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No. 37: Jan-Feb 1985

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The puzzle of the moon's origin

The moon is the closest and best-studied astronomical object. Yet, there is no agreement as to its mode of origin. One might say that planetary scientists have just about thrown in the towel on the three major theories of lunar origin. Two recent articles attest to this discouraging situation.

A Sky and Telescope article provides an excellent review of all three theories, indicating the reasons why each fails to convince a majority of scientists. The theories and the primary reasons for their rejection are:

(1) Fission from earth. Lack of sufficient angular momentum in the earthmoon system and the fact that the moon does not orbit in the plane of the earth's equator. (2) Gravitational capture. The capture of such a large object in a nearly circular orbit is considered too improbable. (3) Earth-moon accretion as a double planet. The compositions of the earth and moon are too different.

This article concludes that the resolution of the problem of lunar origin must await our return to the moon for more scientific exploration.

(Rubin, Alan E.; "Whence Came the Moon?" Sky and Telescope, 68:389, 1984.)

An article in Science also discusses the classical theories of lunar origin and quickly disposes of them for the above reasons. However, a fourth theory makes an appearance, which we might call the Big Splash Theory. The idea is that a Mars-sized object (1/10 the earth's mass) made a grazing collision with the earth when the solar system was more heavily populated with debris. This collision vaporized the projectile's rocky mantle as well as a similar quantity from the earth. This material was flung into orbit by the force of the impact and the expanding hot gases. Beyond the Roche Limit, the expelled debris coalesced into the moon. This fourth theory is now seriously considered by planetary scientists who -- so far -- have found no serious defects.

(Kerr, Richard A.; "Making the Moon from a Big Splash," Science, 226:1060, 1984.)

From Science Frontiers #37, JAN-FEB 1985. � 1985-2000 William R. Corliss