Violent Beginning To History Of Moon Indicated By Tides

Press, Volume CVI, Issue 31320, 16 March 1967, Page 11

Violent Beginning To History Of Moon Indicated By Tides

(By ROBERT C. COWEN, natural science editor of the "Christian Science Monitor") The rhythmic rise and fall of the tide hints at an ancient cosmic drama. It suggests a time when the moon was close to the earth. It implies violent interplay between the two bodies that may have produced some of their most prominent features.

Dr. S. Fred Singer, of the University of Miami, has made one of the latest studies. He thinks that the earth’s oceans and atmosphere may have originated in such an encounter. He thinks also that the dark seas (maria) and the heavy peppering of craters on the moon may be due to the forces involved. Moon’s Spin Slowed

These are the forces that today are typified in the tides. Their action raises tides in the atmosphere and in solid rock, as well as in the sea. As these tides wax and wane, they generate friction that ch inges the rotation of the planet. The spin of the moon has slowed so much that our satellite now turns on its axis only once a month. It keeps the same face aimed at the earth as it goes around in orbit.

The effect on the earth’s spin has been less drastic. Nevertheless, the earth’s rotation seems to be slowing by about one second a century. Dr. Singer thinks there was a time when the day was only five hours long. Orbit Affected

The play of forces also affects the moon’s orbit As th ) planet slows its spin, the satellite swings in r steadily expanding orbit In this way, the moon is thought to have gradually moved away from the earth over several billion years.

The principle of all this is fairly simple. But working out its consequences is a com-

plicated mathematical exercise. Until recently, interested scientists have made do with rough figuring. With high-speed computers, they now are beginning to work out the problem in detail. This has enabled Dr. Singer to trace the history of the earth-moon system as though he were running a movie backward. Contraction Seen

As the “film” reverses, the moon’s orbit contracts. The moon comes so close to the planet that its distance is less than three times the radius of the earth. Today, it is some 60 earth radii away. Then, as the “film” continues to run backward, the moon’s orbit becomes a stretched-out ellipse. Dr. Singer thinks this indicates the moon may have been captured by the earth several billion years ago.

Some scientists think that earth and moon formed initially as a two-body system. They find it hard to understand how the moon could survive being captured as Dr. Singer suggests. Within three earth radii, they point out, the moon would be in a region where tidal forces should tear it apart. But Dr. Singer- says this objection holds only for a body that orbits within that region for some time. In his theory, the moon spent only part of each orbit there. It was not in the region long enough to be destroyed. Captured By Earth

He envisions the moon and earth as formed independently. Then, perhaps three to four billion years ago, the moon passed close enough to the earth to be captured. The earth then may have had no seas. It may have lost its primitive atmosphere.

While the moon’s orbit was elongated so that it passed quickly through the destructive zone on each orbit, tidal forces were severe. It is at this point that Dr. Singer thinks the day was five hours long. The moon also revolved in its orbit once every five hours. The tidal interaction was intense.

Tidal friction would have heated both earth and moon strongly. Perhaps it at least partially melted them. The heat may have driven gases from the rocks. It may have given earth an atmosphere of water vapour, carbon dioxide, and nitrogen. The water vapour would then condense to form oceans. At the same time, chunks may have been pulled off the moon only to fall back on to it.

The impacts of these may have caused the maria and many of the craters on the lunar highlands. Dr. Singer’s calculations, based on tidal theory, merely show how the moon’s orbit probably evolved. The rest of this dramatic story is mere conjecture. “However,” Dr. Singer says, “as a simple minded explanation, I think it most likely.”