The bombardment hypothesis

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The bombardment hypothesis has its roots in explanations for stones falling from the heavens. From earliest times until the start of the nineteenth century, it was widely believed that some types of stone grew in the air and fell from the skies, notably on dark nights and during storms (see Marvin 1986, for a review). The oldest known report of a stone falling from the sky comes from China in 644 BC. Pliny the Elder (AD 23-79), in his NaturalisHistoria, distinguished four classes of stone, the last which, the ceraunius (or ceraunia), was highly prized by Parthian magicians and was found in places that had been struck by lightning. Later writers divided the cerauniae into two varieties: stony and metallic. These stones include true meteorites that, in some cases, observers had witnessed falling from heaven. Stones known to have fallen from the sky were often set in shrines and worshipped. During the third century BC, a largish meteorite fell in Phrygia and was taken to Pessinus and set in a shrine to the goddess Cybele, whose image the stone was thought to resemble. This stone reputedly brought prosperity and victory in battle to its possessor. When Hannibal invaded Italy in 218 BC, the Romans mounted an expedition against the Phyrgian King Attalus, who yielded the stone, which was carried to Rome with much ceremony and was worshipped for more than 500 years. In 47 BC, at Aegos Potamos in Thrace, a mass of iron 'the size of a chariot' fell, which later Anaxagoras declared to be a fragment of the Sun. Embedded in the inside of a wall in the eastern end of Kaaba in Mecca, is the 'Black Stone', a meteorite that probably fell near an encampment of Arabs who made it an object of reverence. In AD 1400, an iron meteorite weighing 105 kg, and thought to be the metamorphosed remains of a local tyrant, fell near Elbogen in Bohemia. On 7 November 1492, a hot mass weighing 127 kg fell, with much noise and flame, into a field in Alsace not far from a party of travellers led by the future Emperor Maximilian I. By his order, it was taken to the church at Ensisheim where it remained until the French Revolution, when it was cut down and distributed to museums.

At the start of the nineteenth century, the origin of meteorites was the subject of intense deliberations. Four views emerged. First, meteorites are of terrestrial origin, thrown up into the heavens by volcanoes or hurricanes. A variation on this theme was the suggestion that meteorites might be ejecta from lunar volcanoes. Second, meteorites form in situ by fusion (vitrification) of terrestrial material when struck by lightning. Third, meteorites are concretions formed in the atmosphere. Fourth, meteorites are masses of matter alien to the planet, in other words, extraterrestrial objects. The first person to prove persuasively that meteorites have a cosmic origin, as popular belief maintained, was a Slovakian physicist who worked in Berlin, Ernst Florens Friederich Chladni (1756-1827). Chladni came into to possession of a fragment of an iron mass that weighed 600 kg, found near the Yenisei River in 1749 by a blacksmith called Medvedev. In a classic book published in 1794, Chladni showed that meteorites are fragments of cosmic bodies that journey through space at enormous speeds, and which the Earth attracts if they should approach it.

At the time of its publication, Chladni's treatise drew scant attention, but exoneration of his ideas on meteorites as stray cosmic wanders was soon to come. In the year after being published, a spectacular meteorite shower fell at Siena; in 1795, large stones fell in Yorkshire; and in 1798, a shower of stones fell at Benares in India. Specimens collected from all these falls, and other 'fallen bodies' from private collections, were, at the behest of Sir Joseph Banks, President of the Royal Society, analysed chemically. By 1802, the chemist Edward C. Howard and the mineralogist Jacques Louis, Comte de Bournon had linked fallen stones with fallen irons by their nickel content, and they had identified several characteristics of meteorites that set them apart from terrestrial rocks and that indicated a cosmic origin (Bournon 1802; Howard 1802). On 26 April 1803, a few months after reading their reports at L'Institut de France in Paris, nearly 3,000 fragments fell over L'Aigle in Normandy. Several town officials witnessed this remarkable event and the cosmic origin of such falling stones seemed incontestable.

For the first three-quarters of the nineteenth century, scientists accepted meteorites as natural phenomena but did not seriously entertain the notion that they might produce sizeable craters on the planet. This was perhaps partly because the meteorites seen to fall, or discovered by chance, throughout the nineteenth century were small bodies that 'produced insignificant pits in the soil' (Marvin 1990, 150). However, the English astronomer Richard A. Proctor (1873, 345) wondered if the innumerable craters of the Moon, which by broad consensus at the time were deemed to have a volcanic origin, might result from the 'plash of meteoric rain'. Geologists, straitjacketed by their inviolable uniformitarian creed, energetically rejected this notion because bodies of huge size would be required and the Earth should bear similar scars to that of the Moon, which, they opined, it does not. Then, in 1891, Grove Karl Gilbert examined a crater in Arizona - later named Meteor Crater - and initially concluded that collision with an asteroid was the cause. Unfortunately, Gilbert could find no evidence to back up his conclusion and he turned to what seemed a more realistic, and unquestionably a more conservative, alternative - that a deep-seated steam explosion was responsible.

For the first half of the twentieth century, most geologists stubbornly resisted the idea of meteorite impact as a significant geological process and ignored the views of a few dissenters. In North America, Daniel Moreau Barringer (1905) argued for the impact origin of Coon Mountain (now called Meteor Crater) and other craters. In Europe, a colossal collision was implicated in the excavation of the Steinheim Basin in southern Germany (Werner 1904). But, a re-examination of this crater basin led to the conclusion that a new brand of 'cryptovolcanism', involving the rupturing of the crust but without an eruption of lava or ash, produced it (Branco and Fraas 1905). In the late 1920s, a flurry of discoveries provided fresh evidence for impact cratering (e.g. Barringer 1929; Alderman 1932; Philby 1933). In 1929, the first expedition returned from Tunguska, the scene of a bolide explosion on the morning of 30 June 1908. A small asteroid, probably 60 m in diameter, travelling from south-east to north-west hurtled over the Podkamennaya-Tunguska River region of Siberia, exploded some 8.5 km above the ground, causing a great fireball about 60 km north-west of the remote trading post of Vanovara. Witnesses saw the fireball 1,000 km away and heard the atmospheric shock at even greater distances. The blast flattened trees within a 40 km radius and ignited dry timber within an 8 km radius. The energy released by the explosion was about 15 Mt of TNT equivalent energy, or roughly the same energy as a very large hydrogen bomb. The Tunguska event was a bona fide instance of an explosive impact seen in operation, though not confirmed until 21 years after the event. Thus, it fulfilled one of the basic requirements of uniformitarianism and it should have led to a general acceptance of impact as a geological process. There followed no conspicuous move toward acceptance, however.

The discovery of asteroids on potential collision courses with the Earth sparked the eventual ascendancy of the bombardment hypothesis. In 1918, Max Wolf discovered the first asteroid with an Earth-crossing orbit, 887 Alinda, which has a diameter of about 5 km. In 1932, astronomers discovered two more - 1221 Amor with a diameter of 1.1 km and 1862 Apollo with a diameter of 1.2 km. In the light of these and later discoveries of Earth-crossing asteroids, it became acceptable to suggest that stray meteorites might collide with the Earth. Fletcher G. Watson (1941) made crude estimates of asteroid impact rates. Others astronomers spelt out the likely consequences of a collision (e.g. Nininger 1942; Baldwin 1949). Harvey Harlow Nininger of the Colorado Museum of Natural History and the American Meteorite Laboratory speculated on what would have happened had the asteroid Hermes, instead of passing by the Earth, as it had just done, had hit the Earth. He argued that a large meteorite impact would cause great changes in shorelines, the elevation and depression of extensive areas, the submergence of some low-lying areas of land, the creation of islands, withdrawal and extension of seas, and widespread and protracted volcanism. He also speculated that the collision between the Earth and planetoids offers an adequate explanation for the successive revolutions of movements in the Earth's crust that have been widely recognized, and for the sudden extinction of biota over large areas, as revealed by the fossil record. Geologists, while accepting the legitimacy of impact craters, did not take such suggestions seriously, and dismissed impact craters as rare curiosities of no importance to global geology. They probably did so because there seemed to be little evidence that large meteorites had actually struck the Earth. True, field research was revealing a large and growing number of crater-like structures, but their impact origin remained questionable.

This situation was to change with the planning and initiation of the space age, which fostered a lively interest in meteorites, the Moon, and planets. As Ursula Marvin (1990, 152) put it, 'Attention catapulted to unprecedented levels after the orbiting of Sputnik I in October 1957'. After that signal event, information on the numbers, magnitudes, and ages of impact sites mushroomed and the discovery of impact signatures enabled the reclassification of most, if not all, the cryptovolcanic features then known in Europe, North America, and Africa as what Robert S. Dietz (1961) styled astroblemes ('star-wounds').

In the early 1960s, Eugene M. Shoemaker and his colleagues developed a model of, and found unique evidence for, the impact origin of Meteor Crater, Arizona (Chao et al. 1960; Shoemaker 1963), more or less settling a controversy that had raged for many decades (Hoyt 1987). Shoemaker had made detailed maps and structural analyses of Meteor Crater during the 1950s (Figure 3.1). In 1960, he sent a rock sample to Ed Chao of the United States Geological Survey Laboratory in Washington, DC, who detected the mineral coesite in the sample, and in further samples. Coesite is a very dense and heavy form of silica that Loring Coes (1953) had made in a laboratory under extremely high pressures. It was unknown in nature. Its discovery in association with a presumed impact crater was exciting and startling. Here in Meteor Crater was firm evidence supporting the view that a meteorite impact had excavated the crater. Only a meteorite impact could produce high enough pressures for coesite to form. Shoemaker's discovery led to a search for coesite in other craters suspected of having an impact origin. The search was successful: coesite turned up in rocks of the Ries Crater, West Germany, and at many other sites.

The proven association of coesite with impact-shocked rocks lent support for the view, first mooted by Nininger in 1956 and developed by Michael E. Lipschutz and Edward Anders (1961), that an impact event formed the diamonds found in iron in the Canyon Diablo Crater, Arizona. It became clear during the early 1960s that the alteration of minerals in target rock, induced by the passage of a shock wave radiating from the point of impact, was a sure signature of an impact event. The enormous pressures generated by a shock wave caused minerals to change instantaneously into glass without melting. Numerous examples of impact metamorphism have since been unearthed, and impact metamorphism is now taken as proof that a crater was produced by a meteorite impact.

Dietz established an independent means of detecting and confirming the origin of impact craters. In 1947, he published a paper in which he showed that the impact of meteorites at hypervelocities created shatter cones (conical fragments of rock with striations that radiate from the apex). Whether other geological processes could produce shatter cones was unclear. Certainly, 'normal' rock formations did not contain shatter cones, nor did rocks subjected to volcanic explosions. The explosives used in quarrying produced crude, irregular fracture cones without striations, while military explosives with a high detonation velocity and high shattering effect produced cones with striations, similar to shatter cones but with a less perfect shape. By the early 1960s, shatter cones at several impact sites had been discovered. They strongly suggested the occurrence of impacts, but did not provide unequivocal evidence, since their origin was not fully understood and the possibility of a geological origin could not be discarded.

From the foundations laid by Dietz, Shoemaker, and other pioneers, a rash of impact studies arose, which eventually led to the general acceptance of the impact origin of the majority of lunar craters and their terrestrial counterparts. A few voices of disagreement were sometimes heard (e.g. Bucher 1963; McCall 1979), mainly because in larger craters no fragments of the impacting body remain, having been vaporized and melted on impact, but also because of the complexity of crater form at larger diameters. The dissenters have suggested a range of internal geological processes to account for crater formation. Walter H. Bucher (1963), for instance, suggested cryptoexplosions of gas. However, in the face of a voluminous literature on impact phenomena, all but a few geologists now question the existence of large terrestrial impact structures. Therefore, from sitting at the bottom of the list of likely causes of catastrophes, bombardment by asteroids, meteoroids, and comets has become the most plausible explanation for some sudden and violent events in Earth's past. It is the case 'that extraterrestrial masses large enough to form vast craters could impact on

Recent alluvium

Kaibab limestone

Recent alluvium

Pleistocene alluvium

Debris from Coconino sandstone and Toroweap formation Debris from Kaibab limestone

Debris from Moenkopi formation

Moenkopi formation: Wupatki member

'Lower' massive sandstone

Kaibab limestone

2 km

Pleistocene and recent sediments Fallback debris Mied debris from target rocks and meteorite Breccia

Moenkopi sandstone

Kaibab limestone

Coconino sandstone Supai sandstone and siltstone Horizontal and vertical scale

Figure 3.1 Meteor Crater, Arizona. This crater has played a major role in basic cratering research. It was also a training site for each of the Apollo astronaut crews. It is one of the youngest impact craters. Fieldwork by Shoemaker (1963) suggests that the crater is 25,000 years old. That means that it was formed just before humans established permanent residence in the southern regions of the Colorado Plateau. The impact event appears to have involved a body, or possibly several bodies, of iron travelling at a hypervelocity of over 11 km per second. On striking the horizontal strata of the region, the iron body released between 5 and 10 megatons of kinetic energy. This energy excavated a large, bowl-shaped crater roughly 1.1 km in diameter and 200 m deep. Surrounding the crater is an extensive sheet of ejected material. Some 175 million tonnes of material were ejected from the crater. The ejecta blanket is up to 25 m thick in places around the rim of the crater. It once extended 2 km from the crater centre but erosion has reduced the range to 1.5 km. The impact caused structural deformation of strata that now form the crater rim. Faulting and folding is pronounced and the strata have been uplifted so that they now dip away from the crater. The structural uplift, which produced steep inward-facing cliffs, was as much as 50 m along the upper crater walls. Source: Adapted from Shoemaker (1974).

Figure 3.1 Meteor Crater, Arizona. This crater has played a major role in basic cratering research. It was also a training site for each of the Apollo astronaut crews. It is one of the youngest impact craters. Fieldwork by Shoemaker (1963) suggests that the crater is 25,000 years old. That means that it was formed just before humans established permanent residence in the southern regions of the Colorado Plateau. The impact event appears to have involved a body, or possibly several bodies, of iron travelling at a hypervelocity of over 11 km per second. On striking the horizontal strata of the region, the iron body released between 5 and 10 megatons of kinetic energy. This energy excavated a large, bowl-shaped crater roughly 1.1 km in diameter and 200 m deep. Surrounding the crater is an extensive sheet of ejected material. Some 175 million tonnes of material were ejected from the crater. The ejecta blanket is up to 25 m thick in places around the rim of the crater. It once extended 2 km from the crater centre but erosion has reduced the range to 1.5 km. The impact caused structural deformation of strata that now form the crater rim. Faulting and folding is pronounced and the strata have been uplifted so that they now dip away from the crater. The structural uplift, which produced steep inward-facing cliffs, was as much as 50 m along the upper crater walls. Source: Adapted from Shoemaker (1974).

the earth or any other solar system body was, at the turn of this [the twentieth] century, an incredible idea; today, meteoritic impact is widely recognised as a fundamental cosmic process' (Hoyt 1987, 366). Or, as Bevan M. French put it:

During the last 30 years, there has been an immense and unexpected revolution in our picture of Earth and its place in the Solar System. What was once a minor astronomical process has become an important part of the geological mainstream. Impacts of extraterrestrial objects on the Earth, once regarded as an exotic but geologically insignificant process, have now been recognized as a major factor in the geological and biological history of the Earth. Scientists and the public have both come to realize that terrestrial impact structures are more abundant, larger, older, more geologically complex, more economically important, and even more biologically significant than anyone would have predicted a few decades ago. Impact events have generated large crustal disturbances, produced huge volumes of igneous rocks, formed major ore deposits, and participated in at least one major biological extinction.

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