Sunday, 25 March 2012

Moon's Origin. Have I Missed Something Here?

Findings Cast Doubt on Moon Origins - ScienceNOW:

I confess to being puzzled by the above article from Science NOW. The argument goes:

Most scientists believe Earth collided with a hypothetical, Mars-sized planet called Theia early in its existence, and the resulting smash-up produced a disc of magma orbiting our planet that later coalesced to form the moon.

One way to test the hypothesis is to look at the isotopes of particular elements in rocks returned from the moon. Atoms of most elements can occur in slightly different forms, called isotopes, with slightly different masses. Oxygen, for example, has three isotopes: 16O, 17O and 18O, indicating differences in the number of neutrons each nucleus contains. Compare any two samples of oxygen found on Earth and you'll find the proportions of 16O, 17O and 18O isotopes are almost identical in the two samples. The proportions found in samples from meteorites and other planets like Mars, however, are usually different. So if you find that a sample has the same oxygen isotope composition as one from Earth, then it's very likely the sample came from our world.

Previous research has established that the oxygen isotope composition of lunar samples is indistinguishable from that of Earth. Since 40% of the moon is supposed to have come from Theia (which presumably would have had a different isotope composition), this might spell trouble for the giant impact hypothesis. But it's possible that Earth may have exchanged oxygen gas with the magma disk that later formed the Moon shortly after the collision, explaining why the results are the same.

In the new research, published online today in Nature Geoscience, geochemists led by Junjun Zhang at the University of Chicago in Illinois, together with a colleague at the University of Bern in Switzerland, looked at titanium isotopes in 24 separate samples of lunar rock and soil. The proportion of 50Ti to 47Ti is another good indicator of whether a sample came from Earth, and, just as with oxygen, the researchers found the moon's proportion was effectively the same as Earth's and different from elsewhere in the solar system. Zhang explains that it's unlikely Earth could have exchanged titanium gas with the magma disk because titanium has a very high boiling point. "The oxygen isotopic composition would be very easily homogenized because oxygen is much more volatile, but we would expect homogenizing titanium to be very difficult."

I'm no cosmologist, but to me it makes perfect sense for the resulting planet and the accretion disc of resulting magma to be fairly well homogenised on impact rather than the accretion disc being derived mostly from one or the other planet, so I would have thought a result showing the the proportions of 50Ti and 47TI being indistinguishable between the lunar and earth samples was a good indicator of a common origin rather than evidence against it.

But maybe I've misunderstood something...

'via Blog this'

1 comment :

  1. I think there are multitudes of models for this collision. In some a glancing blow would leave the moon mainly made out of the impactor and the earth remaining largely made out of... the earth. In a more full-on impact then the two bodies would be incorporated more fully and mixed.

    Of course another factor would be the source of Theia. If it were from a similar part of the sun's accretion disc to earth then its isotope mix would likely be similar to that of earth. If from a different region then more likely to be different.

    All these variables make for plenty of hypothesis which can then be falsified.

    An interesting aside is the fact that the moon has been tidally locked to the earth. If this were not the case then the ancient tribesmen writing the holey babble would have seen it spin - showing that it was spherical . This may have led to far fewer cock-ups in their creation myth.


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