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Science Frontiers ONLINE No. 52: Jul-Aug 1987 |
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Do large meteors/comets come in cycles?
Only a few years ago, geologists refused to recognize any terrestrial meteor craters larger than Arizona's Meteor Crater, which is merely a mile or so in diameter. Now, we have a long list of craters or astroblemes (star wounds), some of which measure hundreds of miles across. In fact, there are enough large dated crters so that some scientists have taken up a time-honored human pastime: Looking for cycles or periodicities in the data. (Humans can find cyclicities in almost any collection of data!) To be more specific, some have claimed that large meteor craters come in clusters dated 28-31 million years apart. These catastrophic events have been correlated with biological extinctions, magnetic field reversals, and basalt flooding.
The astronomical causes of this supposed periodicity range from the solar-system's crossing of the galactic plane, to the perturbations of an unseen solar companion, to regular perturbations of the Oort cloud of comets that is thought to hover at the fringe of the solar system.
In short, a large, interlocking edifice of geological and astronomical speculation has been erected upon a foundation of terrestrial crater ages.
But how well do we really know the ages of these craters? How complete is the cratering record? The answer to the first question is: "Not well at all." Further, we can be certain that many craters still lie undiscovered beneath sediments. In addition, most meteors/comets splashed into the oceans, leaving no record at all.
An updating of the most recent crater data available, such as they are, greatly weakens the case for the popular 28-31 million-year period and strengthens support for a 19-22 million-year period. But neither cycle is in synchronism with the famous K-T (Cretaceous-Tertiary) Boundary, with its perplexing iridium layer and massive biological extinctions. In fact, say V.L. Sharpton et al, the entire known cratering record could well be the consequence of chance encounters between the earth and stray meteors and comets. Thus would pass the periodicities of this world!
(Sharpton, V.L. et al; "Periodicity in the Earth's Cratering Record?" Eos, 68: 344, 1987.)
Reference. More details on crater periodicity may be found in ETC4 in our catalog: Carolina Bays, Mima Mounds. For information on this book, see: here.