Assembled by David Morrison

Kevin Pope's 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 Toon's 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, Pope's 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. Pope's 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 Pope's 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 Pope's 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 doesn't feel that Pope's result is demonstrably wrong, he suggests that several of Pope's 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 Pope's discussion of the current impact hazard, and particularly the threshold for global environmental effects. He writes:
"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:
"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)

Dan

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