As you probably know, the Sep/Oct Skeptical Inquirer (whose cover features Martin Gardner) contains letters by Clark Chapman and Paul Hilfinger, reacting to your May/Jun article on the claimed impact at the Younger Dryas boundary (also with a letter from Mark Bosslough commenting on his sidebar about scientists changing their minds, citing the recent example of Wallace Broecker)
along with your response, which includes the following passage:
“Such huge swarms of super-Tunguska impacts [for the Carolina Bays and the YD impact] are inconsistent with what astronomers know about our planet’s cosmic environment or geologists’ understanding of Earth’s recent impact history.
This is not just improbable; in common usage we would have to call it impossible.”
Dave, I almost hate to say this, but in almost every particular here you are WRONG because
1) the British astronomer Bill Napier has explained this year in MNRAS how Earth might well have encountered such huge swarms of super-Tunguska impactors from the debris of a recently disintegrated comet only 13,000 years ago (despite Chapman seconding you on this point), and;
2) American geologist Adrian Melott et al. have this year presented evidence in GEOLOGY for a large impact at the YDB based on the ammonium signal in the Greenland ice.
The publication of the paper contradicts Chapman’s statement in his letter, “Nor has the Greenland paper been published in any peer-reviewed scientific journal during the subsequent year.”
Furthermore, there need not have been two events IF the admittedly controversial extra-terrestrial origin of the Carolina bays was part of the same impact event at the YDB, but this issue is beyond the scope of my concerns here.
Your failure to take into account these two developments vitiates the thrust of your article.
Also, I find it of passing interest that Hilfinger’s letter correcting you on a probability issue (“That is, Tunguska does not ‘protect’ us from immediate future impacts.”) is the same issue on which my unpublished letter to Sky & Telescope in Sept. 2009 corrected Mark Boslough, i.e., the occurrence of a low probability event does not mean that a second, similar low probability event cannot happen very soon after the first.
The letter below was submitted to Skeptical Inquirer in early July, possibly too late to be included in the Sep/Oct issue, but in light of the tenor of Clark Chapman’s letter (“I wish to expand on David Morrison’s excellent special report . . . on the widely publicized claim that a huge broken-up comet collided with Earth just 13,000 years ago, wiping out mammoths, Clovis culture, and so on. . . .”), excoriating the AAAS for awarding top prize to the NOVA TV documentary on the Younger Dryas Impact shown on PBS.
Perhaps I was too optimistic about SI’s interest in “fair and balanced” scientific reporting.
[ letter to SI by C. Leroy Ellenberger ]
Cosmic Impact at Younger Dryas Boundary
In response to David Morrison’s attempt to quash the possibility that a cosmic impact killed the mammoths approximately 12,900 years ago (SI, May/June 2010), Mark Twain might have responded that the report that such an impact has been disproved is exaggerated.
Certainly Morrison correctly points out scientific problems in some descriptions of the event; but he fails to deal with recent developments that nevertheless support a cosmic impact at the Younger Dryas boundary.
Three issues merit comment here:
Morrison claims, “There was apparently no way to get a swarm of impactors to target North America alone,” while assuming that a comet disintegrated just before encountering Earth; but this objection has been overcome by British astronomer Bill Napier who, in a recent publication, describes how robust meteorid streams (comet debris trails) would be the inevitable result of the disintegration within the past 20-30 KYr of the progenitor of the Taurid Complex in near-Earth space, and that, “encounters with dense swarms of material, sufficient to produce a 12.9 ka cosmic event, are indeed reasonable expectations of recent Earth history.”
Morrison’s criticism concerning impact frequencies does not envision the former existence of a large comet in a short period orbit whose disintegration gave rise to the Earth-orbit-crossing Taurid Complex which then contained myriads of large debris.
Concerning the presence of nanodiamonds, Morrison misstates D.J. Kennett et al.’s position in Science, who stated, “Nanodiamonds . . . are associated with known impacts, during which they may arrive inside the impactor or form through shock metamorphism. . . .”
Morrison improperly ignored the prospect of their arrival inside the impactor, which was the case with the Tagish Lake meteorite (arguably part of the Taurid Complex) that fell over British Columbia in January 2000, and contained the highest abundance of nanodiamonds yet seen.
Conversely, Morrison’s remark, “Most impact experts agree that nanodiamonds were unlikely to have been formed in the impact,” is contradicted by the discovery of nanodiamonds at the Cretaceous-Tertiary boundary, that most likely formed from terrestrial material during the impact.
Finally, airbursts from cometary fragments can be expected to produce copious amounts of ammonium ion, which are captured and preserved in glaciers, especially the Greenland ice cap.
A.L. Melott et al. in the April 2010 Geology analyzed the ammonium signals in the Greenland ice from the Tunguska airburst of June 30, 1908, which cannot be explained by wildfires from the explosion, and also at a depth in the ice corresponding to the onset of the Younger Dryas 12,900 years ago.
This is presumptive evidence for a cosmic impact at that time, regardless Morrison’s attempts to discount such a possibility.
Napier, W. M. 2010.
Palaeolithic extinctions and the Taurid Complex.
Mon.Not. R. Astron. Soc. 405(3): 1901-1906.
Kennett, D. J. et al. 2009.
Nanodiamonds in the Younger Dryas Boundary
Layer. Science 323: 94.
Gilmour, I. et al. 1992.
Terrestrial carbon and nitrogen isotopic ratios
from Cretaceous-Tertiary boundary nanodiamonds.
Science 258: 1624-1626.
Melott, A. L. et al. 2010.
Cometary airbursts and atmospheric chemistry:
Tunguska and a candidate Younger Dryas event.
Geology 38: 355-358.
When Scientists Actually Change Their Minds
by Mark Boslough
Volume 34.3, May / June 2010
Broecker’s esteem among scientists was not diminished when he changed his mind.
The Younger Dryas impact proponents would do well to follow his example.
Did a Cosmic Impact Kill the Mammoths?
by David Morrison
Volume 34.3, May / June 2010
The rise and fall of the theory that cosmic catastrophes altered human prehistory in North America.
Dr. David Morrison is the Senior Scientist at the NASA Astrobiology Institute. His primary interests are the new multidisciplinary science of astrobiology, the protection of Earth from asteroid impacts, and science outreach and education. Dr. David Morrison is the Director of the NASA Lunar Science Institute and Senior Scientist for Astrobiology at the NASA Ames Research Center. He is also the Director of the Carl Sagan Center for Study of Life in the Universe at the SETI Institute, in Mountain View CA.
Dr. Morrison is internationally known for his research on small bodies in the solar system, and has more than 155 technical papers and has published a dozen books, including five university-level textbooks and several popular trade books on space science topics.
In 2005 he received the Carl Sagan medal of the American Astronomical Society for communicating science to the public.
He is a Fellow of the American Association for the Advancement of Science, the California Academy of Sciences, and the Committee for Skeptical Inquiry.
Asteroid 2410 Morrison is named in his honor.
See the Mark Boslough supercomputer 3D simulations of meteor air bursts
becoming complex directed flows of very high temperature and pressure plasma
plumes — 2007 — Dennis Cox gives evidence that this was real on a
continental scale, causing “vertical ablation”.