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I was reading an article today about the 1000th orbit of the Lunar Reconnaissance Orbiter, and as many of you know NASA created an animation that simulates the history of the moon. It is speculated that the moon was formed after a planet the size of mars slammed into the earth and some of that debris and debris from the earth eventually coalesced into the moon. What I am wondering is, is there any speculation as to where this collision with the earth happened, and are there any remnants of that collision?

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I suspect the question may be unanswerable, and possibly even meaningless. As I understand it, the giant impact that resulted in the formation of the Moon would almost certainly have also completely liquefied whatever crust the Earth had at the time, producing a global "magma ocean". Thus, there would've been no traces of the impact left — or rather, the traces would've been spread pretty evenly over the whole planet.

In fact, I just took a closer look at the Wikipedia article I linked to above, and, in the "Energetic aftermath" section, it paints an even more cataclysmic picture of the event:

"In 2007, researchers from the California Institute of Technology showed that the likelihood of Theia having an identical isotopic signature was very small (<1 percent).[12] Instead, they proposed that in the aftermath of the giant impact, while the Earth and the proto-lunar disk were molten and vaporized, the two reservoirs were connected by a common silicate vapour atmosphere, and that the Earth-Moon system was homogenized by convective stirring while the system existed in the form of a continuous fluid."

Yes, apparently these researchers suggest that the Earth's crust was not just liquefied and churned up and splattered all over the place, but that a substantial fraction of it was actually vaporized to temporarily form a common atmosphere of gaseous rock around the Earth and the ejected material that later condensed to form the Moon.

So, no, after such an event I would not expect it to be possible to meaningfully say where on the eventually recondensed blob of rock the initial impact occurred. That would be a bit like pouring some milk into a cup of coffee, stirring it for a while and then asking "which part of the coffee did the milk get poured into?"

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I had a feeling it may be unanswerable, and that was an excellent summary of the wikipedia article that made more sense. –  Chad Ferguson Mar 15 '12 at 0:07
+1 on the coffee analogy. Tomorrow morning I will ask my daughter that question regarding my own cup of coffee! –  dotancohen Jul 19 '12 at 18:14

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