Geoscience Reference
In-Depth Information
million.
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The periodicity of cratering is far less firm, due to the
much smaller sample size, but it has not been falsified. Yet in spite
of the evidence for extinction periodicity, and its importance if true,
interest seems to have waned. One of the reasons is that periodicity
falls deep in the cracks between disciplines—it is not really the
province of geologists, or paleontologists, or astronomers, or statisti-
cians, or anyone—it is a scientific orphan in a world of limited time,
scarce resources, and orthodoxy. Even the pro-impactors need not
endorse it—impact could be of great importance in earth history
and not be periodic.
But there is clearly another reason why periodicity has failed to
continue to excite scientists: No one has been able to find Nemesis,
or Planet X, or any logical reason as to why extinction, and possibly
cratering, should be periodic. We know from the history of science
that we should not reject what seem to be sound observations just
because we cannot account for them, yet it is human nature to do
so. Recently, however, an intriguing explanation has come to light
that may mean that a plausible source mechanism is available.
Astronomers have been studying the effect of passage of the
Oort cloud—that vast reservoir of comets out beyond the solar sys-
tem—through the disk of the Galaxy, and have found previously
unrecognized gravitational effects that could cause comets to strike
periodically and that would repeat every 30 million to 35 million
years.
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Previously, Shoemaker was on record as thinking that cra-
tering periodicity was a statistical fluke, but this new work made a
believer out of him: "Impact surges are real . . . and [the comet flux
is] controlled by the fluctuating galactic tides," he said. The new
work "is a landmark contribution in understanding the history of
bombardment of the earth."
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Add this to the opinion of paleontol-
ogist Douglas Erwin that "The periodic signal continues to shine
through the turmoil, battered but resilient," and we can see that
periodicity of both the fossil record and terrestrial cratering are
hypotheses that are alive and well.
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