Impact event

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 impact crater.

The geology of Earth-impact events

The historical view of creation held by western societies was based on Jewish tradition that the Earth was created 6,000~10,000 years ago, and had been shaped since that time by a number of global cataclysms (see catastrophism). In the course of the first half of the 19th century, the new sciences of geology and paleontology added more contribution to this view, which gradually gave way to a 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 been amended in recent decades to accommodate the fact that the Earth has in fact also gone through periods of abrupt and catastrophic change, some 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 extinction 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 even earlier in Earth's history have 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, a theory known as panspermia.

These modified views 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. Instead, it was widely thought that cratering was the result of volcanism: the Barringer Crater, for example, was ascribed to a prehistoric volcanic explosion (not an unreasonable hypothesis, given that the volcanic San Francisco Peaks 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. In 1994, the first major impact event was directly observed: the collision of the comet Shoemaker-Levy 9 with Jupiter; to date, no such events have been observed on Earth.

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 10,000,000 (ten 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 in the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90% of all species.^[1] 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 a recent report from Ohio State scientists stated that they have located a 483-km diameter impact crater beneath the East Antarctic Ice Sheet which may date back about 250 million years, based on gravity measurements, which might associate it with the Permian-Triassic extinction event.

In 1980, physicist Luis Alvarez, his son, geologist Walter Alvarez, and nuclear chemists Frank Asaro and Helen V. Michael from the University of California, Berkeley 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^{[citation needed]}. Chromium isotopic ratios are homogeneous within the earth, therefore these isotopic anomalies exclude a volcanic origin which was also proposed as a cause for the iridium enrichment. Furthermore the chromium isotopic ratios measured 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 Yucatán 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. None of this takes into account the East Antarctic Ice Sheet crater, which is a recent find.

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 solar system was 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 Henbury crater, in Australia, and Kaali crater in Estonia, apparently produced by objects which broke up before impact. Both are estimated to be 4000-5000 years old.
• the Wabar craters, which apparently formed within the past few hundred years
• the Noerdlinger Ries, a 24 km crater in Germany formed about 15 million years ago.

More recent prehistoric impacts are theorized (Ancient Crash, Epic Wave by Sandra Blakeslee, New York Times, 14 Nov 2006) by the Holocene Impact Working Group, including Dallas Abbott of Lamont-Doherty Earth Observatory in Palisades, N.Y. This group points to four enormous chevron sediment deposits at the southern end of Madagascar, containing deep-ocean microfossils fused with metals typically formed by cosmic impacts. All of the chevrons point toward a spot in the middle of the Indian Ocean where a newly discovered crater, 18 miles in diameter, lies 12,500 feet below the surface. This group posits that a large asteroid or comet impact 4,500--5,000 years ago, produced a mega-tsunami at least 600 feet high. If this and other recent impacts prove correct, the rate of asteroid impacts is much higher than currently thought.

Holocene impact events have been proposed by the dendrochronologist Mike Baillie as a possible cause of several brief (typically 5-10 year) climatic downturns recorded in ancient tree ring patterns. In his book 'Exodus to Arthur: Catastrophic encounters with comets' he highlights four such events and suggests that these might have been caused by the dust veils thrown up by the impact of cometary debris.

Modern impact events

The most significant recorded impact in recent times was the Tunguska event, which occurred in Siberia, Russia, in 1908. This incident involved an explosion that was probably caused by the airburst of an asteroid or comet 5 to 10 kilometers (3–6 mi) above the Earth's surface, felling an estimated 80 million trees over 2,150 square kilometers (830 sq mi). 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 U.S. 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. Noteworthy examples include the Sikhote-Alin Meteorite fall in Primorye, far eastern Russia, in 1947, and the Revelstoke fireball of 1965, which occurred over the snows of British Columbia, Canada. Another fireball blew up over the Australian town of Dubbo in April 1993, shaking things up but causing no harm.

A small number of meteorite falls have been observed with automated cameras and recovered following calculation of the impact point. The first of these was the Pribram meteorite, which fell in Czechoslovakia (now the Czech Republic) in 1959.^[2] In this case, two cameras used to photograph meteors captured images of the fireball. The images were used both to determine the location of the stones on the ground and, more significantly, to calculate for the first time an accurate orbit for a recovered meteorite.

Following the Pribram fall, other nations established automated observing programs aimed at studying infalling meteorites. One of these was the Prairie Network, operated by the Smithsonian Astrophysical Observatory from 1963 to 1975 in the midwestern US. This program also observed a meteorite fall, the Lost City chondrite, allowing its recovery and a calculation of its orbit.^[3] Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977.^[4] Finally, observations by the European Fireball Network, a descendant of the original Czech program that recovered Pribram, led to the discovery and orbit calculations for the Neuschwanstein meteorite in 2002.^[5]

The only reported fatality from meteorite impacts is an Egyptian dog who was killed in 1911, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin.

The first known modern case of a human hit by a space rock [1] occurred on November 30 1954 in Sylacauga, Alabama. There a 4 kg stone chondrite [2] crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed [3] to have been struck by 'meteorites' but no verifiable meteorites have resulted.

A particularly interesting fireball was observed moving north over the Rocky Mountains from the U.S. 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.

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. This blast was also featured on the The Science Channel series Killer Asteroids, with several witness reports from residents in Atlin, British Columbia.

A meteor was observed striking Reisadalen in Nordreisa municipality in Troms County, Norway, on June 7, 2006. Although initial witness reports stated that the resultant fireball was equivalent to the Hiroshima nuclear explosion, scientific analysis places the force of the blast at anywhere from 100-500 tonnes TNT equivalent—at most, around 3% of Hiroshima's yield. [4]

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. In the 21-Nov-2002 edition of the journal Nature, Peter Brown of the University of Western Ontario reported on his study of US early warning satillite records for the proceeding 8 year. He identified 300 flashes caused by 1m to 10m sized meteors in that time period and estimated the rate of Tunguska sized events as once in 400 years [5]. Shoemaker estimated that one of such magnitude occurs about once every 300 years, though more recent analyses have suggested he exaggerated by an order of magnitude. Even at that, this is not a long interval, 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.

Near misses and forecasts

On 19 May 1996 a 300–500-m asteroid, 1997 JA1, passed within 450,000 km of Earth; it had been detected a few days before.

On 18 March 2004 a 30-m asteroid, 2004 FH, passed within 40,000 km of Earth only a few days after it had been detected. This asteroid probably would have detonated in the atmosphere and posed negligible hazard to the surface, had it been on impact course.

On 31 March 2004, a 6m meteoroid, 2004 FU162 made the closest near miss pass ever observed with a separation of only 1.02 Earth radii from the surface (6,500 km). Because this object is certainly too small to pass through the atmosphere, it is classed as a meteoroid rather than an asteroid.

In 2004, a newly discovered 320-m asteroid, 99942 Apophis (previously called 2004 MN4), achieved the highest impact probability of any potentially dangerous object. The probability of collision on 13 April 2029 was estimated to be as high as 1 in 17 by Steve Chesley of NASA's Jet Propulsion Laboratory though the worst published figure was 1 in 37 calculated in December 2004. Later observations showed that the asteroid will miss the earth by 25,600 km (within the orbits of communications satellites) in 2029, but its orbit will be altered unpredictably in a way which does not rule out a collision on 13 or 14 April 2035 or later in the century. These possible future dates have a cumulative probability of 1 in 45,000 for an impact in the 21^stcentury.

Asteroid 2004 VD17, of 580 m, previously was estimated to have a probability of 1 in 63,000 of striking the earth on 4 May 2102 (as of July 2006), with risk 1 on the Torino scale, but further observations lowered the estimate. As of the observation on December 17, 2006, JPL assigns 2004 VD17 a Torino value of 0 and an impact probability of 1 in 41.667 million in the next 100 years.

Asteroid (29075) 1950 DA is predicted to collide with Earth on March 16, 2880. The probability of impact is either 1 in 300 or zero, depending on which one of the two possible directions for the asteroid's spin pole is correct. This asteroid has a mean diameter of about 1.1 km. The energy released by the collision would cause major effects on the climate and biosphere and may be devastating to human civilization.

Relatively small objects that burn up in the atmosphere can be dangerous beyond their own capabilities. In 2002, U.S. Air Force Brig. Gen. Simon P. Worden told members of a U.S. House of Representatives Science subcommittee that the U.S. has instruments that determine if an atmospheric explosion is natural or man-made, but no other nation with nuclear weapons has that detection technology. He said there is concern that some of those countries could mistake a natural explosion for an attack, and launch nuclear retaliation. In the summer of 2001 U.S. satellites had detected over the Mediterranean an atmospheric flash of energy similar to a nuclear weapon, but determined that it was caused by an asteroid.

End of civilization

An impact event is commonly seen as a scenario^{[6] [7]} 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.^[8] 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 more interest from the scientific community and greater public awareness of the possibility of impact events.

Impact events in fiction

The impact event has been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream. Interestingly, it seems, by survey, to be one of the less popular themes in apocalyptic science fiction compared to other possible catastrophes.

Numerous science fiction stories and novels center around an impact event; possibly the best selling was the novel Lucifer's Hammer by Larry Niven and Jerry Pournelle. Arthur C. Clarke's novel Rendezvous with Rama opens with a significant asteroid impact in northern Italy in the year 2077 (interestingly, on September 11) which gives rise to the Spaceguard Project, which later discovers the Rama spacecraft. In 1992 a Congressional study in the U.S.A. led to NASA being directed to undertake a 'Spaceguard Survey' (with the novel being named as the inspiration for the name[6]) to search for Earth-impacting asteroids. This in turn inspired Clarke's 1993 novel The Hammer of God. A variation on the traditional impact story was provided by Jack McDevitt's 1999 novel Moonfall, in which a very large comet travelling at interstellar velocities collides with and partially destroys the Moon, fragments of which then collide with the Earth.

Several disaster movies have also been made, including When Worlds Collide (1951), and Meteor (1979). More recently, in 1998 two high-budget action films were based on preventing extinction-level impact events, caused by different bolides: Deep Impact (a seven-mile-wide comet, later split into two objects 6 mi. and 1.5 mi. wide) and Armageddon (an asteroid described as "the size of Texas", though this is essentially impossible as the largest asteroid known, Ceres, is barely two-thirds this size). Also in 1998, the award winning Canadian film Last Night described the behavior of several characters anticipating the end of the world due to some certain but unstated peril with a known due date - thus resembling a civilzation terminating impact event.

Television shows also used impact events as a plot device, such as the episode "For the World is Hollow and I Have Touched the Sky" from the TV series Star Trek. Animated media have used it as well, including Thundarr the Barbarian (though it's just a comet passing that caused Earth to changed drastically) and Mobile Suit Gundam: Char's Counterattack (though the impact event in Gundam was going to be caused by a gigantic space colony converted from an asteroid, it was still large enough to cause an extinction level event). In "Neon Genesis Evangelion" the 2nd impact was described as a small meteor that crashed into Antarctica at neat the speed of light, melting the polar ice caps and killing millions. In the anime series Stratos 4 the plot turns in which of a large group of asteroids entering the Solar System on a collision course with Earth in a near future.

In the video game The Legend of Zelda: Majora's Mask, Link must prevent the moon from colliding with Termina in three days. In Final Fantasy VI, a falling floating continent causes drastic changes in the planet. And in Final Fantasy VII, a group of people led by Cloud Strife must stop a summoned meteor from colliding with the planet, since a few thousand years ago a previous impact left a large hole at the north pole, wounded the planet, and caused the Cetra (a corruption of the word Settler) civilization to disappear through an indirect way. Also, in Chrono Trigger, the main villain, Lavos, crashes to Earth as a meteorite in 65,000,000 BC. In the LucasArts game The Dig, the 3 main characters have to plant nuclear explosives on an asteroid that is heading directly for Earth to put it in a stable orbit instead. In the Namco game Ace Combat 04, a massive asteroid, codenamed Ulysses 1994 XF-04 struck the fictional contintent of Usea, killing 500,000 people and turning many into refugees.

Notes

1. ^ Permian Extinction
2. ^ Ceplecha, Z. (1961) Multiple fall of Pribram meteorites photographed. Bull. Astron. Inst. Czechoslovakia, 12, 21-46 NASA ADS
3. ^ McCrosky, R.E. et al. (1971) J. Geophys. Res. 76, 4090-4108
4. ^ Campbell-Brown, M. D. and Hildebrand, A. (2005) A new analysis of fireball data from the Meteorite Observation and Recovery Project (MORP). Earth, Moon, and Planets 95, 489 - 499
5. ^ Oberst, J. et al. (2004) The multiple meteorite fall of Neuschwanstein: Circumstances of the event and meteorite search campaigns. Meteoritics & Planetary Science 39, 1627-1641 NASA ADS
6. ^ Armageddon Online. End of civilization scenario.
7. ^ Exit Mundi. End of civilization scenario.
8. ^ "Twenty ways the world could end suddenly". Discover Magazine.

See also

• Spaceguard
• B612 Foundation
• Asteroid deflection strategies
• Torino scale

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