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I don't want to leave you with the impression that we know everything about impacts and extinctions. A question that is under strenuous debate at the moment has to do with reconciling the paleontologists' belief in long term extinction events, (more than one million years) and our demonstration that at least one at the K-T horizon is basically instantaneous. A fascinating paper that addresses this question has just been accepted for publication by NATURE, that has as its co-authors, astronomer Piet Hut, Walter and another geologist, plus four paleontologists and another astronomer. (That is certainly a "first.")

This paper's thesis is that many extinctions were initiated by comet showers, so each one really consists of several sharp extinctions of the kind I've discussed here, but spread out over the few million years it takes for the comet shower to be "cleaned out." This model has been advanced for several years by Rich Muller, and Erie KaufFman has given it the name "stepwise mass extinctions."30 It has much in its favor—not the least of which would be the end to the debate over whether mass extinctions were geologically instantaneous, as I've described today, or spread out over a few million years, as most paleontologists believe. If Rich and the authors of this new paper are right, then the two views are equally valid; it just depends on how you look at the record. (Physicists debated for years about the nature of light, and went through periods when waves or particles seemed to be favored. But everyone is now happy with the idea of duality; you see either particles or waves, depending on how you look.)

So far, we have seen multiple iridium spikes at only one extinction event, the E-O, but there is other evidence for comet showers at several other extinctions. Walter and Rich Muller, as well as Rampino and Stothers showed that large terrestrial craters have nearly the same period and phase as Raup and Sepkoski's 26 million year extinction cycle.31 That could only be true if some bolides were bunched together, as in comet showers.

No one has ever analyzed a comet or an asteroid for its trace element composition, so our knowledge of such objects comes from the analysis of meteorites, the only bolides that survive impact with the earth. As a first approximation, we have assumed that all bolides have the same composition, since they have the same origin, in the original solar nebula. But that is certainly an oversimplification; we know that all the planets have the same origin, but the four solid inner planets bear little resemblance to the gaseous Jovian planets. So we should expect that comets could be quite unlike the asteroids and meteorites; comets are probably mostly ice. (I've learned a great deal about comets from the recent semi-popular book, Comet, by Carl Sagan and Ann Druyanj32 they say, "So it looks very much as though comets really are giant ice balls racing around the sun.") I know of no evidence to contradict the following hypothesis, which I haven't seen advanced in quite this way until now: (but see Ref. 14b) All the major mass extinctions, and all of the secondary ones (addressed by Raup and Sepkoski) were triggered by bolide impact, with most of the bolides being very icy comets, probably in retrograde orbits, to maximize the environmental insult, per unit of incoming iridium.14 (This hypothesis has the advantage of explaining another otherwise very puzzling observation. Strewn fields of micro-tektites, generally believed to be of bolide origin, differ in one interesting way: some are rich in iridium, and others have no detectable iridium.) So, to use words I've used earlier, this is a reasonable hypothesis, that if true, would explain two otherwise puzzling observations, concerning extinctions and micro-tektites. And most importantly, it is testable—by those skilled in the art of finding shocked quartz. If they find shocked quartz at most of the extinction horizons, then I believe the cause of those extinctions will be established beyond all question, and we can all stop talking about the impact theory; it will then be established as a fact, just as the origin of the K-T extinction has been.

I have emphasized the observations that have convinced me, as a physicist, that the K-T extinction was caused by a bolide impact. I'll now try to look at the question from the geological point of view, by reproducing the final table (Figure 17) from a recent paper written by Walter,1 entitled, "Toward a Theory of Impact Crises." You will see that volcanic eruptions of two different types are rejected as possible causes, but impact events are acceptable.

Now that I've told of some problems that remain in our understanding of the other extinctions, I'll return to the more thoroughly studied K-T extinction, and say that for all the reasons I've explored in this talk, I see no way to escape the conclusion that the K-T extinction, including that of the dinosaurs, was triggered by a 10 kilometer diameter bolide impact. And at least from our human point of view, that was arguably one of the most important single events in the 4.5 billion year history of our planet. Had it not taken place, the largest mammals alive today might still resemble the rat-like crcatures that were scurrying around, 65 million years ago, trying to avoid being devoured by dinosaurs.


During the preparation of this talk, I have benefitted from fruitful discussions with Walter Alvarez, Frank Asaro and Richard A. Muller.

Our work on impacts and extinctions is funded predominantly by the director, Office of Energy Research, U.S. Department of Energy. We are very appreciative of the financial support of the M.J. Murdock Charitable Trust for the construction of the new ultra sensitive detector. We are also indebted to the National Space and Aeronautics Administration for funding part of our work for many years. Other important support has come from the

Observational evidence

Impact event

Volcanic Eruptions quiet basaltic violent siliceous


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