CCNet 14/2003 - 10 February 2003
THE GREAT IMPACT DEBATE
>From Astrobiology Magazin, 10 February 2003
Summary: This second in our "Great Debate" series brings together a group of
scientists who are experts on asteroids and comets. Over the course of this
debate, the participants will discuss the past, present, and future effects
of asteroid and comet impacts. Today's session concerns the scientific value
in studying asteroids and comets, and also whether past impacts have caused
some of the mass extinctions of life on Earth.
The Great Impact Debate
Part I: The Benefits of Hard Bodies
The participants are:
Clark Chapman - scientist at the Southwest Research Institute's Department
of Space Studies, in Boulder, Colorado. Member of the MSI/NIS
(imaging/spectrometer) team of the Near Earth Asteroid Rendezvous (NEAR)
mission to Eros.
Alan Harris - senior research scientist at the Space Science Institute, an
affiliate of the University of Colorado at Boulder.
Benny Peiser -social anthropologist at Liverpool John Moores University in
the UK. He has written extensively about the influence of NEO impacts on
human and societal evolution.
Joe Veverka - professor of astronomy at Cornell University in Ithaca, New
York. Principal Investigator for NASA's Comet Nucleus Tour (Contour)
Peter Ward - professor of geology and paleontology at the University of
Washington in Seattle.
Don Yeomans - (debate moderator) - Senior Research Scientist at NASA's Jet
Propulsion Laboratory in Pasadena, California, and manager of NASA's
Near-Earth Object Program Office.
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 with solid central cores
(nuclei) 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 of 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?
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
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
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
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
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
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.
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
Next Week's Installment (Monday, February 17): Great Impact Debate 2 - Much
Ado About Nothing? addresses public perception of the risks.
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