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Artist's impression of a major impact event. The collision between Earth and an asteroid a few kilometers in diameter may release as much energy as several million nuclear bombs detonating.
Impact events are caused by the collision of large meteoroids, asteroids or comets (generically: bolides) with Earth and may sometimes be followed by mass extinctions of life. For discussion of impacts in general, not just on Earth, see crater.
The geology of Earth impact events
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 ancient and that its features reflected gradual changes operating over very long periods of time - a view known as uniformitarianism.
This view has gradually expanded in recent decades to accommodate the fact that the Earth has in fact also 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 plant and animal species. The creation of the Moon is widely attributed to a huge impact early in Earth's history. Impact events have also been credited with creative as well as destructive events; it has been proposed that the water in the Earth's oceans was delivered by impacting comets, and some have suggested that the origins of life may have been influenced by impacting objects bringing organic chemicals to the Earth's surface.
This modified view of the Earth's history did not emerge until relatively recently, chiefly due to a lack of direct observations and the difficulty in recognising the signs of an Earth impact. Large-scale terrestrial impacts of the sort that produced the Barringer Crater in Arizona are rare. The collision of the comet Shoemaker-Levy 9 with Jupiter in 1994 was the first time a major impact event had been directly observed; to date, no such events have been observed on Earth. It was also widely thought that cratering was the result of volcanism. The Barringer Crater was ascribed to a prehistoric volcanic explosion (not an unreasonable hypothesis, given that the volcanic San Francisco Mountains stand only 30 miles to the west). Similarly, the craters on the surface of the Moon were ascribed to volcanism.
It was not until 1903-1905 that the Barringer Crater was correctly identified as being an impact crater, and it was not until as recently as 1963 that research by Eugene Merle Shoemaker conclusively proved this hypothesis. The findings of late 20th century space exploration and the work of scientists such as Shoemaker demonstrated that impact cratering was by far the most widespread geological process at work on the Solar System's solid bodies. As literally every surveyed solid body in the Solar System was found to be cratered, there was no reason to believe that the Earth had somehow escaped bombardment from space.
Based on crater formation rates determined from the Earth's closest celestial partner, the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of five kilometers or more. The smallest of these impactors would release the equivalent of 10 million megatons of TNT and leave a crater 95 kilometers across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.
Mass extinctions and impacts
In the past 600 million years there have been five major mass extinctions that on average extinguished half of all species. The largest mass extinction to have affected life on Earth was the Permian-Triassic one that ended the Permian period 250 million years ago and killed off 90% 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 (K-T) extinction event. There is no solid evidence of impacts leading to the four other major mass extinctions, though many scientists assume that they are at least related to impacts.
In 1980 Luis Alvarez and his son Walter led a team from the University of California, Berkeley that discovered unusually high concentrations of iridium, an element that is rare in the 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 K-T boundary has been found worldwide at 100 different sites. Multidirectionally shocked quartz (coesite), which is only known to form as the result of large impacts or atomic bomb explosions, 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 K-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 K-T boundary are similar to the chromium isotopic ratios found in carbonaceous 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).
Recently, several craters around the world have been dated to approximately the same age as Chicxulub - for example the Silverpit crater in the United Kingdom and the Boltysh crater in Ukraine. This has led to the suggestion that the Chicxulub impact was one of several that occurred almost simultaneously, perhaps due to a disrupted comet impacting the Earth in a similar manner to the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994.
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. One such controversial hypothesis is Tollmann's hypothetical bolide, which claims that the Holocene was initiated by an impact.
Paleontologists Michael Raup and Jack Sepkoski have proposed that an extinction occurs roughly every 26 million years (though many are relatively minor). This led physicist Richard A. Muller to suggest that these extinctions could be due to a hypothetical companion star to the sun called Nemesis periodically disrupting the orbits of comets in the Oort cloud, and leading to a large increase in the number of comets reaching the inner solar system where they might hit Earth.
Indeed, in the early history of the Earth, about four billion years ago, bolide impacts 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.
The leading theory of Moon's origin is the giant impact theory, which states that Earth was once hit by a planetoid the size of Mars; possibly the largest impact Earth has ever suffered.
Recent pre-historic impact events
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:
Barringer Crater, the first crater to be proven the result of an impact
- the Rio Cuarto craters, produced by an asteroid striking Earth at a very low angle
- the Wabar craters, which apparently formed within the past few hundred years
- the Noerdlinger Ries, a 24-km crater in Central Europe, formed about 15 million years ago.
Modern impact events
The most significant recorded impact in recent times was the Tunguska event, which occurred at Tunguska in Russia, in 1908. 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, including 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 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 but causing no harm.
On the dark morning hours of January 18 2000, a fireball exploded over the town of Whitehorse in the Canadian Yukon at an altitude of about 26 kilometers, lighting up the night like day and bringing down a third 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 off 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.
The 1994 impact of Comet Shoemaker-Levy 9 with Jupiter also served as a "wake-up call", and astronomers responded by starting programs such as Lincoln Near-Earth Asteroid Research (LINEAR), Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth Object Search (LONEOS) and several others which have drastically increased the rate of asteroid discovery. However, many objects undoubtedly still remain undetected.
End of civilization
An impact event is commonly seen as a scenario 2 that would bring about the End of Civilization. In 2000 Discover Magazine published a list of 20 likely End of the world scenarios with impact event listed as the number one most likely to occur.3 Until the 1980s this idea was not taken seriously, but all that changed after the discovery of the Chicxulub Crater which was further reinforced by witness to the Comet Shoemaker-Levy 9 event. Since then there has been a lot more interest and funding of studies.
- Smit J., Hertogen J. (1980) An extraterrestrial event at the Cretaceous-Tertiary boundary, Nature 285, 198-200.
- Alvarez L.W, Alvarez W., Asaro F., Michel H.V. (1980) Extraterrestral Cause for the Cretaceous-Tertiary Extinction, Science 208, 1095-1108.
- Shukolyukov A., Lugmair G.W. (1998) Isotopic Evidence for the Cretaceous-Tertiary Impactor and Its Type, Science 282, 927-929.