Artist conception of the giant impact resulting in the formation of our Moon. Image Credit: CORBIS

The most abundant element on Earth is Oxygen, which is responsible for nearly half our planet's mass. Oxygen has three different varieties called isotopes, each with a different mass—Oxygen 16, 17, and 18. Everybody in the solar system has their own unique isotopic signature - Earth is different from Mars which is different from asteroids, which is different from meteorites, etc. With today's technologies, scientists can determine what ratio of these three isotopes are in rocks, thus determining their origin. Imagine their surprise when the the tests indicated that the Earth and the Moon matched.

Thanks to the Apollo program, we have multiple samples of lunar rocks, which are very different from the rocks we have here on Earth. The samples have different mineral abundances and different histories; however, scientists did notice a surprising connection. At the time of its discovery, this link was (and still is) a big deal. Since Oxygen is so prolific, a match like this would not happen by chance.

What The Data Says: 

After analyzing this data, scientists theorized that the Moon was a result of a giant impact with the Earth. Approximately 50 million year after our planet formed, an object roughly the size of Mars, collided with our planet, flinging debris into orbit. This debris eventually, thanks to gravity, combined to form the Moon. However, this model could not fully explain the lunar chemistry - isotopically identical to the Earth, but just slightly different chemically thanks to its formation. The impact theory did fit other parameters such as where the Moon formed, the age of the lunar rocks, and even the rate of the Earth's spin.

Lunar rock sample collected during the Apollo 11 mission. Photo credit: NASA/JSC

This was a sound theory; however, scientists noticed an issue. In order to test the impact theory, scientists used different computer models to simulate the ways in which mass and energy moved during impact. Their results showed the Moon formed from the impactor's material. During the impact, there was a large exchange of mass between the two bodies, and the impactor (known as Theia) should be the main contributor to the Moon. If this is true, why does the Moon's Oxygen isotope concentrations so similar to that of the Earth's? Scientists now believe they have the answer. As technology progresses, we are able to build more accurate equipment and as a result are able to make more precise measurements and detect smaller differences. A research team including Daniel Herwartz, an isotope geochemist at the University of Cologne in Germany, employed a very precise laser-based method in order to record concentrations of Oxygen isotopes in a sample of Earth rocks, meteorites, and the Apollo lunar rock samples. They observed such a minute difference - the lunar sample had 12 parts per million more Oxygen-17 isotopes than the Earth samples. The team suggests that the impactor, Theia, is chemically similar to a class of meteorites known as enstatite chondrites. These meteorites are isotopically similar enough to the Earth, that Theia would not have left any major chemical imprint on the Moon.

What's Next?

Meteorites, like this one found in Botswana in 1999, are often ejected after lunar impacts. Photo credit: Addi Bischoff / Westfälische Wilhelms-Universität

Despite the new measurements, this topic is still a source of debate among scientists. Some say the observed difference is not enough to prove anything significant about how the Moon formed. Others are excited by the findings and believe researchers should reexamine the isotopic differences in other elements such as Silicon and Titanium, looking for any potentially interesting differences. Also the lunar samples collected during the Apollo missions may not be a comprehensive sample and that more samples will need to be collected and analyzed from other locations on the Moon. The team's findings are very exciting, sure to rock the field (pun intended), and is likely to stir up debates. Further research is planned to study ancient Earth rocks, in hopes of determining what (if any) material may have been added to the Earth after this great impact and what sort of significance that may have in these isotopic differences.

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