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What Was the Tunguska Meteor?
What Was the Tunguska Meteor?
Moscow News, 1, 1968 (THE TIME (London))
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One line of speculation about the nature of the Tunguska me­teor is brought to an end by an article in Nature, which amounts lo a demonstration that there has been no perceptible varia­tion in the amount of radioactive carbon in the atmosphere in the past 70 years. This rinding con­tradicts an earlier theory which holds that the phenomenon was associated with intense nuclear reactions in the atmosphere.

The Tunguska meteor seems to have been the most spectac­ular of all those observed from the Earth. It appeared over Si­beria, some 500 miles north of Lake Baikal, on June 30, 1908. Those who saw the meteor in the sky told members of the ex­pedition which investigated the phenomenon that it appeared as a bright fireball in the sky which, according to one observ­er, "made even the light of the sun appear dark."

For all these reasons, it has always been a great surprise that no trace of anything resem­bling a crater has been found in the area beneath the explo­sion.

The most familiar explanation of the phenomenon is that the meteor was really a comet which disintegrated in the at­mosphere, probably at a height of five or six kilometres. There is support for this in the way in which a cloud of dust drifted around the Earth in the weeks after the explosion, increasing the brightness of the night sky and decreasing the intensity of daylight.

Two years ago, however, Pro­cessor VV. F. Libby, of California University, and some of his as­sociates were able to win some public attention for their view that the Tunguska meteor was not a comet at all but, rather, a lump of what is called anti-mat­ter. A comparatively small lump of rock made from anti-matter, probably no more than 10 cen­timetres in diameter, would have been enough to account for the Tunguska explosion.

The success of this argument as an explanation of the Tungus­ka meteor rests on the intensity of the energy released sponta­neously by the interaction of ordinary matter and anti-matter. Weight for weight, this works out at more than a thousand times the energy to be won from nuclear fission or fusion.

Professor Libby and his asso­ciates pointed out that a meteor made of anti-matter would prob­ably be annihilated in the at­mosphere of the Earth, and that the energetic nuclear particles produced in the process would leave lasting effects on the at­mosphere. In particular, the amount of radioactive carbon-14 should be increased by the ex­plosion of the meteor in just the way in which this same iso­tope is continually produced by the interaction of cosmic rays with atoms and molecules in the high atmosphere.

But how is it possible to know whether the atmosphere contained unusually large amounts of car­bon-14 in the years immediate­ly after the explosion of the Tunguska meteor? Professor Libby and his colleagues point­ed out that the analysis of suc­cessive growth-rings in the trunks of trees would provide a means of doing this. There are all kinds of experimental difficul­ties in testing this point, partly because even such a large ex­plosion as the Tunguska meteor would have added a compara­tively small amount to the carbon-14 in the atmosphere already.

There are also problems which arise because living things dis­criminate to a .different degree against the incorporation into their tissues of the various iso­topes of carbon. Nevertheless, Professor Libby considered that they had found evidence of en­hanced carbon-14 in the growth-ring of an oak tree in California corresponding to the year 1909. What has happened now is that a group from the carbon-14 research unit at Groningen Uni­versity have made a careful ana- lysis for carbon-14 in tree-rings in a tree from Norwray, and have found no evidence of the 7 per cent increase of carbon-14 'which Professor Libby and his associates predicted.

The new measurements of tree-rings do, however, show some signs of a periodic fluctua­tion of carbon-14 content with the sun-spot cycle.

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