No. 101: Sep-Oct 1995
Did earth and moon "coaccrete" at the same time? That is, did two clouds of debris simultaneously collect and coalesce into two rough spheres, which then began orbiting about a common center of gravity? Or, perhaps the earth and moon were once a single mass that ultimately fissioned due to the gravitational tugging of a passing massive object. If either of these scenarios were correct, earth and moon would have similar bulk compositions. This, however, does not seem to be the case.
The abundance and distribution of iron on the moon's surface, as measured by the lunar probe Clementine, indicates that the moon is richer than the earth in refractory (high meltingpoint) compounds. The moon, therefore, almost certainly originated elsewhere, contrary to what most astronomers have longbelieved. Given the constraints of celestial mechanics, the most likely hypothesis postulates a colossal impact involving protoearth and the interloping protomoon. After considerable havoc, the two battered spheres settled down into their present configuration. Thus expire the two most popular theories of the moon's origin. (Lucey, Paul G., et al; "Abundance and Distribution of Iron on the Moon," Science, 268:1150, 1995)