Tuesday, March 27, 2012

Questions Over the Standard Theory of Lunar Formation

The moon's chemical composition matches that of the Earth's mantel, which poses a problem for the standard theory of how the moon was formed:
A chemical analysis of lunar rocks may force scientists to revise the leading theory for the Moon's formation: that the satellite was born when a Mars-sized body smacked into the infant Earth some 4.5 billion years ago.

If that were the case, the Moon ought to bear the chemical signature of both Earth and its proposed 'second' parent. But a study published today in Nature Geoscience suggests that the Moon’s isotopic composition reflects only Earth's contribution.

Junjun Zhang at the University of Chicago in Illinois and her colleagues used a mass spectrometer to make the most precise measurement so far of the relative abundance of titanium-50 and titanium-47 in Moon rocks gathered by the Apollo missions in the 1970s. The authors report that the lunar ratio of the two isotopes is identical to that found in Earth’s mantle, within about 4 parts per million.

This presents a conundrum for the lunar-formation model, Zhang says, because any Mars-sized body that might have collided with the fledgling Earth is believed to have been chemically distinct. Studies of meteorites — the modern stand-ins for planet-sized bodies that once roamed the Solar System — indicate that such objects had an isotopic titanium abundance that could have deviated from the terrestrial value by as much as 600 parts per million. And because simulations suggest that the second body contributed more than 40% of the Moon’s bulk, the lunar isotopic ratio shouldn’t mirror the terrestrial value so closely.

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