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Ironclad proof of the moon's origin?
by William R. Corliss
Science Frontiers #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 melting point) compounds. The moon, therefore, almost certainly originated elsewhere, contrary to what most astronomers have long believed. 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)

The abundance of high melting point compounds on the lunar surface shows once again the consequence of near-zero atmosphere, and impact as the dominant force at work on the moon. I'm amused that the supposed match of oxygen isotope ratios on the one hand, and a lack of a match in other materials, are both used to "prove" the impact-fission origin. Heads they win, tails you lose.

Okay, there is at least one advocate of coaccretion:

Did the new moon lose its iron heart?
New Scientist
January 23, 2007
The current theory says that the material that now forms our moon was ejected when Earth was struck by another planet-sized body. But Peter Noerdlinger at Saint Mary's University in Halifax, Canada, says this theory has problems. "The collision has to be implausibly gentle. You practically need someone to hold a Mars-sized object just above Earth and drop it, to avoid messing up Earth's orbit."

The simpler idea that Earth and the moon were both created from the same gas cloud had been rejected because it could not explain why Earth formed an iron core and the moon did not. Now, Noerdlinger has an answer for that.

He suggests that the proto-moon did have an iron core, but that the satellite was ripped apart in a close encounter with Earth. His calculations show that iron from the core would be pulled towards Earth, while the remains of its rocky outer shell reassembled into our iron-free moon.

This fits with evidence that the Earth acquired a veneer of iron after it formed, Noerdlinger says. He presented the work at the American Astronomical Society meeting in Seattle, Washington, last week.

From issue 2587 of New Scientist magazine, 23 January 2007, page 16