CCNet 16/2002 - 26 January 2002

"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."
--David Morrison

"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."
--Kevin Pope

    Kevin O. Pope <>

    David Morrison <>

    Gerta Keller <gkeller@Princeton.EDU>

    Drake A. Mitchell <>


>From Kevin O. Pope <>

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, Maryland 20608, USA

Geology; February 2002; v. 30; no. 2; p. 99-102


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
(<10^14g) 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.


Two decades of research have clearly linked the Cretaceous-Tertiary (K-T) mass
extinction to the catastrophic meteorite impact that formed the Chicxulub crater
in Yucatan, Mexico. Nevertheless, causal factors in this link remain uncertain,
and research continues on the mechanisms by which large impacts disrupt the biosphere.
This paper examines the evidence for the impact extinction mechanism originally
proposed by Alvarez et al. (1980): photosynthesis shutdown by a global cloud of fine
dust. Although several other impact extinction mechanisms have been proposed for
the K-T boundary, the dust hypothesis is perhaps the most widely recognized.
Furthermore, impact dust is one of the key environmental perturbations used to
estimate future hazards from more modest-sized impacts (Chapman and Morrison, 1994).


The original K-T impact extinction hypothesis of Alvarez et al. (1980)
stated that there was a collapse of the global food chain due to the
shutdown of photosynthesis by sun-blocking silicate dust injected into
the stratosphere. The dust-loading threshold for photosynthesis is ~10^16g
of submicrometer-size dust (Gerstl and Zardecki, 1982; Toon et al.,
1982). Below this mass, light levels remain sufficient for photosynthesis.
Thus, the major challenge in evaluating the Alvarez dust hypothesis is
estimating the mass of globally distributed submicrometer-size dust.


Silicate Dust

Toon et al. (1997) used theoretical calculations coupled with energy scaling of
experimental and atomic bomb data, adapted from O'Keefe and Ahrens (1982), to
estimate that ~3 x 10^17g of submicrometer-size dust was lofted into the
stratosphere by the K-T impact. Nevertheless, the data on particle-size
distributions for impacts and atomic blasts used by O'Keefe and Ahrens (1982)
do not cover size ranges below 50 mym. Below 100 mym, these same data show a
sharp drop-off in cumulative mass, suggesting that the target rocks resist
fragmentation below the crystal domain size of 100 mym (e.g., Melosh, 1989).

O'Keefe and Ahrens (1982) assumed a simple exponential decrease in cumulative
mass from 50 cm to 0.5 mym, which indicated that ~0.1% of impact debris would be
<1 mym. Given the evidence for a drop-off in the <100 mym size fraction, a better
estimate of the sub-micrometer-size dust is <0.1%, perhaps much less.

Vapor Condensation

Most of the mass in the fireball of an impact is vapor. Theoretical studies of a
Chicxulub-size asteroid impact indicate that the vapor plume contained 1-3 x 10^18g
of silicate vapor from the target rocks (Toon et al., 1997; Pierazzo et al., 1998).
The plume also contained vapor from the carbonates and sulfates in the target rock
(e.g., Pope et al., 1997). Most of the Ca and Mg in the carbonates and sulfates
prob-ably condensed with the silicate vapors. Given the CO2 and SO2 mass estimated
by Pope et al. (1997), the mass of vaporized Ca and Mg added an additional ~5 x 10^17g
to the plume. Finally, the contribution of the impactor must be considered, which would
add ~1-3 x 10^17g to the plume, or perhaps twice this amount if the impact velocity was
>20 km/s (Pierazzo and Melosh, 2000). The total mass of the vapor plume was
therefore ~2-4 x 10^18g. These vapors, which spread globally and condensed
(e.g., Zahnle, 1990), are the primary source of the global ejecta layer.

O'Keefe and Ahrens (1982) calculated that vapor condensation droplets from a Chicxulub-
size impact would be in the size range of hundreds of micrometers. Ablation of these
spherules upon atmospheric reentry could produce smaller particles (Melosh and
Vickery, 1991), although Zahnle (1990) calculated that the velocity of most
condensates would be too low for significant ablation. Furthermore, the size of the
droplets is close to the 100 mym size limit, below which little ablation occurs
(Melosh, 1989). Therefore, the vapor condensates from a Chicxulub-size impact
probably produce minimal amounts of submicrometer-size particles.


K-T Fireball Condensates

The thickness of the global ejecta layer is ~3 mm (e.g., Smit, 1999). The term "fireball
layer" (Hildebrand and Boynton, 1990) is used here for this global layer. The
fireball layer, which contains shocked quartz, spherules, and an Ir anomaly, is the
only globally distributed K-T ejecta (other ejecta layers have a limited distribution).
Mineralogical studies indicate that the bulk of the fireball layer is altered glass
(Pollastro and Bohor, 1993). Well-preserved examples of the fireball layer are composed
almost entirely of spherules with relict crystalline textures indicative of quenched melt,
and are interpreted to be condensation droplets from the vapor plume (Montanari et al., 1983;
Montanari, 1991; Smit et al., 1992a; Pollastro and Bohor, 1993; Bohor and Glass, 1995;
Kyte and Bohor, 1995). Spherule diameters range from ~20 to 800 mym (Doehne and Margolis,
1990; Montanari, 1991; Kyte et al., 1996); a mean of 250 mym was reported from sites in
Europe (Smit, 1999).

The 3-mm-thick fireball layer represents a global mass of ~3.8 x 10^18g (assuming a
mean density of 2.5 g/cm^3 ), which matches the estimates of the vapor-plume mass
noted here. Likewise, the compo-sition of the fireball layer is consistent with
most of the mass being derived from vapor condensation droplets ~200 mym in diameter.
Nevertheless, these analyses do not prove that there is not a fraction of a percent
of submicrometer-size dust in the fireball layer.

Clastic Debris (''Dust'') in the Fireball Layer

The most complete analysis of clastic (pulverized rock) debris in the fireball layer
comes from the Pacific Ocean (Bostwick and Kyte, 1996). Of the quartz grains examined,
~65% show evidence of impact shock, and these grains have a mean size (d)of 50 mym.
The mass percentage of impact clastic debris in the Pacific K-T fireball layer can
be estimated by assuming (1) all of the clastic quartz grains were originally deposited
in the 3-mm-thick fireball layer; (2) the average mass of quartz grains
= ¼ d^3 x 2.5 g/cm^3, based on the ~1:2 aspect ratio reported by Izett (1990) and the
density of quartz; and (3) the total mass of clastic debris in the fireball layer is
equal to two times the mass of quartz, based on the data in Izett (1990) and the
complex lithology of the target site (Sharpton et al., 1990). Given these assumptions,
and the data reported by Bostwick and Kyte (1996), the mass percentage of clastic debris
in five Pacific sites averages ~0.1% (Fig. 1). The same approach can be used to estimate
the mass of clastic debris from Beloc in Haiti and Frenchman Valley in Saskatchewan,
Canada, based on data reported by Leroux et al. (1995), and from Petriccio, Italy,
based on data reported by Montanari (1991). Haiti has nearly 3% clastic debris in
the fireball layer, Frenchman Valley ~0.3%, and Italy only 0.001% (Fig. 1).

Izett (1990) found 0.02%-0.7% clastic grains (by weight) in the fireball layer from
sites in the Raton basin of Colorado and New Mexico. About half of the clastic grains
are quartz, of which ~50% show impact-shock deformation (Izett, 1990). Sharpton et al.
(1990) found 1% clastic grains in a 2-5-mm-thick fireball layer from the Raton basin.
A clastic mass of 0.5% for the Raton basin is derived by using the methods outlined
here and data reported by Leroux et al. (1995). Taking into account that the lower
percentages probably represent incomplete recovery, the total amount of clastic debris
in the Raton basin fireball layers is estimated to be ~1%.

Izett (1990) found no shocked quartz in an analysis of 15 000 quartz grains from three
sites in New Zealand, although a few grains were found in later analyses. Analyses
of a core from Deep Sea Drilling Project (DSDP) Site 527 (Walvis Ridge) in the
South Atlantic produced only a few shocked-quartz grains, composing ~2% of the quartz
grains in the K-T boundary samples (Huffman et al., 1990). These data are insufficient
to make estimates of the clastic-debris mass in these two Southern Hemisphere sites,
but given the paucity of shocked quartz, the mass is probably less than that found in

The mass of clastic debris in the fireball layer follows an inverse power-law relationship
with distance from Chicxulub (Fig. 1), with the notable exception of Italy (and perhaps
Walvis Ridge and New Zea-land). With Italy omitted, a power-law regression of the mass
(y in %) with distance (x in km) gives the function y = 208 012.3 +/- 2.5x^-1.636 +0.30;
r = 0.91 (95% confidence interval). Another well-known aspect of the shocked quartz in
the fireball layer is that the grains become smaller with distance from North America
(e.g., Bohor, 1990; Izett, 1990). A compilation of data on maximum (24 sites) and mean
(14 sites) shocked-quartz grain sizes is shown in Figure 2. Similar to the mass, there
is a clear pattern of decreasing size with distance from the Chicxulub crater (Fig. 2).
A power-law regression of the maximum size (y in mm) with distance (x in km) gives the
function y = 482.30 +/- 0.70x^-0.87 + 0.08; r = 0.91 (95% confidence interval).


The characteristics of the clastic debris in the fireball layer show clear geographic
patterns that are not readily explained by ballistic transport. Alvarez et al. (1995)
noted that the launch velocity required for ballistic transport of shocked quartz to
distal K-T boundary sites can only be achieved by ejecta that is subjected to shock
pressures that would have annealed or melted the quartz. They explained this apparent
anomaly with a velocity boost imparted to moderately shocked ejecta by the vapor plume.
Nevertheless, models of Chicxulub ballistic ejecta dispersal, with a velocity assist
from the vapor plume (Durda et al., 1998), do not reproduce the mass distributions
of clastic debris shown in Figure 1. Durda et al.'s (1998) model predicts that impacts
produce a distribution of ballistic ejecta that is largely symmetrical around the
crater. The model does not explain why Italy has more than an order of magnitude
less debris than Pacific DSDP Site 576, which is at about the same radial distance from
Chicxulub as Italy. Likewise, ballistic transport of clastic ejecta cannot explain the
size sorting shown in Figure 2. Ballistic transport to distal sites occurs mostly
outside the atmosphere, where no sorting would occur. Once in the atmosphere,  drag
would preferentially reduce the trajectory of smaller particles, producing patterns
inverse to what is observed.

The mass and grain-size distributions of clastic debris in the fire-ball layer are
better explained by (1) ballistic deposition of moderately shocked ejecta on top of the
atmosphere near the crater; (2) subsequent spread of the debris by stratospheric winds;
and (3) gravitational settling of debris as the cloud spreads. Such a process was
proposed by Toon et al. (1997) and Covey et al. (1990). Covey et al. (1990) modeled
the wind dispersal of a cloud of impact dust with an initial loading of 5 x 10^15g
centered (1000 km radius) on the Manson crater in Iowa. After five days, dense clouds
of debris continued to rain down over North America, the northern Atlantic, and the
Pacific; moderate dust loading had spread to central and western Europe; and very
little dust had spread to the southern high latitudes. The speed of westward spread
of the cloud was ~150 km/h. This pattern of dispersal is similar to the spread of the
volcanic plume of the 1982 eruption of El Chichon, located just southwest of the
Chicxulub crater, which spread westward at ~70 km/h and encircled the globe with a
narrow band of debris (Rampino and Self, 1984).

To examine the wind dispersal of ejecta, grain-size distributions were modeled for
three potential dispersal wind speeds: 70, 150, and 400 km/h (Fig. 2), based on the
velocities of the Chichon plume, the Covey et al. (1990) impact simulation, and the
jet stream, respectively. Note that the particle-size distributions found in the K-T
fireball layer follow a power-law relationship similar to the model distributions,
and that these distributions are mostly within the range expected for particles
dispersed by winds with speeds of between 70 and 400 km/h.

The information on ejecta mass in Figure 1 and ejecta size in Figure 2 can be combined
by assuming an initial particle-size distribution. The distribution assumed here
is that measured in pyroclastic deposits (Sheridan, 1979). Pyroclastic deposits
are a reasonable analogue for clastic ejecta and have been well studied down to
the micrometer size range. Figure 3 presents a series of calculations of ejecta
dispersion beginning with an initial mass (10^16g and 10^17g) centered on the crater
with the size distribution noted. Two models of strato-spheric wind dispersion of
ejecta (150 km/h and 400 km/h) were then applied. The calculations assumed that
the clastic ejecta were dispersed in a radial fashion, which the data and Covey
et al.'s (1990) model suggest is not the true case. This simplification will
underestimate the true clastic ejecta mass in the fireball layer, given that the
latitudinal dispersal of ejecta was probably more limited than the radial dispersal
used in the calculations. Calculations were based on the distance particles of
a given size range would travel before settling (Fig. 2); then the mass represented
by that size range (taken from the size distribu-tion) was distributed over the
radial distance covered.

There are two conclusions to be drawn from Figure 3. (1) The distribution of mass
is not highly sensitive to the wind speed, because the 150 km/s and 400 km/s
calculations produced similar results. This insensitivity is because most of the
mass is concentrated in the larger size fraction, so that the different wind speeds
only greatly affect sedimentation near the crater. (2) The mass distribution is
highly sensitive to the initial mass. The pattern of modeled mass dispersal for
an initial loading of 10^16g compares well with the measured mass in the fireball
layer (Fig. 3). Because this simplified model tends to underestimate the mass, the
conclusion to be drawn is that the observed mass in the fireball layer is consistent
with an initial mass of ~10^16g. If the initial mass was 10^17g, much more clastic
debris would be expected than is observed. Note that the mass loading in this model
is that part of the ejecta that was dispersed by winds and does not equate
with the total mass ejected into the atmosphere.

Returning to the issue of the distribution of clastic debris in the K-T fireball
layer, the anomalous small mass of debris in Italy, and perhaps Walvis Ridge and
New Zealand, can be explained by the asymmetrical dispersal patterns of stratospheric
winds. If the impact occurred during summer in the Northern Hemisphere, debris would
be transported mostly westward; thus debris must travel three times further to Italy
than to the western Pacific. Similarly, stratospheric winds are much less effective
in transporting debris latitudinally; hence little debris may have reached New Zealand.


Implications for the K-T Mass Extinction

Although the submicrometer-size component of the fireball layer cannot be directly
examined, it must be very small. Assuming a grain-size distribution typical for
distal volcanic-ash deposits (e.g., Carey and Sigurdsson, 1982), the submicrometer-size
component of the clastic debris in the fireball layer is probably <1%. The total mass
of clastic debris in the fireball layer estimated here is <10^16g. Therefore, the
mass of submicrometer-size dust in the fireball layer is <10^14 g, and is perhaps as
little as 10^13g. This mass is two to three orders of magnitude less than that needed
to shut down photosynthesis. These results shed doubt on the importance of impact dust
in the mass extinction that marks the K-T boundary. A global atmospheric loading of
<10^14g of submicrometer-size dust would not cause the catastrophic impact winter often
proposed (e.g., Covey et al., 1994).

There are, of course, impact hazards other than dust clouds. For the K-T event, the
shutdown of photosynthesis and global cooling are more likely to have been caused
by the impact production of sulfate aerosols from the target rock (e.g., Pope et al.,
1997), and by soot from global wildfires (e.g., Wolbach et al., 1990).

Implications for Impact Hazards

Dust clouds have also been used to estimate the effects of small impacts (Toon et al.,
1997). Given that a Chicxulub-size asteroid (10 km diameter) generates only modest
amounts of fine dust, the dust effects from smaller impacts are probably negligible.
This conclusion has major ramifications for assessments of future impact hazards.
Chapman and Morrison (1994) assumed that the impact of an asteroid between 0.6 and 5
km in diameter would produce enough dust to cause global crop failures leading to the
death of 25% or more of the world's population. The lower and nominal (0.6-1.5 km)
asteroid sizes used in their calculations are much too small to have global consequences
from the dust. Other factors such as sulfate aerosols from the asteroid (Kring et al.,
1996) and soot from fires set by ejecta reentry (Toon et al., 1997) only become
important globally for asteroids $3 km in diameter. Therefore, the often cited
1:20 000 risk of death by impact (Chapman and Morrison, 1994), which assumes mass
mortality during relatively small (1.5 km asteroid) impacts, is greatly overstated.


This research was funded by the National Aeronautics and Space
Exobiology Program contract NASW-96030.


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Copyright 2002, Geological Society of America


>From David Morrison <>

NEO News (1/25/02) Pope on dust

Dear friends and students of NEOs:

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 Pope's paper. (2) The press release issued by Pope and
the Geological Society of America. (2) A summary article by Rob Britt
of that also includes some first responses from other
scientists. (4) Additional discussion of some of the points at issue,
with comments from other scientists. [NOTE: items 1-3 are not included
below because they were posted in yesterdays' CCNet, BJP]

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 on 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


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 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."


Alvarez, L, W. Alvarez, F. Asaro, & H.V. Michel, Extraterrestrial
cause for the Cretaceous-Tertiary extinction, Science 208:1095-1108

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)


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>From Gerta Keller <gkeller@Princeton.EDU>

I am delighted to hear Kevin Pope's skepticism about the impact
dust cloud scenario as primary cause for the K/T mass extinction.
This scenario simply does not fit the paleontological data that show
strong declines in populations for at least the last 0.5-1.0 million
years prior to the K/T impact. Our recent discoveries in Mexico and
Israel of three  glass spherule layers in sediments spanning the last
0.5 m.y. of the Maastrichtian, and which are completely different from
the spherules at the K/T boundary within the same section, suggests
that multiple impacts are a more likely scenario, coupled with Deccan
volcanism and the now well known rapid climate changes during that time.

Gerta Keller, Princeton University

Gerta Keller
Department of Geosciences
Princeton University
Princeton, NJ, 08544, USA
phone: 609 258 4117
fax:   609 258 1671


>From Drake A. Mitchell <>

"New assessments" could indicate equivalent, lesser, or greater risk.
The key statement to decode in the abstract of Dr. Pope's latest paper
seems indeed to be the last sentence: "Furthermore, estimates of future
impact hazards, which rely upon inaccurate impact-dust loadings, are
greatly overstated [1]". I submit that the additional following sentence
may offer less ambiguity: "Nevertheless, subsequent research seeking
to better model several other contributory mechanisms could easily a)
confirm that existing range estimates of global-effects risk are robust
and generally accurate, or b) indicate that current estimates might need
to be adjusted to reflect less risk of global effects, or c) indicate
that current estimates might need to be adjusted to reflect greater risk
of global effects."

Of course, in "contributing mechanisms" in this extinction context I
also am not including the additional imprecisely modeled effects e.g.
climatological, ecological, sociocultural, political, economic, etc.
But in his response statement ("... in my own work I have argued
strongly that sulfate aerosols...", "...combined with the rain of fire
from reentering ejecta...") he does not actually specify where any new
assessments will lead - unchanged, lesser or greater risk [2]. We might
however expect that in the body of the paper itself a specified
calculation is finally offered for the alternative threshold-level(s)
of the "non-dust" sulfate and soot mechanisms.

Additionally, although the results cited by Paine in the 2000 paper by
O'Keefe et al  seem compensatory ("... 6 months of sulfuric acid
haze..." versus "...~one month... ...dust shielding... "), it is not
clear that integrating these results would indicate a changed risk of
global effects greater than the standing 1994 estimate. Does it?
Furthermore, could it be likely that Pope's results are sensitive to the
suspected KT-contemporaneous Shiva astrobleme?

There is strong consensus worldwide that the public, and indeed the
larger science community, needs to be introduced to more information
about the NEO hazard, and in more depth. The occasion of Dr. Pope's
latest analysis presents a golden opportunity for an educational
experience in which "even" undergraduate and high-school students can
participate. With the manageable <1,000 lines of GW Basic code in Prof.
J. S. Lewis's recent text & diskette (Academic Press, 2000), which offers
an "eight-degree" Monte Carlo simulation of the NEO hazard, it should not
be beyond our world's science instructors (ably assisted by our avid NEO
community) to help students adjust and enhance, nay "hack" the code to
reflect parametrized estimates of global effects thresholds [3]. The
possible cases of  "b" and "c" above might be initially roughly modeled
within a semester's project, thereby demonstrating a simulation-based
sensitivity analysis of annualized dollar damage from this compelling
multidisciplinary problem.

If it turns out that the standing global-effects risk estimate should in
fact be downgraded, this would imply that the number of "global killer"
NEAs was judiciously estimated, and that we are closer to completing the
existing survey goal for these large bodies than commonly realized. For
example, Bottke et al [4] estimated in 2000 that if global killers were
>=1 km in diameter, then 32% had been discovered. However, if the
threshold were actually >=3km then the completion as of that dataset's
vintage would have been 56%, or if >=6km then 66%. Of course these
figures will be updated for 2002, perhaps in time for ProSpace's
MarchStorm, and could be expanded to reflect the possibility that NEOs
<1.0km, e.g. a YB5 calving an armada of icebergs from the remains of the
Antarctic icesheet, could also have global effects.

Finally, it has been argued elsewhere that 1) considering 1994's average
annualized damage estimate was ~$300M per year, that 2) in 2000 that
estimate apparently grew five times to ~$1.5B per Lewis's simulation
above[5], and that 3) there are many unfortunate factors complicating
and limiting current research efforts, a more comprehensive future
estimate could possibly be even higher by an order of magnitude. $15
Billion per year in average annualized damage, Enron meltdowns
notwithstanding, may be much too high - either inaccurate, or really
intolerable for the unadjusted reinsurance return periods - but clearly
this is one of several classes of variables that must be targeted for
expedited analysis.

[3] Lewis, J.S., Academic Press 2000, "Comet and Asteroid Impact Hazards
on a Populated Earth." Chapter 9, "Areas Requiring Further Study"
p. 138-9. Subroutines BLOWOFF, CRATER, HAZARD, p. 151-4.
The primary challenge may be starting off simply,
e.g. a statistical model of the geological layer.
[4] Bottke, William F, et al,
"Understanding the Distribution of Near-Earth Asteroids"
Science, Volume 288, Issue 5474, pp. 2190-2194 (2000).
[5] Lewis 2000, "Global Killers" p. 131, plus "Regional Hazards" p.132.

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>From Andrew Glikson <>

Dear Benny,

I have now read Kevin Pope's Geology article "Impact dust not the cause
of the Cretaceous-Tertiary mass extinction" (Geology, 30:99-102), and
like to thank him for sending me a copy of his article.

Kevin questions the long-held assumption of a silicate-dominated dusting
scenario associated with the KT boundary impact. In the following I
suggest that an extrapolation from the coarser-grained (~0.1-1.0 mm)
dust fraction incorporated in the KT-boundary unit to finer fractions,
using volcanic clastic dust size-frequency distribution, is unlikely in
view of (1) intra-crystalline breakdown of shock-metamorphosed mineral
grains in impacted target rocks along planar deformation features (PDF),
and (2) solid-state amorphisation (diaplectic transformation from
crystalline structure to glass) of quartz and feldspar (cf. Stoffler and
Langenhorst, 1994; French, 1998 and references therein), resulting in a
high proportion of low-density micron-scale sub-crystal grains and
silicate glass in the ejecta.

The penetrative development of crystallographically controlled PDF
planes and glass ensues in loss of mechanical coherence of individual
grains and rock fragments. This results in an increased production of
micron to submicron-scale particles. Cores of impact-shocked granite or
sandstone are commonly pulverised on touch and disintegrate upon sample
preparation, for example shocked granitoids from the Woodleigh impact
structure (Mory et al., 2000a,b; Glikson, 2000).

The proportion of shocked grains may be expected to increase with
distance from the impact site, due to high angle ejection of shocked
ejecta from inner-impact aureoles as contrasted with lower angle
ejection of less shocked ejecta from outer crater aureoles. Whereas the
relatively high proportion of shocked quartz grains in the KT boundary
unit at Pacific sites (~65%, Bostwick and Kyte, 1996) may conceivably be
interpreted in such terms, further studies are required to determine the
ratio of shocked to unshocked quartz grains with distance from

Pope (2002) states: "Assuming a grain size distribution typical for
distal volcanic ash deposits, the submicrometer size component of the
clastic debris in the fireball layer is probably <1%". However,
intragranular disintegration of shocked ejecta should result in higher
proportion of submicron ultra-fine dust, and thus in different grain
size distribution frequencies of impact-released particles and volcanic
dust. It follows that extraterrestrial impacts may be more effective
than volcanic activity in producing long-term stratospheric clouding,
and thereby photosynthesis blocking and extinction.

Evidence for impact-produced ultra-fine dust would be difficult to
identify due to (1) fallout of micron-scale dust would postdate the KT
boundary "fireball layer", and may be incorporated in post-impact
sediments, and (2) some or much of the ultra-fine silicate dust may have
been dissolved in the acid atmosphere/acid rains consequent on
flash-triggered oxidation reactions.

Andrew Glikson
Research School of Earth Science
Australian National University
Canberra, ACT 0200

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