Nearly a hundred astronomers, historians, archaeologists and others gathered in Cambridge from 11-13 July 1997 for the second international conference of the Society for Interdisciplinary Studies. The purpose - to discuss the near-simultaneous ending of Bronze Age civilizations world- wide, in particular whether the 'giant comet' hypothesis, associated principally with the names of Clube & Napier and Hoyle & Wickramasinghe, could be substantiated by ground truth in the archaeological, geological, climatological and historical records.
Depending on one's chronology and the geographic region under discussion, the Bronze Age started c.3500 BC and continued for two or three thousand years until approximately 1500 - 500 BC, encompassing not only the main construction phases of Stonehenge and its megalithic counterparts elsewhere, but also the genesis of kingship, priesthood, human and animal sacrifice, the evolution of religious views from polytheism to monotheism and - significantly to astronomers - a burgeoning interest in the celestial sphere.
The heart of the astronomical thesis is that changes in the Earth's near-space astronomical environment, corresponding to the orbital evolution and hierarchical fragmentation of a giant comet - possibly the progenitor of Encke's comet, have produced surges in the rate of accretion of interplanetary dust and associated episodes of bombardment of larger bodies (diameters 10 - 100 m) on time-scales of historical interest (decades to millennia).
Can this line of argument be sustained, or like many a beguiling 'Theory of Everything' will it suffer the usual fate of beautiful theory wrecked by ugly fact? Moreover, even if only some elements of the theory should stand the test of time, what, if any, are the lessons for the future of modern civilization?
The current hazards due to near-Earth objects were considered at a meeting held the previous day at the Royal Greenwich Observatory, Cambridge. The discussion concluded that civilization would be unlikely>to survive the impact of even quite a small astronomical body; and the level of the actuarial risk to society from the observed population of near-Earth asteroids is much greater than that associated with hazards that society routinely considers suitable for detailed assessment and mitigation.
The SIS meeting opened with a keynote presentation by Robert Matthews (Science Correspondent of the Sunday Telegraph), who drew together what he called two 'modern myths': namely, that what happened long ago is complete nonsense; and that the Earth is a Safe Place. These topics, illustrated with examples from mythology and of glyphs and relics, went to the heart of the subject of the conference and demonstrated above all the value of open-minded, multidisciplinary investigations. It was fascinating to hear the fourth estate appeal to academics to talk to journalists ('get the message out as often as possible'); and the justification of the importance of the message, namely 'Why should we care?': because it's fascinating, part of our culture, and directly relevant to our survival on this planet. Quoting George Santayana, Matthews reminded the audience that those who forget the past are condemned to re-live it.
The astronomical backdrop against which changes in human civilization are measured was reviewed by Mark Bailey and Bill Napier (Armagh Observatory). So far as comets are concerned, the key recent discoveries are that (i) 'giants', those with diameters greater than about 100 km, drop into the short-period system roughly once every 20 - 200 thousand years; (ii) their orbital evolution is likely to involve episodes of Sun-grazing, a mechanism able to fragment even a large body; and (iii) Tunguska-class bodies (objects with sizes a few tens of metres) probably arrive at mean intervals ~100 years, randomly if from the main asteroid belt between Mars and Jupiter, and clustered in time if predominantly in streams of cometary origin.
Bill Napier pointed out that the greatest immediate extraterrestrial hazard is probably that due to oceanic impacts, involving objects with dimensions greater than about 200 m (expected to recur on the average about once per 5 - 10 thousand years), producing hemispherically destructive tsunami. The long-term threat from the arrival of a giant comet in the inner solar system is comparable to that from the impact of a 1-km diameter asteroid, in that both have a recurrence interval on the order of 100,000 years, and either could end civilization.
During discussions the debate focused on whether Bronze Age civilizations collapsed simultaneously, and, if so, on a regional or world-wide basis; and whether purely terrestrial or extraterrestrial processes were involved. Is the ultimate cause of the collapse of a civilization simple (as for the single impact hypothesis for the K/T extinction) or complex, involving diverse factors? For example, the combination of climate change, mass migration, war and economic collapse would undoubtedly be worse than one of those factors acting alone. As in the analogous mass- extinction of life debate, the SIS meeting demonstrated that such questions are in principle resolvable by detailed stratigraphical analyses and investigations in the field.
Continuing the astronomical theme, Duncan Steel (Spaceguard Australia) gave an ingenious interpretation of the earliest phases of construction of Stonehenge, pointing out that according to the giant comet hypothesis episodes of enhanced terrestrial bombardment are expected to occur when the orbit of the Encke comet dust trail intersects that ofthe Earth. This leads to the suggestion that pairs of bombardment events will occur a few centuries apart, themselves separated by 2500 - 3000 years, and that such an episode may have occurred c.3600 and 3200 BC.
The astronomical session culminated with a wide-ranging review by Gerrit Verschuur (University of Memphis) of the impact of the 'new astronomy' on human thought and behaviour. There is no doubt that the Earth is repeatedly struck by massive bodies - the late stages of a declining planetary accretion - but the real questions are when was the last major event and when will be the next? Verschuur's tale had a moral: when Pandora opened her box of calamities, there fell out Hope. And 'Hope' gets in the way of devising strategies to deal with the impact hazard to civilization. Whereas, in the chaotic evolution of life on Earth, contingency teaches us that from a biological perspective we are 'lucky' to be here, our key advantage, according to Verschuur, is the human ability to communicate information, and hence more effectively to adapt to rapid environmental change, whether caused by astronomical or purely geological processes.
Subsequent papers, including poster presentations, focused on Archaeology, Geology and Climatology, History and Culture, and the boundary between Archaeology and History. There is no space to review these in detail, but a flavour of the discussion is given below.
Marie-Agnes Courty (CNRS, Grignon) presented new archaeological data concerning a catastrophe inferred to have occurred in the Middle East c.2350 BC. She emphasized the importance of high-time-resolution archaeological investigations in the assessment of natural catastrophes on societal collapse, the data in this case indicating the combination of a burnt surface horizon and air blast, consistent with a Tunguska-like fireball, but possibly also a major volcanic event.
The evidence for regional environmental change at about the same time was confirmed and extended by Mike Baillie (Queen's University Belfast), whose tree-ring analyses of Irish bog oaks showed very significant narrowing of the rings around the year 2345 BC, associated with identified tephra from the Icelandic Hekla 4 volcano, dated to 2310 +/- 20 BC. This suggests a volcanic origin of the c. 2350 BC event identified by Courty, but the period in question is also associated with other events, including floods, the creation of new lakes and even the traditional start of Chinese history! In Baillie's words, 2345 BC 'is a classic marker date, i.e. a date which will show up on a regular basis in studies of various kinds'.
In this instance, as indeed at other times (e.g. 1628 BC, 1159 BC, 948 BC, 430 BC and 540 AD), there are glimpses of possible volcanic signatures and hints of world-wide events, the volcanic cooling often superposed on longer term climatic trends and providing the final straw that broke society's back. This leaves a question, namely what caused the long term colling trends?
Whether the climatic down-turns are associated with impacts or stratospheric dust loading (or neither of these) remains to be seen, although as highlighted by Benny Peiser (Liverpool John Moores University) the period c. 2300 BC crops up surprisingly often, even in studies of dated craters. As for the archaeological record, it is still difficult to know what precise signature to look for: the Tunguska blast (which was undoubtedly devastating) left no crater, whereas the similar-sized Meteor Crater projectile left a hole more than a kilometre across. Moreover, as shown by Amos Nur (Stanford University, California) even earthquakes can produce regional devastation, and may occur bunched in time over a period less than a century, followed by a quiet phase lasting hundreds or thousands of years, rather like the frequency of impacts from a massive disintegrating comet,
The physical connection between impacts or cosmic dust accretion on the one hand and volcanic eruptions, or other manifestations of environmental change on the other, for example, that shown in the climatological record, is at present tenuous, but the marker dates frequently show up. Bas van Geel (University of Amsterdam) highlighted a further connection, this time between the sharp increase in the (14)C content of the atmosphere between c.850 BC and 760 BC and the climatic down-turn in Western Europe, indicating a transition from a relatively warm, dry situation prior to 850 BC to subsequent much cooler, wetter conditions. The suggestion is that reduced sunspot activity allowed an enhanced flux of high-energy galactic cosmic rays to reach the top of the atmosphere, leading to increased cloudiness, and hence lower temperatures and higher precipitation.
Turning briefly to the possible cultural implications of such external forcing of the environment; if humankind has occasionally been exposed to environmental catastrophes having a scale and nature unlike any modern analogue, it is of interest to ask what might be society's reaction to such events. Gunnar Heinsohn (University of Bremen) focused on the start of the Bronze Age as the beginning of the period associated with the onset of kingship and priesthood (the two have subsequently been inextricably linked), and the notion of kings being anointed from heaven. The emergence of the Bronze Age remains an enigma, unless one accepts the premise of a catastrophic environmental change, but the available evidence nevertheless indicates destruction of the previous society followed by a rise of cult-centres and priests. At what time - and why - did the concept of 'god' emerge, and which ghosts were the early rituals intended to lay to rest?
A related theme was pursued by Irving Wolfe (University of Montreal), developing the notion of sharp cultural and religious changes associated with environmental changes observed particularly around the Bronze-Age/Iron-Age boundary c.600 BC. Again much research remains to be carried out, but a broad-brush view is that a possible regional or global natural event during the first millennium BC led to social upheaval which culminated in the pre-Socratic Greek view of the world that eventually laid the distinctive cultural foundations of the modern age.
From: METEORITE! November 1997, 3:4, pp. 34-36