Chunks off the Moon

The earth and its nearby partner the moon live in an orderly neighborhood; only at vast intervals, millions of years apart, is the area blasted by trouble. Then a giant meteor, perhaps a wanderer from the asteroid belt between the orbits of Mars and Jupiter, streaks into range. If it happens to hit the earth, it blasts a crater many miles across, sometimes melting nearby rock and spewing out slaglike material called impactite. If it collides with the moon, the crashing meteor produces glassy objects called tektites, which many scientists believe are knocked out of lunar craters, solidified in space and dropped on earth.

High-level argument about tektites and impactites has hung on for years, but at last week's New York meeting of the Geological Society of America, Drs. Robert L. Fleischer and P. Buford Price of General Electric Co. produced some of the first hard facts about them. Using a new dating method, the G.E. scientists proved that most tektites were formed either 34 million, 15 million or 700,000 years ago, and that known deposits of impactites have the same three ages.

Damaged Spots. The General Electric dating method, which was developed with Air Force backing, depends on the fact that nearly all rocks, including tektites and impactites, contain small traces of uranium. The uranium atoms split, at a slow, known rate and the fission fragments damage the glassy material in which they are embedded. The damaged spots are microscopic, but they can be made visible by a special etching technique. When they are carefully counted and compared with the amount of uranium present, those spots tell how long they have been accumulating and the date when the rock solidified.

The G.E. scientists tested their new dating system on tektites found in Canada and the U.S. All proved to be 34 million years old. Impactites from the Clearwater Lake crater in northern Quebec and from far-off Libya have the same age. Other tests show that tektites found in Czechoslovakia pair up with impactites from an ancient meteor crater in Germany. Both are 15 million years old. An impactite from Tasmania is 700,000 years old, the same age as tektites found in Australia, Indonesia and Southeast Asia.

Three Hits. Why should tektites and impactites have the same ages? One explanation, think Fleischer and Price, is that when very large meteors hit the moon they do more than splash out molten moon-rock that falls to earth as small, harmless tektites. They also detach large chunks of the lunar crust heavy enough to blast craters and form impactite when they hit the earth's surface. This has happened, the scientists think, at least three times in rather recent geological history. And they suspect that a lot of moon-stuff will be found on earth as soon as fellow scientists figure out how to identify it.

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