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Earth Impacts

v1.0.2 / 01 jul 02 / greg goebel / public domain

* Although space is mostly empty, the Earth is not alone in the vacuum. Our planet often crosses paths with chunks of rock, some as large as several kilometers in diameter, and every now and then one of those chunks of rocks slams into the Earth with spectacular results. This document describes the current state of knowledge about such "Earth impacts".



* The only major impact of a celestial object on the Earth recorded in modern history occurred about a quarter after seven on the morning of 30 June 1908, when witnesses observed a huge fireball almost as bright as the Sun plunging across the Siberian sky, terminating in a huge explosion that registered on seismic stations all across Eurasia.

Surprisingly, there was little scientific curiosity about the impact at the time, and due to the subsequent occurrence of war, revolution, and civil war in Russia, it wasn't until the 1920s that anyone performed a serious investigation of what had happened in Siberia in 1908.

In 1921, the Russian minerologist Leonid Kulik visited the Podkamennay Tunguska River basin as part of a survey for the Soviet Academy of Sciences. Locals told him of the great blast, of huge stretches of forest being flattened, of people being blown over by the shock.

The reports were basically consistent with each other, and Kulik was able to persuade the Soviet government to fund an expedition to the Tunguska region. His group reached the "ground zero" of the "event" in 1927. Much to their surprise, there was no crater, just a great region of scorched trees about 50 kilometers across. The trees pointed away from the center of the event, with a few still bizarrely standing upright at ground zero, their branches and bark stripped off.

Over the next ten years, there were three more expeditions to the area, and none of them discovered anything much different from what Kulik and his people had found. Kulik found a little "pothole" bog that he thought might be the crater, but after a laborious exercise in draining the bog, he found there were old stumps on the bottom, ruling out the possibility that it was a crater.

Kulik did manage to arrange a aerial photographic survey of the area in 1938, a few years before his death as a Red Army officer in the Soviet Union's "Great Patriotic War" against Hitler. The aerial survey revealed that the event had knocked over trees in a huge butterfly-shaped pattern that provided information on the direction of the object's motion.

Soviet experiments performed in the mid-1960s with model forests and small explosive charges slid downward on wires that duplicated this pattern suggested that the 1908 object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north, and exploded in mid-air.

Expeditions sent to the area in the 1950s and 1960s did find microscopic glass spheres in siftings of the soil. Chemical analysis showed that the spheres contained high proportions of nickel and iridium, which are found in high concentrations in meteorites, and indicated that they were of extraterrestial origin. However, even this clue could not pin down the nature of the object precisely.

* Many ideas have been proposed for what happened at Tunguska, such as an impact of an antimatter meteor, the passage of a tiny black hole, or even the catastrophic destruction of a nuclear-powered alien spacecraft.

There has never been much evidence for such exotic ideas, and simpler theories were available. In 1930, the British astronomer F.J.W. Whipple suggested that the Tunguska event was the impact of a small comet, which vaporized itself in the explosion and left no obvious trace. Comets are generally believed to be "dirty snowballs" of ices and dust, and one would be quickly destroyed by an impact with the Earth's atmosphere. The idea of a comet impact was reinforced by the fact that there were "skyglows" in the evenings across Europe for several days after the impact, obviously caused by dust dispersed through the upper atmosphere.

The comet idea remained popular for over 50 years, with some astronomers speculating that it might have been a piece of the short-period comet Encke. Materials from Encke apparently make up the stream of sky junk that create the "Beta Perseid" meteor shower, and the Tunguska event coincided with a peak in that shower.

However, in 1983 an astronomer named Zdenek Sekanina, of the US National Aeronautics & Space Administration's Jet Propulsion Laboratory (NASA JPL), published an article that undermined the comet theory. Sekanina pointed out that eyewitness accounts and other evidence point only to one explosion, and that the object passed through the atmosphere at a shallow angle, remaining intact to an altitude of 8.5 kilometers. A dirty snowball of ice and gases would have not got that far in one piece.

Sekanina proposed that the object was a stony "chondritic" asteroid that rammed through the atmosphere until pressures and temperatures reached a point that caused it to abruptly disintegrate in a huge explosion, something like what would happen on a much smaller scale to an aspirin pill smashed with a hammer. The destruction was so complete that no remnants of substantial size survived. The material scattered into the upper atmosphere from the event would have caused the skyglows.

Sekanina's theory was appealing, but it was based on very limited information. Said one critic: "You can't make a sophisticated model from poor data." Sekanina admitted there was "a lot of handwaving" in his ideas. The comet theory still has its partisans, who reply that chemical analyses of the area have showed it to be enriched in cometary material, and suggest that the comet might have been extinct and had formed a tough "mantle" that allowed it to penetrate the atmosphere. In the absence of conclusive evidence, the debate seems likely to continue, but at least nobody is seriously suggesting it was a UFO any more.

* Although the Tunguska impact 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 18 January 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 10 August 1972, and was filmed by a tourist at the Gran 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 the Earth 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 Tunguska fireball was the only major impact event in history to be witnessed and recorded, but there is plenty of evidence for other recent, by geological standards at least, major impact events.

Settlers in the American West had found a great crater in the barrens of the Colorado Plateau in northern Arizona state, about 55 kilometers east of Flagstaff. Some geologists believed it was a volcanic crater, but in 1905 an engineer and businessman named Daniel M. Barringer suggested it was the result of the impact of a large iron-metallic meteorite. Later research would prove him right, as the crater was lined with materials showing the effects of the enormous pressures and high temperatures associated with an impact event.

Impacts produce distinctive "shock-metamorphic" effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:

Barringer had hoped to mine the site for iron, but the meteorite's pieces had been scattered far and wide, and his scheme came to nothing. However, in some compensation, the formation was named the "Barringer Meteor Crater" after him.

* The impact event that formed the Barringer Crater took place about 50,000 years ago. At that time, the climate on the Colorado Plateau was cooler and damper. The area was a grassland dotted with woodlands, and inhabited by wooly mammoths, giant ground sloths, and camels.

One day an iron-metallic meteor about 50 meters across fell from the sky, burning much brighter than the Sun, at a speed faster than 40,000 kilometers per hour (KPH). The object slammed into the ground, producing a massive explosion that was about three times more powerful than the Tunguska event. The explosion dug out 175 million tonnes of rock and left a crater about 1,200 meters across and 170 meters deep. The shock of impact propagated as a hemispherical shock wave that blasted the rock downward and outward from the point of impact, forming the crater.

At ground zero, the impact melted everything that it did not vaporize, and transformed carbon minerals into diamonds. 30 tonne blocks of limestone were tossed outside the crater's rim. The shock of impact sped through the ground, resulting in a magnitude 5.5 earthquake.

Everything within a radius of three to four kilometers was killed immediately. The fireball that formed should have scorched everything within a radius of ten kilometers. A shock wave, moving out at 2,000 KPH, leveled everything from 14 to 22 kilometers, dissipating to hurricane-force winds that persisted to a radius of 40 kilometers. Despite this destruction, the Barringer impact did not throw up enough dust to affect the Earth's climate. The area was likely recolonized by the local flora and fauna within a century.

* In hindsight, the Barringer Crater looks so much like the result of an impact that it is hard to imagine anyone could think it was anything else, but not all remnants of impacts events are so obvious.

In 1990, Captain Ruben Lianza of the Argentine Air Force, an amateur astronomer, provided a report to an astronomy publication that included aerial pictures of a set of odd teardrop-shaped depressions near the city of Rio Cuarto in north-central Argentina. The depressions seemed very similar to the sets of craters produced in laboratory simulations of impacts taking place at low angles. Such features exist on the Moon, Mars, and Venus, but had not been seen on Earth up to that time.

The depressions had been long known to Argentine geologists, but until Captain Lianza came along, nobody had seriously investigated them. Samples of materials obtained from the depressions indicated the presence of shocked materials, as well as pebbles that were clearly of meteoritic origin. A team of American researchers went to Argentina to investigate, collaborating with Captain Lianza and Argentine academics to study the strange depressions.

There were ten depressions, four of them of substantial size. One crater, named the "Drop", was about 200 meters wide and 600 meters long. Two more large craters, the "Eastern Twin" and "Western Twin", both about 700 meters wide and 3.5 kilometers long, were located 5 kilometers to the northeast. Another major crater, the "Northern Basin", about half as big as one of the Twins, was sited 11 kilometers further to the northeast. The long axes of the craters all pointed to the northeast.

The craters were clearly due to a grazing impact of a set of objects at a very low angle, which calculations show to be a rare occurrence. Most impacts will strike at an angle of 45 degrees to the horizontal or less, and the impact craters will always be close to circular, since the shock wave that results from the impact propagates symmetrically.

A grazing impact, however, will form an elliptical crater, with sprays of debris that look like butterfly wings. This has been confirmed by high-velocity guns used for impact experiments, and more recently by computer simulations. On impact, the object may shed chunks of itself that fly further downrange to perform secondary impacts.

Models of the Rio Cuarto event suggest that the object struck at an angle of no more than 15 degrees from the horizontal, with the impact itself having 10 times more explosive energy than the Barringer event and 30 times more than the Tunguska event. Although the age of the craters has not yet been determined precisely, it is believed they are about 10,000 years old.

The object came in from the northeast, bright as a second Sun. The object hit ground at the Northern Basin, creating a mountain of fire about 10 kilometers wide and 50 kilometers long, and scattered off pieces that went downrange to form the Twins and the Drop. The fireball incinerated all life downrange in a firestorm with a parabolic-shaped footprint that created hurricane-force winds, erasing the butterfly-shaped pattern of debris characteristic of such low-angle strikes.

The object was clearly a "carbonaceous chondritic" asteroid, largely made up of simple carbon compounds and resembling something like a big lump of soot. The impact probably released huge clouds of toxic carbon monoxide that killed off wildlife in the area, assisted by heavy concentrations of toxic nitrous oxides created through ionization by the object's fiery passage through the atmosphere. It is likely the impact resulted in serious atmospheric effects and may have even had a short-term effect on global climate.

* Another major impact apparently occurred much more recently. The vast desert wasteland of southern Saudi Arabia known as the "Empty Quarter", or "Rub' al Khali" in Arabic, is one of the most desolate places on Earth. In 1932, a British explorer, Harry Saint John "Abdullah" Philby, father of Communist spy Kim Philby, was hunting for a city named "Ubar", that the Koran claimed had been destroyed by God for defying the Prophet.

Philby mistranslated the name of the city as "Wabar", which in a way was fortunate because he found something else that deserved a different name. After a month's journey through the wastes that was so harsh that even some of the camels died, Philby found was a patch of ground about a half a square kilometer in size, littered with chunks of white sandstone, black glass, and chunks of iron. There were two large circular depressions partially filled with sand.

He brought back one of the chunks of iron. Analysis showed it to be about 90% iron and 5% nickel, with the rest consisting of various elements, including an unusually high concentration of iridium. This implied that the "Wabar" site was a meteorite impact area.

Later research located the town of Ubar elsewhere, but Philby's Wabar site remained intriguing. In 1994, the Zahid Tractor Corporation, a Saudi dealer of the "Hummer" off-road vehicle, decided to stage a publicity stunt of the vehicle by driving several of them across the Empty Quarter in the dead of summer. Few ever went deep into the Empty Quarter in the summer and came back alive.

A US Geological Survey scientist, Jeffrey C. Wynn, was invited to come along. Zahid sponsored a total of three trips into the Empty Quarter in 1994 and 1995, and Wynn went on all of them. Even with modern technology, the trip was a difficult one. Not only were conditions harsh, but the Wabar site was tricky to find, as it sits in the middle of an enormous dune field that has few fixed landmarks.

The Wabar site covers about 500 by 1,000 meters, and features three prominent, roughly circular craters. Two were reported by Philby, and measure 116 and 64 meters wide. The third was discovered on the Zahid expeditions and is 11 meters wide. They are all nearly full of sand.

The surface of the area partly consisted of "impactite" rock, a whitened coarse sandstone, and was littered with black glass slag and pellets. The impactite had a laminated appearance and featured a form of shocked quartz known as "coesite", and seemed clearly the product of an impact event. The impact did not penetrate to bedrock.

The presence of iron fragments at the site also pointed to a meteorite impact, as there are no iron deposits in the region. The iron was in the form of buried fist-sized cracked balls and smooth fragments found on the surface. The largest fragment was recovered in a 1965 visit to Wabar and weighs 2.2 tonnes. It is known as the "Camel's Hump" and is on display at the King Saud University in Riyadh.

The sand was turned into black glass near the craters, and pellets of the glass are scattered all over the area. The glass is about 90% local sand and 10% meteoritic iron and nickel.

The layout of the impact area suggests that the body fell at a shallow angle, and was moving at typical meteorite entry speeds of 40,000 to 60,000 KPH. Its total mass was more than 3,500 tonnes. The shallow angle presented the body with more air resistance than it would have encountered at a steeper angle, and it broke up in the air into at least four pieces. The biggest piece struck with an explosion about as powerful as that produced by the bomb that leveled Hiroshima.

One analysis of glass fragments suggested the Wabar impact took place thousands of years ago, but the fact that the craters have filled up considerably since Philby visited them suggests their origin is much more recent, and different chemical analyses suggest the impact site is no more than a few centuries old. Arab reports of a fireball passing over Riyadh, variously reported as occurring in 1863 or 1891, indicate the impact may have occurred very recently. Fragments scattered from the path of this fireball match samples found at the Wabar site.



* 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. 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. Although it might be assumed that a major impact on the Earth would leave behind absolutely unmistakeable evidence, in fact the gradual processes that change the surface of the Earth tend to cover the effects of impacts. Erosion by wind and water, deposits of wind-blown sand and water-carried sediment, and lava flows in due time tend to obscure or bury the craters left by impacts.

However, some evidence remains, and over 150 major craters have been identified on the Earth. Studies of these craters have allowed geologists to find the remaining traces of other craters that have mostly been obliterated.

* Daniel Barringer was one of the first to identify a geological structure as an impact crater, but at the time his ideas were not widely accepted, and when they were, there was no recognition of the fact that Earth impacts are common in geological terms.

In the 1920s, the American geologist Walter H. Bucher studied a number of craters in the US. He concluded they had been created by some great explosive event, but believed they were the result of some massive volcanic eruption. However, in 1936, the geologists John D. Boon and Claude C. Albritton JR revisited Bucher's studies and concluded the craters he studied were probably formed by impacts.

The issue remained more or less speculative until the 1960s. A number of researchers, most notably Gene Shoemaker, conducted detailed studies of the craters that provided clear evidence that they had been created by impacts, identifying the shock-metamorphic effects uniquely associated with impacts.

Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory in Canada, and Wolf von Engelhardt of the University of Tuebingen in West Germany began a methodical search for "impact structures". By 1970, they had tentatively identified more than 50.

Their work remained controversial, but the American Apollo Moon landings, which were in progress at the time, provided evidence of the rate of impact cratering on the Moon. Processes of erosion on the Moon are minimal and so craters persist almost indefinitely. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.

* The age of known impact craters on the Earth ranges from a few thousand to almost two billion years, though few older than 200 million years have been found as geological processes tend to obliterate older ones. They are also selectively found in the stable interior regions of continents. Few underwater craters have been discovered because of the difficulty of surveying the sea floor; the rapid rate of change of the ocean bottom; and the "subduction" of the ocean floor into the Earth's interior by processes of continental drift.

Current estimates of the rate of cratering on the Earth suggest that from one to three craters with a width greater than 20 kilometers are created every million years. This indicates that there are far more relatively young craters on the planet than have been discovered so far.

An asteroid falls onto the Earth at a speed of about 40,000 to 60,000 KPH. If the object weighs more than 1,000 tonnes, the atmosphere does not do much to slow it down, though smaller bodies can be substantially slowed by atmospheric drag, as they have a higher ratio of surface area to mass. In any case, the temperatures and pressures on the object are extremely high. They can destroy chondritic or carbonaceous chondritic bodies before they ever reach ground, but iron-metallic asteroids have more structural integrity and can strike the surface of the Earth in a violent explosion.

The result is a crater. There are two forms, "simple" and "complex". The Barringer crater in Arizona is a perfect example of a simple crater, a straightforward bowl in the ground. Simple craters are generally less than four kilometers across.

Complex craters are larger, and have uplifted centers that are surrounded by a trough, plus broken rims. The uplifted center is due to the "rebound" of the earth after the impact. It is something like the ripple pattern created by a drop of water into a pool, frozen into the Earth when the melted rock cooled and solidified.

In either case, the size of the crater depends on the material in the impact regions. Relatively soft materials yield smaller craters than brittle materials. Erosion and other geological activities quickly hide impact craters on the Earth. The Barringer Crater is in superlative shape, but it is only about 50,000 years old.

Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides impact craters. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.



* In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez; his son, geologist Walter Alvarez; and a group of colleagues published an article in the scientific press. The article pointed out that fossilized sedimentary layers found all over the world dating back to the end of the Cretaceous era, 65.7 million years ago, contain a high proportion of iridium, which is relatively common in asteroids.

The iridium concentration was almost two orders of magnitude greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs and was in general a period of extraordinary mass extinction, leading to the Tertiary era, in which mammals began to predominate on the Earth. The paper suggested that the dinosaurs had been killed off by the impact of a ten-kilometer-wide asteroid on the Earth.

The resulting blast would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, possibly created a hurricane of unimaginable fury, and would have certainly thrown massive amounts of dust and vapor into the upper atmosphere or even into space. The worldwide cloud would have choked off sunlight for years, resulting in a "long winter" that wiped out many existing species, as well as creating "acid rains" that would have inflicted further hardship on the environment.

* Although further studies of the "Cretaceous-Tertiary" or "K-T" layer ("K" is used by geologists to refer to the Cretaceous era because "C" is taken by the "Cambrian" era) consistently showed the excess of iridium, the idea that the dinosaurs had been exterminated by an asteroid remained a matter of controversy among geologists for over a decade.

There was the problem that no crater was known that matched the event. This was not a lethal blow to the theory. Although the crater resulting from the impact would have been 150 to 200 kilometers in diameter, as mentioned in the previous section the Earth's geological processes tend to hide craters over time. Still, finding a crater would obviously have buttressed the "Alvarez hypothesis", as it came to be known.

In early 1990, Alan K. Hildebrand, a graduate student at the University of Arizona, visited a small mountain village named Beloc in Haiti. He was investigating certain K-T deposits that include thick, jumbled deposits of coarse rock fragments, which were apparently scoured up from one location and deposited elsewhere by kilometers-high "tsunamis", giant sea waves, that most likely resulted from an Earth impact. Such deposits occur in many locations, but seem to be concentrated in the Caribbean basin.

Hildebrand found a greenish brown clay with an excess of iridium, and containing shocked quartz grains and small beads of weathered glass that appeared to be tektikes. He and his faculty adviser William V. Boynton published the results of the research in the scientific press, suggesting not only that the deposits were the result of an Earth impact, but that the impact couldn't have been more than 1,000 kilometers away.

This was particularly puzzling, because no crater of any size was known to exist in the Caribbean basin. Hildebrand and Boynton also reported their findings to an international geological conference, sparking substantial interest.

Evidence pointed to possible crater sites off the north coast of Columbia or near the western tip of Cuba. Then Carlos Byars, a reporter for the HOUSTON CHRONICLE, contacted Hildebrand and told him that a geophysicist named Glen Penfield had discovered what might be the impact crater in 1978, buried under the northern Yucatan Peninsula.

In that year, Penfield had been working for Petroleos Mexicanos (PEMEX, the Mexican state-owned oil company), as a staff member for a airborne magnetic survey of the Yucatan peninsula. When Penfield examined the survey data, he found buried in the noisy data a huge underground "arc", with its ends pointing south, in the Caribbean off the Yucatan that was inconsistent with what he knew about the region's geology.

Penfield was intrigued, and managed to obtain a gravity map of the Yucatan that had been made in the 1960s and was gathering dust in PEMEX's archives. He found another arc, but this one was on the Yucatan itself, and its ends pointed north. He matched up the two maps and found that the two arcs joined up in a neat circle, 180 kilometers wide, with its center at the the village of Puerto Chicxulub.

Penfield was an amateur astronomer and had a good idea of what he was looking at. Although PEMEX would not allow him to release specific data, the company did allow him and a PEMEX official named Antonio Camargo to present their results at a geological conference in 1981. Unfortunately, that particular conference was under-attended in that year, ironically because most geologists were attending a workshop on Earth impacts, and their report attracted very little attention, though it did get back to Byars.

Penfield didn't give up. He knew that PEMEX had drilled exploratory wells in the region in 1951. One of the wells had bored into a thick layer of igneous rock known as "andesite" about 1.3 kilometers down. Such a structure could have resulted from the intense heat and pressures of an Earth impact, but at the time of the borings it had been written off as a "volcanic dome", even though such a feature was out of place in the geology of the region.

Further studies of the archived well cores would have resolved the issue, but unfortunately most of them had been destroyed in a warehouse fire in 1979. Penfield presently flew down to the Yucatan to see if he could find anything out from the "tailings" left by the wellheads. This idea didn't pan out, and in one case Penfield found himself digging through a communal pigsty that had been set up on a wellhead site, a task he described as "unpleasant and unrewarding."

After Hildebrand got in touch with Penfield, however, the two men were able to locate two separate samples from the wells drilled by PEMEX in 1951. Analysis of the samples clearly showed shock-metamorphic materials. Studies by other geologists of the debris found in Haiti at Beloc also showed it to be clearly the result of an impact.

This research was persuasive, and received a major boost when a team of California researchers, including Kevin O. Pope, Adriana C. Ocampo, and Charles E. Duller, conducted a survey of satellite images of the region. They found that there was a nearly perfect ring of sinkholes centered on Puerto Chicxulub that matched the ring Penfield had found in his data. The sinkholes were likely caused by subsidence of the crater's wall.

This evidence was enough to get most of the geological community on the bandwagon, and further studies have reinforced the consensus. Indeed, some evidence has accumulated that the actual crater is 300 kilometers wide, and the 180 kilometer ring is just an inner wall.

* However, paleontologists remained skeptical, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, instead occurring gradually over about ten million years. There was also a certain general distrust of a group of physicists intruding into their domain of expertise.

Luis Alvarez, who died in 1988, replied that that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to come around to the idea that the mass extinctions at the end of the Cretaceous were largely due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as drop in sea level and massive volcanic eruptions in India, that may have also contributed to the extinctions.

Skeptics remain. 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.



* The general acceptance of the Alvarez hypothesis has raised the awareness of the possibility of future Earth impacts with asteroids that cross the Earth's orbit. A few hundred such "near-Earth asteroids (NEAs)" are known, ranging in size up to four kilometers. Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometer in diameter at up to 2,000.

There are three "families" of NEAs:

Astronomers believe that NEAs only survive in their orbits for 10 million to 100 million years. They are eventually eliminated either by collisions with the inner planets, or by being ejected from the solar system by near misses with the planets. Such processes should have eliminated them all long ago, but it appears they are resupplied on a regular basis.

Some of the NEAs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEAs still show faint comet-like tails. These NEAs were likely derived from the "Kuiper Belt", a repository of comets residing beyond the orbit of Neptune. The rest of the NEAs appear to be true asteroids, driven out of the asteroid belt by gravitational interactions with Jupiter.

There is also a threat of impacts by comets falling into the inner Solar System after having been disturbed from their orbits in the "Oort Cloud", a huge, tenuous sphere of comets surrounding the Solar System. Such "long period" comets are only infrequent visitors into the inner Solar System and they do not generally fall in orbits in the same plane as that of Earth, but there is nothing to rule out the possibility that one might collide with the Earth. The impact velocity of a long-period comet would likely be several times greater than that of an NEA, making it much more destructive.

The threat of an Earth impact was emphasized by the collision of the comet Shoemaker-Levy 9 with Jupiter on 16 July 1994, resulting in explosive impacts that would have been catastrophic on Earth. To be sure, Jupiter is far larger and more massive than the Earth and so undergoes far more impacts, but the event still provided an illustration that such things do happen and can be unimaginably destructive.

Although there have been a few false alarms, a number of asteroids are definitely known to be threats to the Earth. Asteroid 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow plotting its orbit, and then rediscovered on 31 December 2000. Proper calculation of its orbit then demonstrated that it has a 1 in 300 chance of hitting the Earth on 16 March 2880. This probability is a thousand times greater than any other known asteroid threat, and 50% greater than all other known asteroid threats combined. 1950 DA has a diameter of a kilometer.

It is difficult to determine the chances of its impact better than that. The uncertainty is due to minor irregularities in the Sun's shape, and so its gravitational field; weakening of the Sun's gravity through mass loss from the solar wind of particles that streams out from its atmosphere; uncertainties in the masses and so the gravitational pull of the planets; variations in the tidal pull of the surrounding galaxy; the subtle pressure of sunlight; and, in particular, a phenomenon known as the "Yarkovsky effect".

This effect was discovered by a Russian engineer named I.O. Yarkovsky a century ago. It is a subtle process: the heating of the asteroid's surface causes it to emit thermal radiation, which creates a slight amount of thrust. It is somewhat unpredictable, since an asteroid's ability to soak up heat from the Sun depends on its terrain, and the effect is also influenced by the asteroid's spin orientation and rotation rate.

* Astronomers have been conducting surveys to locate the NEAs. One of the best-known is the "Spacewatch" project, which uses an old 90 centimeter telescope sited at the Kitt Peak Observatory in Arizona, updated with automatic pointing, imaging, and analysis gear to search the skies for intruders. The project was set up in 1980 by Tom Gehrels and Dr. Robert S. McMillan of the Lunar & Planetary Laboratory of the University of Arizona in Tucson, and is now being operated by Dr. McMillan.

The Spacewatch project has acquired a 1.8 meter telescope, also at Kitt Peak, to hunt for NEAs, and has provided the old 90 centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability. These new resources promise to increase the rate of NEA discoveries by Spacewatch from 20 to 30 a year to 200 or more.

NASA has considered a more extensive network of six 2.5-meter automated telescopes named "Spaceguard" to watch the skies for such intruders, but at present astronomers are still assessing the actual level of the threat posed by NEOs, and moving to such a "defensive" network is not generally regarded as justified at present.

Nonetheless, the fact that an impact of an NEA a kilometer or more in size would be a catastrophe unparalleled in human history has kept the idea of a defensive network alive, as well as led to speculations on how to divert objects that might be a threat. Detonating a nuclear weapon above the surface of an NEA would be one option, with the blast vaporizing part of the surface of the object and nudging it off course with the reaction.

However, it is becoming increasingly obvious that many asteroids are "flying rubble piles" that are loosely glued together, and a nuclear detonation might just break up the object without adjusting its course. This has led to a variety of other ideas for dealing with the threat:

Thinking on the matter continues, and if there is no prospect of immediate action, the issue isn't going away, either.



* One of the better-known stories associated with Earth impacts is the observation of a great streak of fire associated with the Moon, recorded by the monk Gervase of Cantebury in June 1178. Since the 1970s, this was suspected as evidence of a major lunar impact event on the time, resulting the creation of the young crater "Giordano Bruno", which is about 22 kilometers across.

Recent studies have suggested that it was more likely to have been the observation of a large fireball passing through the Earth's atmosphere that happened to cross the line-of-sight to the Moon. There are no other records of such an event at the time, suggesting the local nature of the event; the crater Giordano Bruno appears to be substantially older than 800 years; and a major impact on the Moon would have showered the Earth with fragments, resulting in spectacular meteor showers, for which there is no record.

* Incidentally, in the early 1970s I heard a word-of-mouth report of the 1972 Teton fireball. The tale made it out to be a UFO that had run up the Rockies, gone in circles near the Canadian border, and then flown back out into space. I didn't actually learn about the actual incident for years, and then this story came back to me, illustrating just how wildly facts can be garbled in their retelling.

* Sources include:

* Revision history:

   v1.0   / 01 jun 01 / gvg
   v1.0.1 / 01 dec 01 / gvg / Minor cosmetic update.
   v1.0.2 / 01 jul 02 / gvg / Added comments on SD 1950, cleanup.
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