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The participants are:
Don Yeomans: Comets and asteroids are the most numerous bodies within our solar system. They are thought to represent the leftover bits and pieces from the planetary formation process that began some 4.6 billion years ago.
Most asteroids formed in the inner solar system, and the inner rocky planets (Mercury, Venus, Earth, Mars) formed as collections of asteroid-like material. Asteroids today range from a few meters to several hundred kilometers in size. The structures of asteroids can range from very weak rubble piles of rock to chunks of solid iron. Because asteroids formed closer to the early sun, they are largely devoid of the ices that are characteristic of comets.
Comets formed in the region where the large planets - Jupiter, Saturn, Uranus and Neptune - now reside, so these planets are thought to have formed from comet-like material. Comets are icy dirt balls ranging in size from less than a kilometer to several tens of kilometers. The tails of comets form when they approach the sun and some of their ices vaporize, releasing glowing gases and dust particles that reflect sunlight. Many comets are fragile bodies that can split into pieces for no obvious reason. After many passages by the sun, a comet exhausts most of its ices and either disintegrates into a cloud of dust particles or turns into an extinct comet that looks like an inactive asteroid.
On a daily basis, an average of about 400 tons of cometary and asteroidal material rains down upon the Earth, mostly in the form of particles far too small to be noticed. Whenever you see a shooting star, you are likely witnessing a particle about the size of a grain of sand colliding with the Earth's upper atmosphere and causing the air molecules to glow as the particle burns up.
Near-Earth objects (NEOs) are defined as those comets and asteroids that, in their orbits about the sun, approach within 1.3 times the Earth's distance from the sun. Some of these objects can, and do, closely approach the Earth. Because NEOs can approach and even hit the Earth from time to time, there have been press reports over the last few years about a number of objects that have a remote chance of threatening Earth.
Before we get into whether or not these objects do represent a significant threat to Earth, let's discuss some positive aspects of these objects, such as why they are scientifically important. Also, what role (if any) did they play in the formation and evolution of life on Earth?
|Artist's depiction of the Chicxulub impact crater. About 2,225 near-Earth objects (NEOs) have been detected, primarily by ground-based optical searches, in the size range between 10 meters and 30 kilometers, out of a total estimated population of about one million; some information about the physical size and composition of these NEOs is available for only 300 objects. The total number of objects a kilometer in diameter or larger, a size that could cause global catastrophe upon Earth impact, is now estimated to range between 900 and 1,230.
Clark Chapman: In many ways, if asteroids and comets did not exist, we would not exist. It seems clear that the Earth and other planets were created, in whole or in part, by the gravitational gathering together ("accretion") of countless asteroids and comets. The later ones to fall in may have been particularly responsible for the life-giving waters in Earth's oceans and atmosphere.
A benefit of early giant impacts is that they have helped shape the geology of our world, which modern civilization is now exploiting. There is a decided correlation between giant impact craters and important raw materials like oil. Some articles in petroleum-science journals claim that impacts, like the Chicxulub impact 65 million years ago, break up subterranean rocks and provide regions where oil is likely to collect. The Chicxulub impact is why Mexico is in the second-tier of oil-producing nations. I also don't think it is a coincidence that one of the world's largest supplies of nickel comes from the giant Sudbury impact crater in Canada.
Comets and asteroids will have a vital role to play in the near future, as mankind eventually ventures into space. Because they are so numerous and have so little gravity, they provide the most accessible sources of raw materials for use in interplanetary space endeavors - for shielding astronauts from cosmic radiation, for fuel, and even for sustenance of interplanetary travelers. Because of asteroids and comets, we won't have to haul these necessary resources up from the surface of the Earth.
Joe Veverka: There is a real potential that we will be able to exploit some NEOs for space resources, probably not immediately but very likely in a few decades from now. Compared to many solar system objects, NEOs are relatively accessible.
NEOs also offer one of the best means of answering some fundamental questions about the evolution of our solar system in general, and of our Earth in particular. There is a strong likelihood that among the numerous NEOs are pieces and fragments of relatively rare and distant bodies, and that these fragments contain important clues about the formation and evolution of the solar system and its planets. Given an increasingly complete inventory and a gradually improving ability to characterize NEOs, we soon will be able to identify such key objects among the vast population and mount efforts to explore and sample them.
Clark Chapman: As Joe Veverka said, the small bodies that remain in the solar system provide precious information about how our planetary system formed. That's because the planets have all, to varying degrees, been subjected to heating and melting, chemical reactions, geophysical forces, and other processes that have virtually erased all evidence of how the planets formed and the first several hundred million years of planetary evolution. Even relatively recent epochs on bodies like Venus, Io, and Europa have been erased by rampant geological forces. Most comets and asteroids, by contrast, are relatively pristine bodies, little changed over the aeons. They are like time-travelers from the distant past for scientists to examine.
|According to the hypothesis of David Raup of the University of Chicago, mass extinctions may well have been due to comet and asteroid impacts.
Image Credit: NASA
Benny Peiser: I find it very difficult to see any positive traits in comets or asteroids. From a psychological perspective, it is understandable that we try to put a positive spin on the ultimate threat NEOs pose to human survival. In the 17th century, Isaac Newton was the first to suggest that cometary impacts were essential for the preservation of the world since they "refurbished" and "replenished" the planets, the sun, and the stars. While the general public at the time regarded comets as harbingers of doom and disaster, Newton claimed they were "absolutely necessary for the watering of the Earth, and the production and nourishment of vegetables." So much for wishful thinking.
As we have discovered during the last 50 years, the sad truth is that asteroids and comets have been the foremost agents of environmental annihilation and the key obstacles to the evolution of life. Life has taken root on Earth not because of cometary deliveries of organic material and volatiles but in spite of extensive NEO bombardment. We only need to look at the other pockmarked planets of our solar system to recognize that impacts essentially extinguish the chances for the evolution of life. Complex forms of life have survived on Earth because we seem to be uniquely situated in a habitable niche that comprises relatively decent protection from colossal, life-exterminating impacts. We should bear in mind that 99.9% of all species that ever dwelled on Earth were wiped out, most likely, as a result of large impacts.
As long as we remain incompetent to take full control over these destructive forces, any over-optimistic undertone regarding asteroids and comets seems untimely to me. We need to get our priorities right first. The anticipated opportunities for science to take advantage of and exploit NEOs for space exploration remains an ambition for the distant future. In order to bring this long-term goal to fruition, we need to learn how to reshuffle the cosmic game of dice to our advantage.
Clark Chapman: Here I differ with Benny Peiser. Even the catastrophic influence of asteroids has been mainly beneficial to mankind. We mammals have definitely benefited from the evolutionary competition unleashed 65 million years ago when the Chicxulub impact caused the Cretaceous-Tertiary (K/T) mass extinction of dinosaurs and other dominant species. My understanding of the post-Cambrian (last ~600 million years) evolution of life on this planet is that evolution has been profoundly influenced by major epochs of sudden upheaval due to mass extinctions.
According to the hypothesis of David Raup of the University of Chicago, these mass extinctions may well have been due to comet and asteroid impacts. All signs point to an impact as the cause of the Permian extinction (the greatest-ever mass-extinction), as well as lesser ones like the K/T extinction. While such events have wiped out many species of life, they have provided the environmental niches for evolutionary change. As the late Stephen Jay Gould argued, evolution favors the more randomly selected species that are able to adapt to unexpected sudden changes rather than those that slowly evolve in competition with their competitors in a nearly constant world. I would expect Benny Peiser to applaud this attribute of impacts. In any case, I agree that we should avoid becoming the next dinosaurs. But I hope and expect that no major impacts will happen during the present century.
Alan Harris: First of all, in contradiction to Benny Peiser's remarks, Peter Ward has presented data showing that while it is true that the majority of species that have ever existed are now extinct, only a minority of those, a few percent in fact, were victims of mass extinctions. Instead, most extinct species have come to an end at some random time between mass extinctions. The point is, extinctions happen all the time. A mass extinction may produce a very high spike in the momentary rate of extinctions, but averaged over a very long time constitute only a minor fraction of the total extinctions.
The second contradictory point Ward has made, this time to one of Chapman's comments, is that the evidence that the Permian extinction is due to an impact is close to nil. To be sure, this is still an unsettled question, and by no means has anyone offered a definitive alternative to impacts, but the evidence supporting an impact cause is very weak.
|Walter Alvarez (shown above) believes that there is little or no evidence that the Permian extinction is due to an impact.
Image Credit: Steve Dutch
I attended a meeting in which Walter Alvarez, one of the originators of the K/T impact scenario, gave essentially the same evaluation of the evidence (or lack thereof) regarding the Permian extinction. He reflected on his battle to gain acceptance on the impact cause of the K/T extinction, and was almost apologetic in observing that everyone now assumes other extinctions must be caused by impacts. In fact, the evidence is really persuasive for only one extinction - the K/T one that ended the reign of the dinosaurs. Combining these two points - that most species meet extinction not as a part of a mass extinction, and that only one out of four major (and none of many minor) extinctions can be definitely attributed to an impact - it appears that death by asteroid is not the most likely end in store for our species. We should not be obsessed with it to the exclusion of other environmental and societal concerns.
Peter Ward: The 1980 discovery that a mass extinction had been caused by an asteroid impact was revolutionary. Questions then arose regarding the frequency of asteroid and comet impacts on Earth. By examining the size and frequency of meteor impact craters, Gene Shoemaker and others calculated that we might expect a K/T-sized impact every 100 million years. This frequency roughly fits the facts on Earth: there have been five major extinctions in the past 500 million years. But the K/T extinction is the only one undoubtedly caused by an impact. And a salient fact remains - the K/T asteroid came nowhere near wiping out all animals and plant species. We took this hit, reeled a bit, and got back to business relatively quickly.
In the 500 million years of animal life on the planet, the most consequential extinction of all was at the end of the Permian. This event resulted in an enormous amount of extinction, but came nowhere near to eliminating animal and planet life. For all of its wreckage, within ten million years the world had caught up to its prior biodiversity, and then surpassed it. But there is no credible evidence that this extinction was caused by an impact.
Over the past decade a new truth seems evident: instead of finding ever more mass extinctions of greater severity, we are finding fewer. The so called "Big Five" - the mass extinctions of the Ordovician, Devonian, Permian, Triassic, and Cretaceous - have been whittled down to a "Big Three." New work by Mike Foote and his students at the University of Chicago suggests that the first two of these events, while undoubtedly major crises of some sort, were not in the class of the later Mesozoic events. My own new work on another of the so-called "Big Five," the extinction at the end of the Triassic, also suggests that it was never a threat to ending animal life on the planet. The K/T event seems unique. None of the other major extinctions appear to have been wholly or even partially caused by impacts.
Clark Chapman: If, by his last statement, Peter means that there is no undisputed proof that the end-Permian extinction was caused by an impact, then that is true. But surely the correct approach is to assume that it was caused by impact, unless proven otherwise. The debates on the Alvarez hypothesis about the K/T must have taught us that the *fact* of asteroids and comets is at least as weighty as interpretations of facts in the stratigraphic record...and that impacts should no longer be relegated to being the explanation of last resort.
It is unlikely that there will be as conclusive a 'smoking gun' as the Chicxulub crater provides for the K/T extinction. Most of the Earth's surface has been recycled by plate tectonics since then, so any resulting crater has likely been destroyed. But huge asteroid impacts *must* occur on 100-million-year time scales. They *must* do unimaginably enormous damage to the ecosystem, and there is simply no other plausible killing mechanism to explain the Permian mass extinction. No other disruption of our planet can approach the sudden global devastation guaranteed by such an impact. Each year, research shows that the great extinctions happened more and more instantaneously -- a prime attribute of an impact disaster. Indeed, it is the extraordinary immediacy of the global environmental holocaust that makes impacts such an effective mass-killer. Living things simply have no time and nowhere to run to escape death.
Peter Ward: Two years ago the work by Luann Becker and others seemed to show that the end-Permian event was impact-caused, as evidenced by their finding of fullerenes ("bucky balls" with trapped Helium 3). However no lab has been able to replicate these results, and there is no other evidence for an impact. Furthermore, new work by Greg Retallack in Antarctica, Roger Buick in Australia, and my own work in South Africa using stable isotopes shows that the Permian extinction may have had multiple causes. The K/T extinction, on the other hand, is presumed to be caused by a single event -- the signature of an impact-induced mass extinction.
There is better evidence for an impact as the cause of the Triassic event, but new evidence suggests it played a minor role in this extinction. The finding of Paul Olsen and his colleagues of an iridium layer at the Triassic/Jurassic (T/J) boundary is evidence for an impact. But most sections (unlike the K/T event) show no evidence at all of large body impact - for example, iridium, shocked quartz, sperules, or a single carbon isotope anomaly. The only large crater of about T/J age, the 100-km diameter Manicouagan crater in Quebec, is slightly older than the extinction and thus predates it. My own work, and that of my team in western Canada, can find no evidence of an impact at the T/J boundary in spite of intensive searching by experts.
Next Week's Installment (Monday, February 17): Great Impact Debate 2 - Much Ado About Nothing? addresses public perception of the risks
Related Web Pages
Impact Hazards Website
NASA/JPL Near Earth Object Program