Ted Nield reports from Nancy on the Giant Impact Hypothesis, which seemed to be moving forward, but is now moving backward.
Geoscientist Online 15 July 2009
Ever since 1984, the chemical and isotopic similarities (and differences) between the Earth and its satellite all apparently lent broad support to the widely accepted hypothesis that our satellite was created by a giant impact, early in Earth history. However, isotope geochemists have now become so good at their jobs that they are now creating difficulties for theoreticians, Professor Jay Melosh (University of Arizona) told delegates today at the Meteoritical Society’s 72nd Annual Meeting, Nancy, France.
It used to be all so neat. The Giant Impact Hypothesis explained so much – from the angular momentum of the Earth-Moon system, to the lack of metallic iron in the Moon’s makeup. The Moon, it was said, was exactly what you’d get if you took some Earth Mantle, vaporised it and allowed it to condense in the vacuum of space so that the volatiles were lost. The proposed giant impact created a magma disc, of perhaps two lunar masses - and from that the Moon condensed. Computer simulations suggested that 70% of the Moon’s mass would have come from the impactor, while only 10% of the Earth’s total mass was contributed by the interloper.
And it is from pulling at that last thread of evidence, that the theory has now begun to unravel, says Melosh. In 1984, our knowledge of the oxygen isotope composition of the Moon was such that the obvious similarities to Earth were not an embarrassment. Because if 70% of the moon came from the impactor, one would expect to see some dissimilarities. Oxygen isotope compositions vary widely between different solar system bodies, and it would be extremely unlikely that a wandering impactor would just happen to possess exactly the same oxygen isotope profile as the object it hit.
But then the isotope measurements got more accurate. Now they are accurate to five parts per million, and the similarities have not gone away. If anything, the increase in accuracy has only served to underline the common identity of the Earth and Moon’s oxygen isotope profiles. So how can this be?
In 2007 Pahlevan and Stevenson, in an influential paper in Earth and Planetary Science Letters, sought to explain how the two bodies could have become isotopically homogeneous. They envisaged a turbulent “atmosphere” of vapour enveloping both proto-Earth and orbiting disc (the proto-Moon). This, they thought, might recycle material from the rapidly spinning Earth, through the vapour phase and into the disc – and back again, thus gradually homogenising the oxygen isotopes between the two. By the time the disc collapsed and the Earth and Moon became finally established, they would have become isotopically identical – while preserving their chemical differences. Hey presto: model saved.