In a scientific paper published this week in the journal Geology, Kevin Pope criticizes the previous scientific consensus that small dust particles in the stratosphere produced a prolonged period of darkness following the KT impact 65 million years ago, and that similar but much smaller amounts of dust represent the primary environmental threat from more frequent impacts such as those by kilometer-size NEAs. Pope is a geologist whose past work includes research on the identification of the Chicxulub Crater in Mexico as the smoking gun of the KT mass extinction. His paper is entitled Impact dust did not cause the Cretaceous-Tertiary mass extinction. Both the paper and the accompanying news release specifically challenge the original Alvarez hypothesis that stratospheric dust played a major role in the KT mass extinction. He also questions the subsequent atmospheric modeling of dust injection and its persistence as presented in several papers by Brian Toon and others, as well as the estimates of the contemporary hazard of kilometer-size NEAs by Chapman and Morrison, which were based in part on the Toon models. Kevin Pope did not measure the dust particles in the KT boundary layer. In fact, no one has detected or measured this dust. All estimates, including that of Pope, are based on theoretical modeling and extrapolation from the larger particles measured in the KT boundary layer. The dispute is between different models and is strongly related to the data (from smaller impacts or volcanic eruptions) that form the basis of the extrapolation to the environmental conditions following a large impact. Below are four items that relate to this issue. (1) The published abstract of Popes paper. (2) The press release issued by Pope and the Geological Society of America. (2) A summary article by Rob Britt of Space.com that also includes some first responses from other scientists. (4) Additional discussion of some of the points at issue, with comments from other scientists. I note that these responses are somewhat disjoint. None of the groups whose work is questioned by Pope (including Chapman and Morrison) had received preprints of the paper or knew that a press release was about to be issued. Also, this work had not been presented and debated at scientific meetings in asteroid impacts. Most comments are therefore based on the press release itself or on a very quick read of the paper, once copies began to circulate yesterday afternoon. This is not an ideal way to conduct a scientific dialog. Nevertheless, it seems appropriate to note some initial discussions since the issues that Pope addresses are basic to our understanding of the environmental consequences of impacts, ranging from the KT extinction-level event to the smaller events that constitute the contemporary impact hazard. David Morrison
Abstract from Geology: Vol. 30, No. 2, pp. 99-102. IMPACT DUST NOT THE CAUSE OF THE CRETACEOUS-TERTIARY MASS EXTINCTION Kevin O. Pope, Geo Eco Arc Research, 16305 St. Mary's Church Road, Aquasco MD 20608 ABSTRACT Most of the 3-mm-thick globally distributed Chicxulub ejecta layer found at the Cretaceous-Tertiary (K-T) boundary was deposited as condensation droplets from the impact vapor plume. A small fraction of this layer (<1%) is clastic debris. Theoretical calculations, coupled with observations of the coarse dust fraction, indicate that very little (<1014 g) was submicrometer-size dust. The global mass and grain-size distribution of the clastic debris indicate that stratospheric winds spread the debris from North America, over the Pacific Ocean, to Europe, and little debris reached high southern latitudes. These findings indicate that the original K-T impact extinction hypothesis-the shutdown of photosynthesis by submicrometer-size dust-is not valid, because it requires more than two orders of magnitude more fine dust than is estimated here. Furthermore, estimates of future impact hazards, which rely upon inaccurate impact-dust loadings, are greatly overstated. © Copyright by Geological Society of America 2002
THE K-T IMPACT EXTINCTIONS: DUST DIDN'T DO IT GSA Release No. 02-04, January 23, 2002 Geological Society of America, Boulder, Colorado Contact information: Kevin O. Pope Geo Eco Arc Research Phone: 301-888-1048 E-mail: kpope@starband.net Ann Cairns Director - Communications and Marketing acairns@geosociety.org, 303-357-1056 By Kara LeBeau, GSA Staff Writer Scientists basically agree that an asteroid struck the Earth some 65 million years ago and its impact created the Chicxulub crater in Yucatan, Mexico. More controversial is the link between this impact and a major mass extinction of species that happened at the geological (K-T) boundary marked by the impact. But what mechanism did the impact trigger to cause mass extinction? The conventional theory is that impact dust obscured the sun, shutting down photosynthesis and snuffing out life. Kevin Pope from Geo Eco Arc Research shows in the February issue of GEOLOGY that the assumptions behind this theory are amiss, and therefore damage estimates from future asteroid impacts are also amiss. This latter point became a recent issue when a large asteroid passed near the Earth on January 7 and news reports exaggerated its potential impact effects. "Based on the old, inaccurate dust numbers, which erroneously suggested that a medium-sized asteroid (1-2 km in diameter) could cause global climate change and famine, scientists calculated that one's chance of getting killed by an asteroid impact were about the same as dying in a plane crash," Pope said. "My new impact dust estimates indicate that death by an asteroid is far less likely and that such medium-sized asteroid impacts would not have catastrophic global effects. But of course the regional effects would still be devastating." To truly understand the influence of impact dust, scientists need to find a way to directly measure the amount of small dust particles in such places as the K-T boundary. In the meantime, Pope studied patterns of coarse dust particles to create a model that showed how the small dust particles were dispersed. Incorporating these geological observations with new theoretical work, Pope asserts that very few of the particles are of the size that it would take to shut down photosynthesis for any significant length of time and therefore the original K-T impact extinction hypothesis is not valid. He believes it may have been sulfate aerosols produced from impacted rocks and soot from global fires that could have shut down photosynthesis and caused global cooling. "The original studies of the clay layer found at the K-T boundary assumed much or all of this layer was derived from fine impact dust," he said. "More recent studies of this layer have shown this not to be the case. Furthermore, earlier estimates were based on extrapolations of data from surface atomic bomb blasts, which had about 100 million times less energy than the Chicxulub impact. Extrapolation over eight orders of magnitude is risky business." Pope was involved in the "discovery" of the Chicxulub crater in 1989-1990 when he worked at the NASA Ames Research Center. (Oil geologists had discovered the crater and reported the finding in 1981, but it was basically ignored.) He was using satellite images to map water resources in the Yucatan with Adriana Ocampo and Charles Duller when they found the semi-circular ring of sink holes. They thought the crater might be the K-T impact site and published their hypothesis in the May 1991 issue of NATURE.
AIRPLANES MORE DANGEROUS THAN ASTEROIDS, NEW STUDY CLAIMS From SPACE.com (by Rob Britt) A new study claims the potential climate effects of an asteroid impact are overstated and that a person's chances of death by asteroid -- commonly said to be equal to dying in a plane crash -- are therefore lower. The study directly challenges the prevailing view among leading asteroid experts, and it has generated much interest and fresh debate. Some experts said they simply did not yet know what to make of the new research. The study does not change a basic equation: If an asteroid lands on your head, you are dead. So might be thousands or millions of your neighbors if the space rock is large enough. What the new research does question is the widely held belief that the impact of a large asteroid would have global repercussions, threatening civilization as we know it. The study was done by former NASA scientist Kevin Pope, now of Geo Eco Arc Research, and is detailed in the February issue of Geology. In a telephone interview, Pope said the chances of any U.S. resident suffering death by asteroid are about 1-in-100,000 instead of 1-in-20,000, odds which are frequently cited by asteroid experts and equal to the risk of dying in an airplane crash. The odds of being killed in a car in the United States are about 1-in-100. "It's still a very remote thing," Pope said of the asteroid threat, "but it can happen." Cosmic winter Pope's work is based on new mathematical models and a review of scientific literature regarding a thin, global layer of dust determined to be 65 million years old. The dust was generated when a roughly 6-mile-wide (10-km) asteroid slammed into Mexico's Yucatan Peninsula, where a crater was found more than a decade ago. The regional effects would have been devastating -- no one argues that point. But most researchers have assumed that the dust blotted out the Sun for months on end, creating a "cosmic winter" that prevented photosynthesis in plants -- the basic chemical transaction that supports most life on Earth. Widespread extinctions resulted, including that of most dinosaurs. Not everyone subscribes to the dust hypothesis, which is about 20 years old. I have been skeptical of the 'darkness at noon' scenario for years," says Jay Melosh, a University of Arizona researcher who models asteroid impacts. Melosh did not comment on Pope's study, but he said some researchers, himself included, have shown that impacts might not generate enough fine dust to have blocked the Sun enough to cut off photosynthesis. Only tiny dust, which remains aloft longer than larger dust grains, could have such an effect. That's exactly what Pope's study concludes. "The [previous] studies were in error," Pope says of work that supports the cosmic winter hypothesis. "They really overestimated the effects of dust." Threat to humans Pope speculates on how his results might apply to impacts of smaller rocks, which pose a more real threat because they are more likely. While no immediate threat to Earth exists based on the present census of asteroids, rocks are out there that will eventually hit the planet, Pope and other researchers agree. Asteroids larger than a half-mile (1 kilometer) are thought to hit Earth once every 100,000 to 300,000 years. Most asteroid experts -- including top NASA researchers -- have said that such an impact could cause global devastation and might threaten human civilization, in part as a result of climate effects on crop production. "My new impact dust estimates indicate that death by an asteroid is far less likely and that such medium-sized asteroid impacts would not have catastrophic global effects," Pope said. "This seems extremely over-optimistic and goes against a widely held scientific consensus," said Benny Peiser, an expert on asteroid risk at Liverpool John Moores University in the UK. "Crucially, [Pope] ignores the social and economic knock-on effects of such a global disaster. While we as a species would not become extinct as a result of such an impact, it is almost certain that the world as a whole would suffer to the extent of civilization collapse and Dark Age conditions." Peiser said, however, that the new study is likely to stimulate vigorous debate and trigger new research designed to further pin down the risks. Kevin Pope doesn't entirely disagree with Peiser's assessment. "There's a whole other set of processes that could be set in motion" by an asteroid impact, he said, referring to potential economic consequences, especially if multiple large cities were wiped out. But as for a dust-induced cosmic winter, Pope said previous assumptions are flawed. The data supporting such a worldwide catastrophe is only partly based on dust from impacts, because such dust is hard to find. Researchers have therefore used what's known about atomic bomb blasts to help support the claim. But it would take many atomic bombs to equal the effects of one large asteroid. So researchers have to dramatically scale up the bomb data, and Pope said this scaling creates errors. Pope did not do any new field research. But his review of recent studies of real impact dust, which he says were more revealing than previous studies, show that much of the dust in the layer is larger than had been thought -- too large to stay aloft long enough to generate the hypothesized climate catastrophe. Concerns Pope's case is not airtight. And several researchers told SPACE.com they are concerned the study may be flawed. Pope examined patterns of coarse dust bits to create a model that showed how the small dust particles were dispersed. To truly understand the influence of impact dust, he says, scientists need to find a way to directly measure the amount of small dust particles. The work was made available to reporters in a press release put out Jan. 23 by the Geological Society of America. But scientists said that from what they could see, the study did not appear promising, yet they needed more time to fully digest Pope's full scientific paper, which they had not seen. These researchers were dismayed that such a strong challenge to a well-established idea was made available via a press release before they had had time to read the actual scientific paper and thus be prepared to comment on its validity. "If you wanted to make a press release of this nature, you really ought to notify some colleagues," said Alan Harris, an asteroid researcher at NASA's Jet Propulsion Laboratory. After having discussed the press release and an abstract of Pope's work with several colleagues, Harris said the jury was still out on its merits. Brian Toon, a University of Colorado atmospheric scientist, helped work out some of the widely accepted notions that Pope's work refutes. Toon was also part of a group that peer-reviewed, on behalf of Geology, an early version of Pope's paper. Toon recommended rejecting the paper for publication because, he said, small dust particles "have no simple relation to large ones." He said it was akin to saying "I saw a panther today, lions must be going extinct." Toon said Geology did not let him see the final paper or Pope's responses to his criticisms. Bottom line Amid all this uncertainty is one solid fact: The extinctions that occurred 65 million years ago are well documented by paleontologists. The fossils exist in the same layer of soil where the dust is found, the so-called K-T boundary that marks a division between Cretaceous and Tertiary periods of geologic history. Noted University of Chicago paleontologist David Raup has been embroiled in these issues for more than two decades and now views it all from the comfortable distance of retirement from active research. "The strong implication [of Pope's study] is that the impact explanation of the K-T extinction will fall if the dust cloud hypothesis falls," Raup told SPACE.com. Raup said there is a tendency to look at the dust cloud hypothesis as a critical element in those extinctions. But he points out that many other scenarios have been suggested. These range from the possibility of a global firestorm fueled by an asteroid impact to climate change or acid rain unrelated to any objects coming from space. "Given the paucity of hard data, who knows which one to choose?" Raup wonders. Indeed, if Pope's work is to be verified or refuted, there is likely some more field work to do.
SCIENTISTS COMMENT ON POPES PAPER assembled by David Morrison Kevin Popes paper Impact Dust Not the Cause of the Cretaceous-Tertiary Mass Extinction deals primarily with the quantity of fine dust that would be injected into the stratosphere from impacts. Although it represents only a tiny fraction of the total ejected mass, this stratospheric dust plays a key role in the environmental aftermath of an impact. Because it has a long lifetime, it can create long-term darkening, lasting months or even years. Pope challenges the fundamental hypothesis of Alvarez and colleagues in 1980 (see reference list below) that ejected dust blocked sunlight after the KT impact and played a key role in the KT mass extinction. He also questions various subsequent atmospheric models for the injection and distribution of this dust in impacts that range from the KT (hundred million megatons of energy) down to the kilometer-scale impacts that contribute most to the current impact hazard. This challenge is primarily to the work of Brian Toon of the University of Colorado and his colleagues. The results from Toons work are summarized in three major papers listed in the references below: Covey and others, Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth (1994); Toon and others, Environmental perturbations caused by impacts (1994); and Toon and others, Environmental perturbations caused by the impacts of asteroids and comets (1997). Since the estimates of the current impact hazard are based in significant part on the environmental effects of stratospheric dust as derived by Toon and his colleagues, Popes result also challenges the premise of the NASA Spaceguard report in 1992 and the UK NEO Task Force report in 2000. Papers that summarize this hazard estimate include Chapman and Morrison, Impacts on the Earth by asteroids and comets: Assessing the hazard (1994); Morrison, Chapman, and Slovic: The impact hazard (1994); and Morrison and others, Dealing with the impact hazard (2002). In spite of its importance to the post-impact environment, no one has succeeded in measuring the small (micrometer and submicrometer) dust in the boundary layer that marks the KT impact event. Presumably this dust would be deposited on the top of the large ejecta, since it was the last component to fall out of the atmosphere. However, even the models of Toon and his colleagues suggest that this layer would be less than a millimeter in thickness. Geologists have identified other components in the boundary layer, including shocked rock, soot from the global firestorm that followed the impact, and of course the famous iridium and other rare elements that are the signature of extraterrestrial material. However, in view of the small size of these stratospheric particles and the thinness of this layer, it is not surprising that it has not been detected. Popes estimate is more than a factor of a thousand less than that of Toon and colleagues, far below the threshold for direct detection. This difference between Popes result and that of previous workers is thus a matter of theory, without the comfort of an anchor to direct measurements. It depends on the models used to extrapolate from the observed large particles in the KT boundary layer (which did not remain long in the atmosphere) down to the fine particles that are hypothesized to have made a major contribution to the environmental shock of the KT impact. Additional questions concerning the threshold for global environmental damage from smaller impacts are related but different. Many mechanisms, such as a global firestorm, that are important for the KT extinction do not play a major role for smaller impacts, such as those from kilometer-scale asteroids. Brian Toon has noted that he reviewed Popes paper and recommend that it be rejected. He felt that Pope made an inappropriate extrapolation from data on very large particles to his conclusions about small ones. Kevin Zahnle of NASA Ames Research Center, who has worked with Toon on these questions, has looked for the key differences in the way Pope approached the problem. Zahnle notes that absence of evidence is not evidence of absence . That is, the fact that we do not see fine dust in the KT boundary layer is not an indication that it is not there in small quantities, which is all that are expected in any case. The KT boundary clay is made up mostly of large (100 micrometer or so) spherules (presumably condensed from rock vapor). Zahnle also writes that the fine dust is but one of three important opacity sources for the post-impact stratosphere. The others we are aware of are smoke from wildfires and sulfates from the sulfur content of the impactor. In our calculations (Toon et al. 1997), for asteroid impacts the submicrometer dust is about as important as the other two effects added together. Subtracting the dust would only make a factor two change in our estimates (equivalent to a 30% change in the diameter of the threshold asteroid) . How did Pope arrive at his low estimate for the stratospheric dust? Zahnle notes that Pope got the mass of fine dust by estimating the total mass of clastics and assuming that the size distribution appropriate to volcanic ash applies to impact ejecta. Zahnle questions this volcano analogy, for a variety of reasons. He also feels that Pope uses a rather low estimate of the amount of clastic material, which then becomes the basis for his extrapolation to smaller sizes. Thus while he certainly doesnt feel that Popes result is demonstrably wrong, he suggests that several of Popes assumptions need a critical review. In the case of the KT impact, there are many causes of the environmental disaster, not just one. While it has been assumed that the fine dust dominates at least for the collapse of the marine ecosystem (where photosynthesis ceases in the months of darkness), it is not the only disaster by any means. The firestorm that swept across the land was presumably the main killing agent for the dinosaurs and other terrestrial creatures (see, for example, Melosh and others, Ignition of global wildfires at the Cretaceous/Tertiary boundary, 1990), and soot from those fires could have contributed to the global darkness even if there were less stratospheric dust. But the work of Toon and colleagues has shown that these global wildfires are not important for impacts with energy smaller than 10 million megatons (about 5 km diameter asteroids). For the smaller impacts there may not be so many other killing agents, and thus the dust issue may be even more important. Clark Chapman of Southwest Research Institute addresses Popes discussion of the current impact hazard, and particularly the threshold for global environmental effects. He writes that while it is true that there is much more than dust, most analyses (including Toon et al.'s 1997 Reviews of Geophysics review) on which Morrison and I relied, had dust as the global environmental consequence that sets in "first" (i.e. for the smallest impactor). While many other phenomena (Pope himself refers to sulfate aerosols and global firestorms) were pertinent to the K/T boundary, I suspect that the threshold for a modern-day catastrophe does involve the collapse of global agriculture and hence is dependent on the threshold for the global distribution of dust....Britt quotes Pope as lowering the chances of a civilization-destroying impact by a factor of 5 (although I don't see that buttressed in the article itself). A point to be made is that the uncertainties are large. Our original paper (Chapman and Morrison 1994) tabulates not only the 1-in-20,000 chances of dying but also 1-in-3000 to 1-in-250,000 range (the latter limit even beyond Pope's new number). We have always said, in our viewgraphs for instance, that the impact frequency was the most solid information we had, but that there are larger uncertainties in what the environmental effects might be (e.g. the dust cloud), and still larger ones about the effects on civilization and hence the chances of death. Peter Ward, geologist at the University of Washington, adds that I would suggest that the disruption of global human agriculture becomes a major issue in all of this. While one may not be able to invoke widespread species extinction from smaller impacts, the fragility of crop yields in the face of volcanic events of far less energy and consequence than even a small body hit should be warning enough. We are headed toward a global population of 9 to 12 billion in the next century. Heroic efforts in agriculture will be required to sustain that number. Any disruption would be disastrous. Benny Peiser of Liverpool John Moores University makes a similar point from the perspective of a social anthropologist: "Crucially, Pope ignores the social and economic knock-on effects of such a global disaster. While we as a species would not become extinct as a result of such an impact, it is almost certain that the world as a whole would suffer to the extent of civilization collapse and Dark Age conditions. Alan Harris of JPL summarizes the situation as follows: Pope did not prove anything in his paper; he presented a new estimate, different from previous ones, but he did not find a fatal mistake in earlier estimates, only differences of opinion. He did not find something that would leave previous researchers saying, how silly of us to overlook that, of course he's right. Lacking such certainty, he is simply opening a debate. Kevin Pope has the last word today, in a message to Clark Chapman: What is needed now are more in-depth studies of the impact dust issue, and other affects from modest-sized asteroids. It may turn out that I am wrong about the dust, perhaps there is a flaw in my analysis, or maybe some new data will come to light that will change the conclusions. I had a frustrating time with this paper because none of the [peer] reviews provided any detailed criticism, only arm waving saying this can't be right. In this vein I would appreciate a critical reading of the paper from you or your colleagues. References Alvarez, L, W. Alvarez, F. Asaro, & H.V. Michel, Extraterrestrial cause for the Cretaceous-Tertiary extinction, Science 208:1095-1108 (1980) Chapman, C.R. and D. Morrison: Impacts on the Earth by asteroids and comets: Assessing the hazard. Nature 367:33-39 (1994) Covey, C. et al.: Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth. Global and Planetary Change 9: 263-273 (1994) Melosh, H.J., N.M.Schneider, K. Zahnle, and D, Latham, Ignition of global wildfires at the Cretaceous/Tertiary boundary, Nature 343:251-254 (1990) Morrison, D., C.R. Chapman, and P. Slovoc: The impact hazard. In Hazards Due to Comets and Asteroids (T. Gehrels, editor), University of Arizona Press, pp 59-92 (1994) Morrison, D., A.W. Harris, G. Sommer, C.R. Chapman, A. Carusi, Dealing with the impact hazard. In W. Bottke and others, editors, Asteroids III, Univ. of Arizona Press, Tucson. (2002) Toon, O.B., K. Zahnle, R.P. Turco, and C. Covey: Environmental perturbations caused by impacts. In Hazards Due to Comets and Asteroids (T. Gehrels, editor), University of Arizona Press, pp 791-826 (1994) Toon, O.B., K. Zahnle, D. Morrison, R.P. Turco, and C. Covey: Environmental pertubations caused by the impacts of asteroids and comets. Reviews of Geophysics 35: 41-78 (1997) |
