Scientists Meet to Debate Lunar Origins

By 1964, the question of how the moon originated had generated sufficient interest to merit a conference devoted entirely to that subject. At that meeting in New York City, much attention was paid to the fission theory proposed by George Darwin. Darwin's suggestion that the moon was created from molten material spun off by the Earth seemed plausible for a time when our planet rotated much faster than it does now, possibly once every 4 hours instead of once every 24 hours. By the time of the conference, however, calculations had shown that even such a high rotational speed would not have been sufficient to form the moon. Friction in the molten rock would not have permitted a bulge of matter to rise high enough to be flung free of Earth's gravity.

In light of that finding, proponents of the fission theory at the conference suggested another possibility. The material that formed the moon, they said, might have been spun off when the Earth was rotating not once every 4 hours but once every 2.1 hours. If a molten Earth had ever rotated that fast, they pointed out, its mechanical strength and gravity would simply not have been able to hold the planet together. The Earth would have hurled great amounts of matter into space in a process called rotational instability.

Proponents of the modern fission theory calculated exactly how such a process could take place. The birth of the moon would not have occurred as a catastrophic outburst, such as an explosion, but as a progressive change in Earth's shape. First, the Earth's rapid rotation would have flattened the planet at the poles, deforming it into an oblate shape—wider at the equator than across the poles. Next, the spinning, flattened Earth would have produced a neck of material from a point on its equatorial bulge. The neck would have flown off into space as a large blob trailed by smaller blobs that would soon have fused into one. But a major flaw in the fission theory, other scientists noted, was that in order for fission to occur, the Earth would have been spinning so fast that it would still be turning much more rapidly than it is today.