Friday, Feb. 03, 1967

New Twist for an Old Theory

Although the moon has lately been giving up many of its ancient secrets to prying spacecraft, it has clung stubbornly to one--the genesis of its own existence as an earth satellite. With monotonous regularity, scientists have punched holes in theories that the moon was torn, Eve-like, from the earth's side; that the earth and moon condensed simultaneously, as neighbors, from the same blob of primordial dust; or that the moon was a planetary interloper accidentally captured by the earth's gravity. Says Nobel Laureate Chemist Harold Urey: "All explanations for the origin of the moon are improbable."

Airless Orb. Undaunted, a University of Miami scientist with credentials in the field of cosmogony-has resurrected one of the old theories and given it a new twist that he feels will enable it to pass the mathematical and dynamical tests its predecessors failed. Physicist S. (for Siegfried) Fred Singer suggests that the moon first evolved as a minor planet, independent of the earth and following its own orbit around the sun. About four billion years ago, he believes, its path carried it on a near-collision course with the earth, which at that time was an atmosphereless orb revolving once every five hours. Captured by terrestrial gravity, the moon was pulled into an elliptical orbit around the earth, passing as close as 10,000 miles away at perigee and swinging hundreds of thousands of miles out at apogee.

The gravitational force of the new satellite was so powerful that it raised great "tides" in the solid earth, literally causing it to bulge in the direction of the moon (a smaller bulge was raised on the opposite side of the earth). Because the moon moved more slowly across the sky when it was at its far-out apogee than the surface of the earth revolved below it, the bulge tended to lead the moon. Its gravity thus pulled forward on the moon. At perigee, when the moon was moving across the sky faster than the earth's rotation, the bulge lagged behind, exerting a backward pull. When these effects are taken into account, Singer says, some of the old mathematical and dynamical objections to the moon-capture theory disappear.

Pock-Marked Face. The lunar tides when the moon was near produced friction and violent heating of the interior and surface layers of the earth, Singer believes. This could well have led to the sudden degassing of rocks, volcanic activity and the creation of an atmosphere that probably consisted of water vapor, carbon dioxide and nitrogen. Thus, the capture of the moon by the earth may well have produced an atmosphere much earlier in the earth's history than anyone had heretofore believed-- and led to the evolution of life itself. Terrestrial gravity had an even more spectacular effect on the newly arrived moon. In addition to tidal heating and volcanic activity on the moon as it approached perigee, great chunks may well have been torn from the lunar surface--only to fall back onto the moon near apogee, when the earth's gravitational force was lessened. This recurring bombardment could account for the moon's pock-marked face. Singer calculates that within a few thousand years after the encounter, the moon's orbit decayed from an elongated ellipse into a near-circular path only about 10,000 miles above the earth. At this point, it was in near-synchronous orbit.

Soon afterward, the moon's orbit began gradually spiraling outward to its present 239,000 miles. The tidal phenomena, though substantially reduced by distance, are still at work. The moon is still receding from the earth by about one inch every year. And the tidal braking effects are still at work increasing the length of the earth's day by .0018 seconds every century.

Further confirmation of the lunar capture theory may have to await the arrival of men on the moon, Singer believes. When samples of the lunar surface can be returned to earth and analyzed, it will be possible to determine when substantial heating and volcanic activity first occurred. If it turns out to be four billion years ago--the same time that the earth is known to have been subjected to intense heating --Singer will have powerful evidence that a single event was responsible: the capture of the moon by the earth.

* The study of the origin and development of the universe and its members.

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