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Moon rocks yield up first direct evidence of ancient impactor, Theia

Credit: NASA/JPL-Caltech

Credit: NASA/JPL-Caltech

Scott Sutherland
Meteorologist/Science Writer

Thursday, June 5, 2014, 6:28 PM - Billions of years ago, as our planet had just finished forming, a Mars-sized object slammed into it, melting and merging with the Earth, but also blowing off more than a billion-trillion tons of combined material which eventually coalesced to form the Moon. Until now, this has simply been a hypothesis, based on theory and computer simulations, but a new study of Moon rocks has revealed the very first direct physical evidence of this massive impactor.

The breakthrough came from a study of isotopes of oxygen - that is, oxygen atoms with a different number of neutrons than your 'standard' oxygen. The vast majority of oxygen we encounter (in our atmosphere or elsewhere) is oxygen-16, with 8 protons and 8 neutrons, but there are other stable isotopes of oxygen, specifically oxygen-17 (8 protons and 9 neutrons) and oxygen-18 (8 protons and 10 neutrons).

Although scientists believe that that this object - dubbed Theia - formed from roughly the same materials as the Earth, they have expected that it would have at least some small differences in the isotopes of elements in its composition, since no two planets in the solar system have exactly the same composition. However, studies looking for this difference in Moon rocks, either ones that fell to Earth as meteorites long ago or the ones brought back by NASA astronauts, have always turned up nothing... until now, that is.

A team of German researchers, led by Daniel Herwartz, of the University of Cologne, came up with a new technique of analysis, one that Herwartz told Reuters "guarantees perfect separation" of oxygen isotopes. Their analysis discovered that the Moon rocks returned by the Apollo missions had around 12 parts per million more oxygen-17 than is found in rocks from Earth. This is such a small amount that it simply wasn't detected before now.

"The differences are small and difficult to detect, but they are there," Herwartz told Reuters in an email.

Past simulations haven't been very good at narrowing down how much of the Moon came from Theia - it's ranged anywhere from 8 per cent all the way up to 90 per cent. Based on these latest results, though, the researchers believe it's somewhere in the middle, closer to 50 per cent Theia and 50 per cent Earth. It will take further study to narrow it down any further, but in the mean time, knowing this difference in isotopes will help researchers adjust the size and composition of Theia in their simulations.

According to Robin Canup, a planetary scientist with the Southwest Research Institute in Boulder, Colorado, the small difference in oxygen isotopes found will likely bring about some debate as to whether the results are real. 

"Because this is such an important issue," she told Discovery News, "this paper is likely to prompt additional work and critical debate on this topic, which will be great for the field no matter the final answer."

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