Many observers of the science press have noted an increasing tendency for both press releases and printed stories about science topics to exaggerate the uniqueness and impact of new research. The writer of a press release does this to increase the probability that the media will cover the story, and the media reporter will go along with this hyperbole or perhaps expand it further in order to get the story approved for publication by the editors or other gatekeepers.
The field of impacts (and impact hazards) is not immune to these trends. However, in this webpage I try to apply a filter to reduce the noise level in media reports. The following personal essay explains why many of the impact-related stories reported elsewhere have not been discussed in these news reports.
I acknowledge Benny Peiser's http://abob.libs.uga.edu/bobk/cccmenu.html" target="_blank">CCNet for comprehensive reporting of these stories and their media coverage. Peiser has also frequently noted the same kind of contradictions I am writing about today. The coverage can produce a whipsaw effect, with different scientists successively emphasizing apparently contradictory results. Often, each story is discussed with little reference to the context or possible mitigating evidence that should soften the conclusions and make them more tentative.
This is not intended as a general criticism of science reporting. There are many excellent science journalists who understand the issues and provide well-reasoned discussions of context for news stories. Overall, the reporting by science journalists of NEO stories has been excellent. But sometimes even the best writers can get caught by a hyperbolic headline added without their knowledge before publication.
As Carl Sagan often said, "Extraordinary claims require extraordinary evidence." A similar admonition might be that before revolutionary theories are widely publicized, they need to be given a reality check. This is best done by the scientists deciding whether to issue a press release. But if the scientists are not self-policing, the burden falls upon the journalists to filter the signal from the noise.
Did the KT impact cause the extinction of the dinosaurs?
This is by now an old story, but sometimes the press still report dramatically opposed conclusions as if this were a contentious debate within the scientific community today. Certainly the issue was contentious when impact extinction was first proposed by the Alvarez team 25 years ago, but a scientific consensus had emerged by the early 1990s. This progress of the Alvarez theory, increasingly supported by new evidence (such as the discovery of the Chicxulub Crater), has been chronicled is several excellent trade books, such as "Night Comes to the Cretaceous" by James Powell, "T. Rex and the Crater of Doom" by Walter Alvarez, "The End of the Dinosaurs" by Charles Frankel, and "When Life Nearly Died" by Michael Benton.
In spite of the scientific consensus, there has been substantial media coverage in 2004 discussing alternative hypotheses of dinosaur extinction. Major stories have arisen from the work of paleontologist Gerta Keller at Princeton, who has been challenging the impact theory for more than two decades. Recently she has decided that impacts may indeed be implicated, but probably not the Chicxulub impact. One hypothesis she has suggested is that while the 100-million-megaton Chicxulub impact was insufficient to kill the dinosaurs, a smaller impact 300,000 years later may have done so. The problem here is not her research, which is legitimate peer-reviewed science, but he way it is sometimes handled in the press, with bold headlines such as "KT Mass Extinction Debate Wide Open and in Full Swing", "The Space Rock Was Framed: Asteroid Cleared in Dinosaurs' Death", and "Asteroid Couldn't Have Wiped Out Dinos".
One characteristic of media hype is to suggest that all science dealing with the KT extinction is about dinosaurs. Dinosaur fossils, which are relatively rare, do not define the mass extinction boundary; that is precisely marked in the marine fossil record by changes in single-celled protists such as forams, as well as by the global layer of extraterrestrial material and shocked quartz from the impact.
On the other hand, some scientists discuss the dinosaur extinction without reference to the simultaneous global mass extinction in which more than half of all families were lost. This attitude is reflected in the remark by paleontologist David Penny that "We agree completely with the geophysicists that an extraterrestrial impact marks the end of the Cretaceous. But after 25 years [scientists] have still not provided a single piece of evidence that this was the primary reason for the decline of the dinosaurs." This quote is from a perceptive article (http://www.space.com/scienceastronomy/dinosaur_debate_041014.html" target="_blank">In Extinction Debate, Dinosaurs and Science Writers are the Losers) by Rob Britt in Space.com, 14 October 2004.
Most scientists consider it to be exceedingly unlikely that the dinosaur extinction was unrelated to the global KT event. In addition to the coincidence in time, we understand how the huge KT impact killed large land animals by a combination of brief global firestorm followed by months of cold. Neglecting this relationship is one fatal flaw in this year's widely reported hypothesis that dinosaurs went extinct because of disparity in the numbers of males and females born. Perhaps in this case the publicity was stimulated by the word "sex", as in the Washington Times headline "Why Dinosaurs Died ý It's all about Sex".
Was the end-Permian (PT) mass extinction caused by an impact?
No one knows. The past year has seen several new scientific results, many associated with claims and counter-claims concerning the Bedout impact (or non-impact) structure that might (or might not) be the "smoking gun" crater. I have no quarrel with the coverage of these issues, except where press releases claim that the problem has been definitively solved. There is no consensus concerning the cause of the PT extinction, and hence every reason to follow the debates as they happen.
Do meteorites cause fires?
No. Yet it is common to find news reports that a bright meteor fell to ground and started a fire. This past year has seen publicity for the old idea that both the Great Chicago Fire of 1871 and another blaze the same night more than a hundred miles north in Wisconsin were started by hot stones from the sky. This coincidence seems striking. Yet in the cases where we have been able to estimate the surface temperature of just-fallen meteorites (such as where they land on snow or ice), the data indicate that they are cool. While there may be exceptions, this webpage follows the rule of thumb that if a meteor or meteorite is reported to have started a fire, the claim is probably mistaken and need not be reported unless the meteorite itself is recovered.
Is the recent impact rate much higher than is given by conventional wisdom?
A recurring feature of the tabloid press and of some websites is the assertion that we are at great risk from impacts, because impacts happen much more frequently than the scientists claim. Usually the "evidence" is related to the discovery of recent large impacts.
One report (The Guardian, 19 August) concerned huge craters under the Antarctic ice sheet said to be caused by an asteroid as big as the one that wiped out the dinosaurs, but striking about 780,000 years ago. The evidence supposedly shows that an asteroid measuring 3-7 miles across broke up in the atmosphere with five large pieces creating multiple craters over an area measuring 1,300 by 2,400 miles. Supposedly this impact caused a reversal in the Earth's magnetic field (a highly suspect claim) but little other damage. Obviously the description of this event is inconsistent with what is known about cosmic impacts. It is also impossible for atmospheric breakup (occurring within a few tens of kilometers of the surface) to distribute the impacts over more than 1000 km. Yet this was reported seriously.
The "Sirente crater", a lake near Abruzzi , Italy , has also been widely speculated to be an impact from the Roman period. This year an article in Tumbling Stone magazine suggests, however, that this is an anthropogenic feature and not the result of an impact.
The on-line website of Astronomy Magazine in October published a report on identification of a field of meteorites and impact craters near Lake Chiemsee in southeastern Bavaria , Germany . This crater field, which falls within an ellipse 58 by 27 km, is said to hold at least 81 impact craters ranging from 3 to 370 meters in size. The authors, using historical and archeological evidence, conclude that an asteroid or comet fragment exploded above southeastern Germany in the Celtic-Roman period, probably around 200 BCE. They estimate the projectile had a diameter of about 1 km. Since this report is new and has not been published in a refereed journal, it is difficult for me to judge how reliable these conclusions are, in terms of either the identification of impact craters or their probable date of formation.
Another story this month, further discussed below, suggests that there are many very dark, unseen comets that constitute a previously unrecognized threat. In recent years there has also been publicity given to claims of various 20 th century Tunguska-like impacts, which have not been verified.
I am not able from press reports to assess the reliability of most of these individual impact claims. However, any one of them is unexpected from the known average impact rates, which are based on NEO observations as well as the long-term cratering history of the Earth and Moon. For example, the proposed Lake Chiemsee impactor of 2000 years ago is claimed to have been about 1 km in diameter, whereas on average a NEO this large hits Earth only once in about 500,000 years. While any one impact proposal might be true (as a statistical fluke), it is hard to believe that several of these stories are correct. I remain skeptical.
Are we in danger from super-dark "stealth" comets?
The most recent publicity is about a paper in Monthly Notices of the Royal Astronomical Society by W.M. Napier, J.T. Wickramasinghe and N.C.Wickramasinghe on "Extreme albedo comets and the impact hazard". Based on a dynamical argument, they conclude that there should be more than 1000 times more Halley-type comets than are actually observed. They therefore suggest that the comets become invisible, and that the NEO population is dominated by bodies too dark to be seen with current NEO surveys. If this were correct, the time between km-scale (global catastrophe) impacts would be only a few hundred years.
I believe that one should be skeptical of a theoretical result that has no data to support it. One should be even more skeptical if it seems inconsistent with the data we do have, resolved (in this case) only by postulating a new class of "invisible" comets. But further, the idea that NEO surveys might miss huge numbers of such such "stealth" objects is largely beside the point. We know about the scarcity of small comets in part because of the low cratering rates implied by the dearth of small craters on the Galilean satellites, especially Europa. Don Yeomans summarized the data on the small comet population in the 2003 NASA STD report on sub-km impacts. When we consider the entire problem, not just the NEO surveys, it becomes clear that there is not a large population of stealth comets to worry us.