Friday, April 6, 2012

Does New Data Challenge Giant Impact Theory?

Image Credit: Joe Tucciarone

Sometimes referred to as "the big splat" or "the big splash" hypothesis, the giant impact hypothesis is currently the most widely accepted explanation for the formation of Earth's only moon. According to this theory, sometime in our planet's distant past -as in, around 4.6 billion years ago- Earth was struck by a Mars-sized object dubbed Theia. This impact resulted in the ejection of a portion of material from the Earth's surface, which, along with Theia's remains, eventually coalesced to form the moon we see today. While there are, of course, several popular alternate explanations for the formation of our moon, none of them are as well supported by either physical observations or experimental results as the Giant impact theory; hence it's place of prominence on the issue.

One of the methods of testing the giant impact theory is to compare the isotope structure of basic elements found within rocks both on the moon and here on Earth. Because the the number of neutrons contained within each elemental isotope's nucleus can change, finding, say, an oxygen isotope with same atomic structure to be present in both samples, is a strong indicator that the samples in question originated from the same place. Which is exactly what previous analysis of lunar materials have shown.

But if 40% of the moon is made of the remains of the hypothetical object known as Theia, as is proposed by the giant impact theory. Then one would expect to eventually find some variance in the isotope structure of elements found in materials on the moon vs those on Earth, as those originating from Theia would presumably have been slightly different. But so far, that isn't the case. In fact, a recent analysis conducted by geochemists led by Junjun Zhang at the University of Chicago in Illinois, together with a colleague at the University of Bern in Switzerland, found, not only oxygen isotopes, but that also titanium isotopes taken from lunar materials, were identical to those found on Earth, which is a significant result. Because, while oxygen isotopes on each body could have been homogenized as a result of the formation process (meaning, oxygen isotopes originating from Theia cold basically have been "burnt off" and replaced) the high boiling point of titanium makes the same explanation an unlikely one for the presence of identical titanium isotopes.

So if the Giant impact hypothesis is correct, Thia really was a thing, and it's remains now account for 40% of the material on the moon, then why haven't we found any pieces of it yet?

The answer, of course, could be as simple as scientists having over estimated the size of Theia, that we merely haven't managed to collect a sample of it's remains yet, or some other variable or detail that we have yet to discover. But while it's an absolute certainty that the details of the Giant impact theory will change over time as new discoveries are made, and I think instances like this are both interesting and important examples of how the scientific process works. I think it's unlikely that this single data set will lead to the dethroning of the theory, and I'm personally not even convinced that it's a significant contradiction of previous findings, as the giant impact theory still accounts for more observed physical effects both on the Earth as well as the moon, than any of the alternate lunar formation theories around today.


VIA: ScienceNOW

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