longer time period).
It was the lack of high concentrations iridium and shocked quartz which has prevented the acceptance of the idea that the Permian extinction (so-called mother of mass extinctions) was also caused by an impact. However, during the late Permian all the continents were combined into one supercontinent named Pangaea and all the oceans formed one superocean, Panthalassa. If an impact occurred in the ocean and not on land at all, then there would be little shocked quartz released (since oceanic crust has relatively little silica) and much less material
Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other impacts of the same order of magnitude that did not result in any mass extinctions, and in fact there is no clear linkage between an impact and any other incident of mass extinction.
Nonetheless it is now widely believed, if a little on faith, that mass extinctions due to impacts are an occasional event in the history of the Earth. Indeed, in the early history of the Earth, about four billion years ago, they were almost certainly common since the skies were far more full of "junk" than at present. Such impacts could have included strikes by asteroids hundreds of kilometers in diameter, with explosions so powerful that they vaporized all the Earth's oceans. It was not until this "hard rain" began to slacken, so it seems, that life could have begun to evolve on Earth.
In addition to the extremely large impacts that happen every few tens of millions of years, there are many smaller impacts that occur much more frequently but which leave correspondingly smaller traces behind. Due to the strong forces of erosion at work on Earth, only relatively recent examples of these smaller impacts are known. A few of the more famous or interesting examples are:
The most significant recorded impact in recent times occurred at Tunguska in Russia, in 1908; see that article for more details. But although the Tunguska event was both spectacular and unparalleled in any historical record, it no longer seems as unique and unusual as it once did. We now know that Earth impacts, fairly big ones, are happening all the time.
The late Eugene Shoemaker of the US Geological Survey came up with an estimate of the rate of Earth impacts, and suggested that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious, but they generally go unnoticed for a number of reasons: the majority of the Earth's surface is covered by water; a good portion of the land surface is uninhabited; and the explosions generally occur at relatively high altitude, resulting in a huge flash and thunderclap but no real damage.
Some of have been observed, such as the Revelstoke[?] fireball of 1965, which occurred over the snows of northern Canada. Another fireball blew up over the Australian town of Dubbo[?] in April 1993, shaking things up up a bit but causing no harm.
On the dark morning hours of January 18, 2000, a fireball exploded over the town of White Horse[?] in the Canadian Yukon at an altitude of about 26 kilometers, lighting up the night like day and bringing down a third of of the Yukon's electrical power grid, due to the "electromagnetic pulse" created by the blast. The meteor that produced the fireball was estimated to be about 4.6 meters in diameter and with a weight of 180 tonnes.
A particularly interesting fireball was observed moving north over the Rocky Mountains from the US Southwest to Canada on August 10 1972, and was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera. The object was in the range of size from a car to a house and should have ended its life in a Hiroshima-sized blast, but there was never any explosion, much less a crater. Analysis of the trajectory indicated that it never came much lower than 58 kilometers of the ground, and the conclusion was that it had grazed Earth's atmosphere for about 100 seconds, then skipped back out of the atmosphere to return to its orbit around the Sun.
Many impact events occur without being observed by anyone on the ground. Between 1975 and 1992, American missile early warning satellites picked up 136 major explosions in the upper atmosphere.
The Tunguska event was about a thousand times more powerful than such events. Shoemaker estimated that one of such magnitude occurs about once every 300 years. This is not a long interval even by historical standards, and it is a somewhat nerve-wracking question to consider when the next "Big One" will be, and more to the point, where.