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The message from the Meteoritical Society Meeting today is that impact craters, in particular the small ones, are much more difficult to identify than you might think. Although craters are circular depressions in the Earth this does not necessarily mean that all circular depressions on the Earth are craters. Our planet has such an active surface, on which wind, water and ice scour away the rocks, that circular features can form in many ways. Gathering the evidence to needed prove that a structure has formed due to impact turns out to be a difficult and even frustrating business.
Geologist Wakefield Dort, from the University of Kansas, knows how difficult crater hunting can be. Dort and his colleages have been studying a 1.5 km wide circular depression, known as the Merna Crater, in Nebraska since 1991. It certainly has the outward appearance of a crater with a flat floor and a circular raised rim, however, this is not enough to prove that it formed due to the collision of a Near Earth Object. Dort can, however, estimate when the Merna depression was formed since it cuts ridges formed by the wind on the fine silica-rich soil on which it lies. The ridges formed 12,000 years ago and so the circular depression must be younger. In places around the depression, Dort has found pieces of silica glass that may have formed by melting of the fine soil in the depression, however, no trace of an asteroid to be found. So far no signs of the enormous pressures that are generated in collisions have been discovered at Merna and thus crater experts at the meeting are doubtful that the depression really is a crater. It may, therefore, take much more work to show whether an asteroid collided with the Earth in the middle of Nebraska.
Christian Koeberl, from the University of Vienna, has had much experience in the business of seeking out impact structures and presented the first results of his study of two craters in Libya. The craters, known as the BP and Oasis structures have only ever been visited once by scientists and much of what is known about them has been gleaned from satellite images. Koeberl and colleages, however, this year obtained permission from the Libyan authorities to spend several weeks studying the craters which they have now mapped in detail and sampled their rocks. The reason for their interest in the two craters is that mysterious green Libyan Desert glass is found only 100 km to the east and they wish to test whether the glass was formed from molten rocks thrown out from one of them. The glass contains materials from an asteroid, and thus was definitely formed in a collision, and has an age, measured from its low-levels of radioactive elements, of 28 million years. Koeberl is to test whether either of the craters formed at the same time and hopes, once and for all, to prove where the Libyan Desert Glass comes from.
Today’s presentations on impacts have been enlightening and show that it is only through careful and meticulous work that the secrets of impact craters can be discovered.
Matthew Genge
at the Meteoritical Society Meeting in Los Angeles
More info: The Meteoritical Society
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