Astronomy: Capture of the Moon

To the casual reader, the story may sound like a far-out effort at science fiction. But the moon tale told by Swedish Physicist Hannes Alfven amounts to much more than an imaginative voyage into the distant past; it is an ingenious effort to reconstruct a cosmic catastrophe that changed the composition of man's earth and set a new course for the moon more than 2 billion years ago.

Dr. Alfven's theory reaches back to a time when the moon was not yet a satellite of earth, when it soared around the sun like any other planet on its own independent orbit. Trouble was, its orbit took the moon near its large neighbor, the earth. In Icarus, International Journal of the Solar System, Alfven suggests that eventually the moon ventured too close and was captured by the earth's gravity.

Strange Discrepancy. That far, the theory is disarmingly simple. But astronomers can calculate the new orbit of such a recent captive, and the moon does not move along the expected path. Instead of curving along an eccentric ellipse far out into space and then close to earth again, the moon moves along a mildly deformed circle. Such a course would be explainable for a satellite that was moving in the opposite direction from the earth's rotation at the time it was captured. But the moon now revolves in the same direction, and to prove his theory correct. Dr. Alfven somehow had to account for the change.

When the moon was first captured. Dr. Alfven believes, it did indeed curve through space in the opposite direction from the earth's rotation. And, as expected of such a satellite, it drew gradually closer to the earth. Its orbit became circular. About 2.5 billion years ago, the earth-moon system passed through a violent crisis. The approaching moon exerted more and more gravitational pull on the earth's oceans. Tides miles high swept around the globe in a few hours. At last the moon reached Roche's limit,* the closest that a satellite can come to its parent body without being torn to bits by gravitational forces. When the moon passed this boundary, fragments of all sizes began flying off it. Some fell on the earth, heating its atmosphere, churning its surface, forming a halo of dust around it. Any life that existed on earth at the time was probably exterminated. So much moon matter fell on the earth that its high-speed impact changed the earth's rotation.

Buoyant Granite. When the giant meteors stopped falling, and the atmosphere cooled off, the diminished moon, having lost about half its mass, was once more just outside Roche's limit. It was now revolving in the same direction as the earth, and as a result it withdrew slowly to its present distance (240,000 miles).

To support this fast-spinning theory, Alfven points out that the earth's continents are made of comparatively light granitic rock that floats on the heavy basalt underlying the oceans. The basalt, he says, may be the original earth. But the buoyant granite of the continents has about the same density as the moon and nearly equals the moon's present mass. He suspects that it is moon-stuff that dived to earth through the fiery atmosphere 2.5 billion years ago.