It has been something of a struggle over the last few centuries for exponents of the theory to convince both scientists and the public that the millions of craters that pockmark the face of the Moon are the result not of volcanic explosions but of collisions with objects from space. As long ago as the early nineteenth century, the German natural philosopher Baron von Paula Gruithuisen's declaration that the lunar craters were a consequence of 'a cosmic bombardment in past ages' was treated with contempt by 'serious' scientists. (No doubt his further claims to have uncovered evidence for the existence of humans and animals on the surface of the Moon had a little to do with this.) At the other end of the nineteenth century, the US geologist Grove Karl Gilbert tried to simulate in the laboratory the formation of the lunar craters by firing objects into powder or mud. Gilbert was perplexed, however, by the observation that only objects fired vertically produced circular craters like those that cover the lunar surface. In light of this, W. M. Smart proclaimed, in 1927, that the craters of the Moon could not be caused by impacts because 'there is no a priori reason why meteors should always fall vertically'. It was only after observing the effects of the billions of tonnes of bombs dropped in the Second World War that it began to dawn on geologists that given a violent enough explosion, a circular crater would always be formed— whatever the angle of the trajectory. In other words, the tremendous explosion generated when an object hit the Moon virtually always resulted in a circular crater. Remarkably, it took another quarter of a century for the impact origin of lunar craters to gain widespread acceptance, and even today one or two maverick scientists still support a volcanic origin in the face of overpowering evidence to the contrary. Getting any new paradigm accepted in science is a battle, and geology is no exception. Just as the proponents of the revolutionary theory of plate tectonics had initially to fight hard against reactionary forces, so those scientists who claimed that the Earth, as well as the Moon, had also taken a battering found the going difficult.
As long ago as 1905, Benjamin Tilghman proposed that Arizona's famous Barringer Crater (also now known as Meteor Crater) was the result of 'the impact of a meteor of enormous and hitherto unprecedented size'. This suggestion failed to convince, however, because a quarter of a century of excavation by Tilghman and his engineer colleague D. M. Barringer failed to find the impactor itself. We now know that this had been essentially vaporized by the enormous heat generated by the collision, but at the time the absence of a 'smoking gun' simply lent credence to those who suggested an alternative mechanism of formation.
Until well after the end of the Second World War, many Earth scientists suffered an extraordinary failure of the imagination, accepting an impact origin for the lunar craters but grabbing at any straw in order not to support an impact origin for crater structures on the surface of our own planet. Given that, due to the Earth's much greater size and stronger gravity field, it must have been struck perhaps 30 times more frequently than its nearest neighbour, this denial is even more extraordinary. Perhaps not entirely surprising, however, when we consider that the enormously dynamic nature of our planet is far from suited to the preservation of impact craters, particularly those of any great age. Because of plate tectonics, and in particular the process of subduction, through which the basaltic oceanic plates are continuously being consumed within the Earth's hot interior, some two-thirds of the Earth's surface is only a few hundred million years old. Bearing in mind that the most intense phase of bombardment occurred during the first few billion years of our planet's history, then evidence for this can now only be found in the ancient hearts of the granite continents that are immune to the subduction process. Because they have succumbed to erosion and weathering, perhaps for aeons, these craters are notoriously difficult to spot. Also, the oldest rocks, which are likely to support the most craters, are in remote areas such as Siberia, northern Canada, and Australia, and some craters are so big that their true form can only be seen from space. Today, satellites have helped in the identification of over 165 impact craters all over the world, and the idea that the Earth is susceptible to bombardment from space is now as accepted as plate tectonics.
Controversy has certainly not gone away, however, and argument continues amongst the scientific community, particularly about the frequency and regularity of impacts and— probably of most interest to the layman-about the effects of the next large impact on our civilization. The question of frequency is far from straightforward and serious disagreement exists between schools of thought that, on the one hand, support a constant flux of impactors and, on the other, advocate so-called impact clustering. Notwithstanding the very heavy bombardment of the Earth's early history, followers of impact uniformitariunism support a strike rate that is uniform and invariable. This is at variance with rival groups of scien-tistswho are promoting an alternative theory of coherentcatas-trophism, within which the Earth, for one reason or another, periodically comes under attack from increased numbers of asteroids or comets.
If we are realistically to assess the threat of future impacts to our civilization, then clearly it is vital that we resolve as soon as we can whether the number of collisions continues at its current rate or whether we have a nasty shock in store somewhere down the line. If the former proves to be correct, we can expect business as usual, meaning a collision with a 50-metre potential city-destroyer every 50 years or so, a half-kilometre small-country obliterator every ten millennia, and a 1-kilometre global impact event every 100,000 to 333,000 years—depending on whose figures you accept. Fortunately for us, gigantic extinction level events (ELEs), such as that
33 Over 165 impact craters have now been identified on Earth, many in the ancient hearts of the continents caused by the lo-kilometre monster that ended the reign of the dinosaurs 65 million years ago, appear to happen every 50 to 100 million years, so the chances of one striking the Earth soon are tiny. Based upon the above impactor strike rates, proponents of the threat from asteroids and comets come up with probabilities of dying due to an impact that really make one think. If you were able to construct a time machine and hurtled forwards to the year 1,000,2002 where you sought out and consulted the Centre for Planetary Records you would come up with a fascinating fact. The number of people killed in air (and no doubt space) crashes during the intervening million years — probably between 1 and 1.5 billion — would be less than those killed by impact events, which could total 3 billion or more, assuming two or three collisions with 1-kilometre objects. What this amounts to is that during your lifetime your chance of dying due to an asteroid or comet impact could be twice as great as being killed in an air crash; a pretty sobering thought if ever there was one. Looked at another way, if you gamble, your chance of being killed during an asteroid or comet strike is 750 times more likely than winning the UK lottery. Maybe this scares the wits out of you, but the true situation may actually be worse. If the coherent catastrophists are correct then there are certain periods in the Earth's history when our planet, or perhaps even our entire solar system, travels through a region of space containing substantially more debris than normal, resulting in a significant increase in impact events on all scales.
A number of theories lay the blame for this periodic increase in Earth-threatening space debris on the episodic disruption of the so-called Oort Cloud, a great spherical cloud of comets that envelops the entire solar system far beyond the orbit of Pluto. Typically, comets in the cloud travel along such huge orbits, which take some a quarter of the way to the nearest star, that they rarely visit the inner solar system, and then only in ones and twos. However, if some external influence were to interfere with the cloud, so the thinking goes, hundreds or thousands could have their orbits changed encouraging them to plunge Sunwards, greatly raising the threat of collision with the planets — including our own. A number of suggestions have been put forward for how the
34 The Barringer Crater (also known as Meteor Crater) in Arizona is the legacy of a collision with a small asteroid 50,000 years ago
34 The Barringer Crater (also known as Meteor Crater) in Arizona is the legacy of a collision with a small asteroid 50,000 years ago
Oort Cloud might be periodically disrupted, including due to the passage through the cloud of the mythical and much sought after planet X, which some scientists think may be orbiting far beyond frozen Pluto, or to a dark and distant stellar companion of our own Sun.
An alternative and intriguing theory, known as the Shiva hypothesis after the Hindu god of destruction and renewal, has been vigorously promoted by Mike Rampino of New York University and his colleagues, who believe that the great extinctions recognized in the Earth's geological record are the result of major impact events that happen pretty regularly every 26-30 million years. Rampino and his colleagues link this to the orbit of our solar system — including the Earth — about the centre of our Milky Way galaxy, an orbit that moves up and down in a wave-like motion. Every 30 million years or so, this undulating path takes the Sun and its offspring through the plane of our disc-like galaxy, when the gravitational pull from the huge mass of stars at the galaxy's core provides an extra tug. This, say the Rampino school, is sufficient to disturb the orbits of the Oort Cloud comets to an extent sufficient to send an influx of new comets into the heart of the solar system, dramatically raising the frequency of large impacts on the Earth. It is just a few million years now since our system last plunged through the galactic plane-could a phalanx of comets be heading for us at this very moment? By the time we find out it might very well be too late.
The Shiva hypothesis calls for a periodicity operating on truly geological timescales, and for this reason is rarely addressed in discussions of the immediate threat from impact events. Much more relevant to considerations of our own safety and survival — and that of our immediate descendants — is a proposal by UK astronomers Victor Clube and Bill Napier that the Earth is struck by clusters of objects every few thousand years, and that our planet took a serious pounding as recently as the Bronze Age—just 4,000 years ago. To find out what might cause such a worryingly recent bombardment we need to return to the Oort Cloud in deep— est space. Leaving aside disturbance of the cloud due to the passage of the solar system around the galaxy, normality sees a new comet from the cloud every now and again falling in towards the inner solar system — maybe as frequently as every 20,000 years. The newcomer is rapidly 'captured' and torn apart by the strong gravitational fields of either the Sun or Jupiter, forming a ring of debris spread out along its orbit, but concentrated particularly around the position of the ori— ginal comet itself. A large comet, broken up in this way, can 'seed' the inner solar system with perhaps a million 1-kilometre sized lumps of rock, dramatically increasing the numbers of Earth—threatening objects, and significantly rais— ing the chances of our planet being hit. Clube, Napier, and others of this particular coherent catastrophist school pro— pose that the last giant comet from the Oort Cloud entered our solar system towards the end of the last Ice Age—a mere 10,000 years or so ago— breaking up to form a mass of debris known as the Taurid Complex. Every December the Earth passes through part of this debris stream, resulting in the sometimes spectacular light show put on by the Taurid meteor storm, as small rocky fragments and gravel-sized stones burn up in the upper atmosphere. These innocuous bits and pieces only represent the tail end of the Taurid Complex, however, the heart of which contains a 5-kilometre wide Earth-crossing comet known as Encke and at least 40 accompanying asteroids any one of which would create global havoc if it struck our planet.
The distribution of debris along the Taurid Complex orbit about the Sun is rather like that of runners in a 10,000-metre race; while the majority are clustered together in a pack, the rest are dotted here and there around the track. Mostyears— according to the coherent catastrophists — the Earth's orbit crosses that of the Taurid Complex at a point where there is little debris, resulting in a pre-Christmas spectacle and little else. Every 2,500-3,000 years or so, the Earth passes through the equivalent of the runners' pack — and finds itself on the receiving end of a volley of rocky chunks perhaps up to 200-300 metres across. Benny Peiser, a social anthropologist at Liverpool's John Moores University, thinks thatjustsuch a bombardment around 4,000 years ago led to the fall of many early civilizations during the third millennium BC. He and others have interpreted contemporary accounts in terms of a succession of impacts, too small to have a global impact but quite sufficient to cause mayhem in the ancient worlds, largely through generating destructive atmospheric shock waves, earthquakes, tsunamis, and wildfires. Many urban centres in Europe, Africa, and Asia appear to have collapsed almost simultaneously around 2350 BC, and records abound of flood, fire, quake, and general chaos. These sometimes fanciful accounts are, of course, open to alternative interpretation, and hard evidence for bombardment from space around this time remains elusive. Having said this, seven impact craters in Australia, Estonia, and Argentina have been allocated ages of 4,000-5,000 years and the search goes on for others. Even more difficult to defend are propositions by some that the collapse of the Roman Empire and the onset of the Dark Ages may somehow have been triggered by increased numbers of impacts when the Earth last passed through the dense part of the Taurid Complex between 400 and 600 AD. Hard evidence for these is weak and periods of deteriorated climate attributed to impacts around this time can equallywell be explained by large volcanic explosions. In recent years there has, in fact, been a worrying tendency amongst archaeologists, anthropologists, and historians to attempt to explain every historical event in terms of a natural catastrophe of some sort-whether asteroid impact, volcanic eruption, or earthquake— many on the basis of the flimsiest of evidence. As the aim of thisvolume is to shed light on how natural catastrophes can affect us all, I would be foolish to argue that past civilizations have not suffered many times at the hands of nature. Attributing everything from the English Civil War and the French Revolution to the fall of Rome and the westward march of Genghis Khan to natural disasters only serves, however, to devalue the potentially cataclysmic effects of natural hazards and to trivialize the role of nature in shaping the course of civilization.
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This is common knowledge that disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.