News reports on the Torino impact hazard scale, on the generation of tsunami by impacts, and on the sudden extinction of the dinosaurs at the end of the Cretaceous.

Below are two abstracts of papers published in the refereed technical literature, one by Binzel on the Torino Impact Hazard Scale and the other by Ward and Asphaug on impact tsunamis and the level of risk they represent to coastal populations. Finally is an article (taken from Benny Peiser's CCNet e-mail list) concerning the apparent rapid (geologically instantaneous) extinction of the dinosaurs at the end of the Cretaceous; the author of the research is quoted as saying "The asteroid impact bought a sudden and very abrupt demise to species that were healthy and doing well."

THE TORINO IMPACT HAZARD SCALE

R.P. Binzel: The Torino Impact Hazard Scale. PLANETARY ANDSPACE SCIENCE, 2000, Vol.48, No.4, pp.297-303

Newly discovered asteroids and comets have inherent uncertainties in their orbit determinations owing to the natural limits of positional measurement precision and the finite lengths of orbital arcs over which determinations are made. For some objects makings predictable future close approaches to the Earth, orbital uncertainties may be such that a collision with the Earth cannot be ruled out. Careful and responsible communication between astronomers and the public is required for reporting these predictions and a 0.10 point hazard scale, reported inseparably with the date of close encounter, is recommended as a simple and efficient tool for this purpose. The goal of this scale, endorsed as the Torino Impact Hazard Scale, is to place into context the level of public concern that is warranted for any close encounter event within the next century. Concomitant reporting of the close encounter date further conveys the sense of urgency that is warranted. The Torino Scale value for a close approach event is based upon both collision probability and the estimated kinetic energy (collision consequence), where the scale value can change as probability and energy estimates are refined by further data. On the scale, Category 1 corresponds to collision probabilities that are comparable to the current annual chance for any given size impactor. Categories 8-10 correspond to certain (probability >99%) collisions having increasingly dire consequences. While close approaches falling Category 0 may be no cause for noteworthy public concern, there remains a professional responsibility to further refine orbital parameters for such objects and a figure of merit is suggested for evaluating such objects. Because impact predictions represent a multi-dimensional problem, there is no unique or perfect translation into a one-dimensional system such as the Torino Scale. These limitations are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.

ASTEROID IMPACT TSUNAMI: A PROBABILISTIC HAZARD ASSESSMENT

S.N. Ward and E. Asphaug: Asteroid impact tsunami: A probabilistic hazard assessment. ICARUS, Volume 145, Number 1, May 2000, 64f.

We investigate the generation, propagation, and probabilistic hazard of tsunami spawned by oceanic asteroid impacts. The process first links the depth and diameter of parabolic impact cavities to asteroid density, radius, and impact velocity by means of elementary energy arguments and crater scaling rules. Then, linear tsunami theory illustrates how these transient cavities evolve into vertical sea surface waveforms at distant positions and times. By measuring maximum wave amplitude at many distances for a variety of impactor sizes, we derive simplified attenuation relations that account both for geometrical spreading and frequency dispersion of tsunami on uniform depth oceans. In general, the tsunami wavelengths contributing to the peak amplitude coincide closely with the diameter of the transient impact cavity. For the moderate size impactors of interest here (those smaller than a few hundred meters radius), cavity widths are less than or comparable to mid-ocean depths. As a consequence, dispersion increases the long-wave decay rate to nearly 1/r for tsunami from these sources. In the final step, linear shoaling theory applied at the frequency associated with peak tsunami amplitude corrects for amplifications as the waves near land. By coupling this tsunami amplitude/distance information with the statistics of asteroid falls, the probabilistic hazard of impact tsunami is assessed in much the same way as probabilistic seismic hazard, by integrating contributions over all admissible impactor sizes and impact locations. In particular, tsunami hazard, expressed as the Poissonian probability of being inundated by waves from 2 to 50 m in height in a 1000-year interval, is computed at both generic (generalized geography) and specific (real geography) sites. For example, a typical generic site with 1808 of ocean exposure and a reach of 6000 km, admits a 1-in-14 chance of an impact tsunami exceeding 2-m in height in 1000 years. The likelihood drops to 1-in-35 for a 5-m wave, and to 1-in-345 for a 25-m wave. Specific sites of Tokyo and New York have 1-in-24 and 1-in-47 chances, respectively, of suffering an impact tsunami greater than 5 m in the next millennium. (c) 2000: Academic Press

STUDY SUPPORTS QUICK END OF DINOSAURS FROM ASTEROID IMPACT

Study Backs Quick End of Dinosaurs By PAUL RECER, AP Science Writer

WASHINGTON (AP) Dinosaurs died quickly, snuffed out by the impact of an asteroid that sent a wall of fire and death racing across North America, an analysis of fossils found in Montana and North Dakota concludes. The finding casts doubt on a theory the dinosaurs died out slowly and that the asteroid impact was simply an end-the-misery trauma for an almost-vanished species, said Peter M. Sheehan of the Milwaukee Public Museum, first author of the study appearing Thursday in the journal Geology.

Researchers analyzed the number and distribution of fossils across large parts of the two states, where the animals roamed some 65 million years ago. "What we found suggests that the dinosaurs were thriving, that they were doing extremely well during that time," Sheehan said. "The asteroid impact bought a sudden and very abrupt demise to species that were healthy and doing well."

The research adds weight on one side of a debate among experts who study the dinosaur and how the huge animals died. One group, often called the gradualists, believes the dinosaurs were slowly dying out, that they were weak and beginning to disappear when the asteroid hit. William A. Clemens of the University of California, Berkeley, a leader of the gradualists, said the Sheehan study fails to prove the asteroid theory of dinosaur extinction. Sheehan and others believe it was the asteroid impact alone that killed the dinosaurs in one, swift fiery eruption, followed by weeks of deep cold.

The gradualists base their argument on a 20-year-old study that found few dinosaur fossils in the top 9 feet of a rock deposit, called the Hell Creek Formation, that was laid down in North Dakota and Montana during the last two million years before the asteroid impact. Based on the scarcity of fossils, the gradualists believe the 200-million-year reign of the terrible lizard was already drawing to a close when the asteroid arrived.

But Sheehan said a three-year survey of outcroppings of the Hell Creek Formation shows fossils throughout the deposit and that dinosaurs lived there in vigorous numbers and varieties until the very end.

"We looked at the community of dinosaurs in the Hell Creek formation and found they were not changing," Sheehan said. "If they were going through a gradual extinction, we would have expected to see some change. We found no evidence of a decline.'" Sheehan said that through the whole 180-foot depth of the Hell Creek formation, the species mix and numbers of dinosaurs were the same, with Tyrannosaurus as the most common carnivore and the Triceratops the most common plant eater. This was true, he said, right up to the 2 centimeter layer that marks the impact.

This layer, found virtually everywhere on Earth, is rich in iridium, a rare element brought to Earth by the asteroid. The iridium layer sits atop the Hell Creek formation. "The abundance of dinosaur fossils in the upper three meters of sediment immediately underlying the impact layer is well within the range of many intervals lower in the Hell Creek formation," the study says. After the impact layer, there are no dinosaur fossils.

To gather the data, scores of volunteers spent three summers combing more than 11 million square meters of North Dakota and Montana, walking shoulder-to-shoulder in a search for dinosaur fossils. They found the bones of almost a thousand dinosaurs sprinkled throughout the exposed levels of the Hell Creek formation. Clemens said that the weakness of the Sheehan study is that it fails to go back far enough in history. He said that deposits five million and six million years old contain a much richer variety and number of dinosaur fossils, suggesting the animals were declining when the Hell Creek formation was deposited. Clemens also said the Sheehan study does not consider the effect an asteroid extinction would have on other species. ``You need to consider the whole fauna,'' Clemens said. "Why did amphibians go through this period unaffected? There was a diversity of birds and they go through this period unaffected."