Earth Impacts

v1.1.1 / 01 apr 15 / greg goebel

* 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".

Earth impact



* 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 mineralogist 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, which would have been surprising if the bog had been ground zero of the impact. 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. Tidy 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, indicating extraterrestrial 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 have traditionally been seen as "dirty snowballs" of ices and dust; modern examination by space probes has shown them to be more dust than ice, suggesting that the name "snowy dirtball" would be more appropriate, and also shown that they are low-density objects, full of voids, with one astronomer calling them "cosmic dust bunnies". A comet 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 point out 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 seriously thinks it was a UFO.

* 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:

Some have been observed, such as the Revelstoke fireball of 1965, which occurred over the snows of northern Canada. 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.

Another fireball blew up over the Australian town of Dubbo in April 1993, shaking things 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 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.

At midmorning on 15 February 2013, a fireball streaked over the sky of the Chelyabinsk region in central Russia, finally breaking up in a stratospheric explosion. The meteor was estimated to have a mass of about ten tonnes. The fireball lit up the sky like a second Sun; many residents went to their windows to see what was going on, to be badly cut up when the shockwave arrived and shattered the glass into shards. At least one small crater was found, created by the fragments of the meteor's disintegration. Many video recordings were made of the event by dashboard cameras, common in Russia.

Chelyabinsk meteor trail

Analysis suggested the meteor was about 18 meters in diameter, with a mass of about 11,000 tonnes, and hit the Earth's atmosphere at a speed of about 68,000 KPH (42,000 MPH). It exploded at an altitude of about 24 kilometers, generating a blast equivalent to over 400 kilotonnes of TNT -- about 30 times more powerful than the nuclear weapon that destroyed Hiroshima. It was the largest known meteor strike since the Tunguska event. The Chelyabinsk meteor was clearly a stony object, not a rarer iron-nickel one, which would have penetrated more deeply and caused far more damage.

Many impact events occur without being observed by anyone on the ground. Between 1975 and 1992, American missile launch 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 -- though a more recent study, published by MIT Lincoln Laboratory researcher J. Scott Stuart in 2003, suggested that Shoemaker's estimate was exaggerated by an order of magnitude, and that the interval is more like 3,000 years. This is still not that long an 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 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, its rim making it visible above the flatlands around it. 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, since the crater was lined with materials showing the effects of the enormous pressures and high temperatures associated with an impact event.

Barringer Crater, external view

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 woolly 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, leaving a crater about 1,200 meters across and 170 meters deep. The energy of the impact propagated as a hemispherical shock wave that blasted the rock downward and outward from the point of impact, forming the crater.

Barringer 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 impact 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 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 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 no more than 45 degrees from the vertical, 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 had a short-term effect on global climate.

* In the mid-1980s, satellite photography revealed a neat circular structure in the Bolivian lowlands about 8 kilometers across that seemed to be an impact crater. The region was somewhat inaccessible, but expeditions finally confirmed that the structure was in fact an impact crater, produced by the impact of a body about 30,000 years ago with the yield of about a gigatonne of TNT.

In 2004, German researchers announced that studies had shown a large body hit the Earth's atmosphere over southern Bavaria in 200 BC. The size of the body was estimated at about 1.1 kilometers and the impact produced an enormous explosion that dwarfed the Tunguska blast. However, the damage was modest relative to the size of the object, since the object broke up at high altitude, leading the researchers to believe it was a comet. Large fragments did fall to Earth over an elliptical area, leaving a large number of craters, the largest now being occupied by Lake Tuettensee, which is about 370 meters across. The actual crater appears to be about twice that big. Roman authors of the time do make references to tales of fires in the sky and stones falling from the sky.

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 Red 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 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 came back with 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, since 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, since 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 other 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.

It wasn't until the 1980s that it was realized that Chesapeake Bay, breaking up the US state of Maryland, was found to be the remnant of an impact crater. About 35 million years ago, a giant meteor slammed into the Earth at what is now the mouth of the bay, blasting a crater almost 90 kilometers in diameter, incinerating much of what is now the US East Coast, and sending tsunamis across the Atlantic. The geology of the crater appears to be contributing to the unusually rapid rise of the sea at locales in the area, such at Norfolk, due to the continued settling of the material that eventually mostly filled up the crater -- and the subsidence of ground that was pushed up around its rim.



* 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 evidence that the Earth has in fact gone through periods of abrupt and catastrophic change, in some cases 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 events, but believed they were the result of massive volcanic eruptions. 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 discovered since 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 there are far more relatively young craters on the planet than have been discovered to date.

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, since 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 a paper in the scientific press. The paper 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 particular 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 tektites. 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 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. The 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 then 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 revealed 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 persuade the geological community there was really something going on worth paying attention to.

The K-T impact event has been described as a disaster of Biblical proportions. Along with the simple explosive force of the collision and the tidal waves that it generated, it tossed up hot debris to an altitude almost a quarter of the way to the Moon. When the debris came back down over the next few days, it started fires almost everywhere it landed, with only the northern latitudes escaping complete destruction. The marker of this global firestorm is a layer of soot along with the iridium.

Between the material thrown up by the impact itself and the smoke generated by the fires, the Earth was shrouded in dark clouds that cut off sunlight for months or years. Once the clouds faded away, there was so much carbon dioxide in the atmosphere that global temperatures skyrocketed. It took anywhere from centuries to millennia for the global ecosystem to stabilize again, and when it did stabilize it was a different ecosystem from the one that had existed before the day of fire.

* However, although the K-T impact theory did acquire a considerable following, skepticism over the theory, at least as it was proposed, has been on the increase in recent years. Nobody seriously doubts that the Chicxulub impact occurred and that it was cataclysmic, but there is doubt that it really killed off the dinosaurs.

Paleontologists have been doubtful, since 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 were also odd patterns to the extinctions: some species that would have been expected to die out under the conditions postulated for the impact cataclysm survived, while others that might have been expected to survive died out. Luis Alvarez, who died in 1988, did acknowledge that other factors, such as a drop in sea level and massive volcanic eruptions, might have contributed to the mass extinction, but he maintained that the impact was of primary importance and that paleontologists were being misled by sparse data.

One interesting proposal is that there was a much bigger impact a few hundred thousand years earlier, its remnant being a large undersea depression off the west coast of India. Advocates of the "Shiva impact", as it has been named in honor of the Hindu god of destruction, claim the object was four times bigger than the object that struck the Yucatan. They claim that the single iridium layer observed in strata actually consists of two closely-spaced layers; analyses are being performed to see if that idea holds up.

It is tempting to link impact events for other mass extinctions in the history of life on Earth, but the evidence for such is even more ambiguous than for the K-T event. The biggest known mass extinction was at the end of the Permian period, 250 million years ago, with about 90% of species wiped out, including the well-known "sailfin" or "sailback" reptiles. This mass extinction was long thought to be due to slow climate change and not some global cataclysm, but more recently evidence of huge volcanic eruptions was uncovered, suggesting that some catastrophe was involved. Recent studies have suggested that an impact event might have occurred and identified craters as possibly being created by the event.

To further confuse matters, remnants have been found of other major impacts that are not linked to any mass extinctions. Nobody doubts the impacts of course, and few deny the possibility that extinctions due to such impacts may have occurred during 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 emerged on the surface of the Earth -- though there are speculations that it could have been brewing underground before the end of the impact era.



* The publicity generated by 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 objects (NEOs)" are known, ranging in size up to four kilometers. Tens of thousands probably exist, with estimates placing the number of NEOs larger than one kilometer in diameter at up to 1,000. Earlier estimates placed the number of such large NEOs at about 2,000, but closer examination of NEOs showed that their reflectivity had been underestimated slightly, making them seem bigger than they really were.

Astronomers believe that NEOs 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 NEOs with highly eccentric orbits appear to actually be extinct "short period" comets that have lost all their volatiles, and in fact a few NEOs still show faint comet-like tails. These NEOs were likely derived from the "Kuiper Belt", a repository of comets residing beyond the orbit of Neptune. The rest of the NEOs 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 NEO, making it much more destructive.

The threat of an Earth impact was emphasized by the collision of fragments 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 -- obviously an asteroid that is spinning rapidly along an axis at a right angle to the Sun will tend to radiate heat more or less evenly in all directions, while one that is spinning slowly or along an axis pointing to the Sun will emit heat towards the Sun.

* Astronomers have been conducting surveys to locate the NEOs. One of the first to go into operation was the "Spacewatch" project, which used 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 McMillan.

The Spacewatch project later acquired a 1.8 meter telescope, also at Kitt Peak, to hunt for NEOs, and has provided the old 90 centimeter telescope with an improved electronic imaging system with much greater resolution, improving its search capability. Another asteroid survey project, the "Lincoln Near-Earth Asteroid Research (LINEAR) Project", was initiated in 1998 with NASA and USAF funding, and has cataloged large numbers of NEOs.

There are now at least half a dozen loosely-affiliated NEO search efforts going on around the world. The most ambitious survey effort is the University of Hawaii's "Panoramic Survey Telescope And Rapid Response System (PAN-STARRS)", which went online in 2010 on Mauna Kea; it also consists of a 1.8 meter wide-field telescope. A second telescope, "PAN-STARRS 2", has been set up nearby, with two more planned, though they haven't been funded.

The four PAN-STARRS telescopes will be independent optical systems, but they will all be steered together, with each telescope slightly skewed from the common boresight to provide a field of view of 3 degrees, six times greater than the full Moon. In completion, PAN-STARRS 4 will, in the course of conducting a general all-sky survey, map all NEOs larger than 140 meters that can be observed from Hawaii.

Space-based surveys are now being conducted. The first, the Canadian Space Agency's "Near Earth Object Surveillance Satellite (NEOSSat)" microsatellite, was launched as one of a set of payloads by an Indian Polar Satellite Launch Vehicle on 25 February 2013. NEOSSat was a space observatory with a launch mass of about 72 kilograms; it carried a 15 centimeter (6 inch) telescope to scan for asteroids crossing Earth's orbital path, with a precise pointing system permitting long exposures.


A second NEO search is being conducted by NASA as a secondary mission for the agency's "Widefield Infrared Survey Explorer (WISE)" satellite, launched on 14 December 2009. WISE used a cryogenically cooled telescope to perform an infrared sky survey, that survey including the observation of tens of thousands of asteroids. After its hydrogen coolant ran out in October 2010, WISE was still able to make observations with degraded sensitivity, and spent several months scanning the sky for NEOs, the mission being renamed "NEOWISE". The spacecraft was put into hibernation in early 2011, to be revived in late 2013 for a follow-on NEOWISE survey. At last notice, it was about halfway through.

* The search for threatening asteroids implies some means of diverting those objects likely to strike the Earth. Detonating a nuclear weapon above the surface of an NEO 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 less drastic ideas for dealing with the threat:

Thinking on the matter continues, with an international collaboration named "NEOShield" initiated in 2012. The NEOShield research effort is intended to perform technical studies and provide recommendations for policy-makers on the subject.



* 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 Canterbury 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 Gervase had probably observed a large fireball passing through the Earth's atmosphere that happened to cross his 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.

However, one impact on the Moon has been more or less verified. On 15 February 1953, an amateur astronomer in Oklahoma, Dr. Leon Stuart, photographed a flash in the center of the face of the Moon that he believed to be due to an impact. There was a great deal of skepticism over "Stuart's Event", as it came to be known, with some suggesting that he had actually photographed a meteor strike in the Earth's atmosphere that just happened to be in the line-of-sight to the Moon. The matter was unresolved when Stuart died in 1968.

NASA researchers decided to inspect images returned by lunar orbiting probes to see if they could find a fresh crater that might have been caused by such an event. They estimated from the size of the flash on Stuart's photograph that the meteor had to have been about 20 meters across and would have left a crater a kilometer or two across. The lunar surface is subjected to a form of "space weathering" over time, and a fresh crater would be brighter than neighboring craters.

Examination of images taken by the NASA Lunar Orbiter probes in the 1960s turned up nothing particularly interesting, but inspection of better images returned by the NASA Clementine probe, launched in 1994, revealed a bright crater about 1.5 kilometers in diameter right in the target area of Stuart's photograph. The NASA researchers calculated that the impact that produced the crater had an explosive yield of 500 kilotonnes. Absolute proof would probably require a close-up inspection of the crater, but the finding did make Stuart's belief that he had witnessed an impact much more plausible.

* 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 urban myths get started.

* Sources include:

* Revision history:

   v1.0   / 01 jun 01 
   v1.0.1 / 01 dec 01 / Review & polish.
   v1.0.2 / 01 jul 02 / Added comments on SD 1950, cleanup.
   v1.0.3 / 01 jul 04 / Added Stuart's Event.
   v1.0.4 / 01 mar 06 / Review & polish.
   v1.0.5 / 01 apr 06 / A few tweaky changes.
   v1.0.6 / 01 feb 08 / Minor updates.
   v1.0.7 / 01 jan 10 / Comments on Shiva.
   v1.0.8 / 01 dec 11 / Review & polish.
   v1.1.0 / 01 nov 13 / Chelyabinsk fireball, NEOSSAT, NEOWISE.
   v1.1.1 / 01 may 15 / Review & polish.