(This is a preliminary version of this book review, written by Clark R. Chapman.)
James Lawrence Powell, W.H. Freeman and Co., New York, xvi + 250 pp, ISBN 0- 7167-3117-7, 1998, $22.95.
In the 1960s and 70s, geology witnessed an intellectual revolution. Recognition that seafloors spread and continents drift led to plate tectonics. "Night Comes to the Cretaceous" addresses a second, still-developing revolution: the view that impact cratering and other catastrophes may be the dominant phenomena driving evolution of the biosphere. The specific case analyzed is the hypothesis of the late Nobel laureate, physicist Luis Alvarez, and his colleagues that the Cretaceous/Tertiary (K-T) mass extinctions resulted from impact of a 10 km diameter asteroid or comet. Once Alan Hildebrand focussed geologists' attentions on the immense Chicxulub crater in the Yucatan, K-T deposits bearing impact signatures were found to be arrayed radially to the crater which, itself, was dated precisely at the K-T age of 65 million years. By the early 1990s, the Alvarez hypothesis was thereby proved, despite protests of a few hold-outs, like Dartmouth's Charles Officer, whose scientific sloppiness is repeatedly scorned by this book's author, Jim Powell, director of the Los Angeles County Museum of Natural History.
It remains unsettled, however, whether or not impact is the *chief* cause of the "punctuations" in the punctuated equilibrium model of Darwinian evolution, espoused by Stephen Jay Gould and others. As University of Chicago paleontologist David Raup has argued, the statistics of species diversity during the Proterozoic are consistent with the hypothesis that *all* mass extinctions, large and small, are due to impact. So after the initial frustration of the origin of life by giant impacts, continuing evolution has perhaps been catalyzed by impacts. Conceivably, cosmic bombardment not only controlled the origin and evolution of life on Earth, but in other planetary systems, as well. Should this new paradigm be accepted eventually, it could be much more profound than even plate tectonics.
Powell argues that it required a physicist's cross-disciplinary "invasion" of geology to promote such a radical hypothesis for the K-T. There was fertile ground in which it could take root. For instance, the dramatic photos by the Mariner and Voyager spacecraft had revealed landscapes dominated by craters on most solid-surfaced planets and satellites. And with no generally accepted explanation for the dinosaurs' sudden demise, there was no broad, unified defense of an alternative to the Alvarez proposal. Nevertheless, as Powell documents, it was no easy road to acceptance of the idea, especially among paleontologists. One prominent astronomer even argued against the impact hypothesis.
Powell's documentation of the Alvarez debates is selective, emphasizing the geological evidence. It is perhaps just as well that he doesn't branch out into the often equally significant research in planetary, atmospheric, and environmental sciences, with which he is less familiar. When he does briefly venture into planetary astronomy, he makes his worst mistakes. For example, comet tails do not simply trail behind a comet, but are always driven away from the Sun. And despite Powell's description of comet Shoemaker-Levy 9 as breaking up after discovery, it was actually tidally disrupted by Jupiter about a year *before* discovery, rendering it bright enough for the Shoemakers and David Levy to discover it.
As a planetary scientist, I am rather exasperated by Powell's pedagogy: the tortured, step-by-step proofs of impact, involving "predictions" and "findings". Perhaps such logic was needed to convince many geologists of the truth of the hypothesis. But from an astronomer's perspective, the plausibility of impacts seems self-evident without requiring the discovery of Chicxulub (which could well have been subducted by now). Impacts are hardly a *deus ex machina*. After all, asteroids and comets are *observed* in the inner solar system and it is elementary to calculate their impact frequency. As early as 1941, Harvard astronomer Fletcher Watson calculated a duration of "a hundred thousand years between collisions with ... these flying mountains." Recognizing that "...asteroids still abound in space and occasionally come close to the Earth," Ralph Baldwin noted as early as 1949 that "the explosion that formed the crater Tycho...would, anywhere on Earth, be a horrifying thing, almost inconceivable in its monstrosity."
Unfortunately, through the 1970s the next step was never taken to quantify just how monstrous the environmental consequences of impact might be; such scenarios were left to science fiction. Yet by the 1980s, in the wake of the environmental movement and research on the influences of volcanos and nuclear warfare on global climate, the burden of proof should have shifted to geologists and biologists to demonstrate how life on the land and the seas could possibly survive an explosion equivalent to 100 million megatons of TNT. Paralleling the geological debate was a developing literature, largely unrecorded by Powell (except for a section based on a single 1994 review paper), on the environmental consequences of major impacts. Even the proceedings of the first Snowbird conference contains a whole section on "Physical, Chemical, and Biological Models of Atmospheric Effects of Large Impacts."
After detailed treatment of the K-T boundary, Powell steps lightly through other mass extinctions, reflecting the lesser research done on these often-more-ancient cases. We need another definitive case of an impact holocaust before there can be broad acceptance of impact as the usual (rather than exceptional) motive force for evolutionary change. Consider the end of the Permian, when over 95% of species were extinguished. Recognizing that volcanism, sea level changes, and climate changes are each inadequate, the Smithsonian's Douglas Erwin [discussed by Powell, but also see the July 1996 issue of *Scientific American*] proposed an unlucky (and unlikely) combination of all of them, acting over many hundreds of thousands of years. (Erwin recounted how estimates of the pace of extinctions had shrunk from 5 to 1 million years.) In May 1998, Erwin coauthored a new study of end-Permian geochronology, which demonstrates that extinctions took 165,000 years or less. The more sudden the extinctions, the more likely that an impact is the explanation. A truly instantaneous catastrophe is far more capable of extinguishing species than slowly acting processes like sea-level or climate changes. Species can adapt and migrate in response to changes taking thousands to millions of years. Dramatic aftereffects of an impact begin in just an hour; others peak within months. Probably such abrupt changes are required for mass extinctions unless life is far more fragile than the robust recoveries following local devastations, like the Mt. St. Helens explosion, seem to imply.
Late in "Night Comes to the Cretaceous," Powell finally realizes that the burden of proof has shifted to the anti-impactors: "...since it is inescapable that the earth has been bombarded by meteorites of a range of sizes...and since even a modest-sized crater releases huge amounts of energy, how are we to escape the conclusion that...impact has caused many extinctions?" This is a well-written, intelligent book, accessible to the interested layperson but also fully footnoted for geoscientists who want more technical details. It is a thorough account of that portion of the K-T battle, now won, that was fought on a geological turf. Whether or not the continuing input from astronomy, physics, atmospheric science, evolutionary biology, and other disciplines will eventually lead to acceptance of the new paradigm in its broadest form -- a victory for a neocatastrophist model for the evolution of our planet's ecosphere -- remains to be seen. Powell's book sets the stage for the continuing research and debates.
--Clark R. Chapman, Space Studies Department, Southwest Research Institute, Suite 426, 1050 Walnut St., Boulder CO 80302 USA.
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