Science: New Evidence that Collision Produced Earth's Moon

The study provides the first isotopic evidence that the moon formed from a collision between Earth and a mysterious planetary object.
 
Was the moon born in a cosmic smash-up, or slowly shaped out of stardust like the Earth? Learn more in this episode of Science with a Twist, a new video series from AAAS. | AAAS/Carla Schaffer

The origins of the moon have been as murky as a black hole's interior, with several hypotheses competing to explain this pockmarked body's beginnings. But, a new series of measurements published in the 6 June issue of Science makes one hypothesis the clear leader.

This analysis of lunar samples shows for the first time that the moon's chemical composition is distinct from that of Earth. The finding supports the prevailing origins story for the moon called the Giant Impact Hypothesis, in which the moon was born out of a collision between a young Earth and a solid object of unknown composition called Theia around 4.5 billion years ago.

"I think that most people believe in the Giant Impact Hypothesis," said the study's lead author, Daniel Herwartz, a geologist at Georg-August-Universität in Göttingen, Germany, "and now that we solved one of the last gaps in explaining it, those who criticize it will have a hard time."

Debris from both the Earth and Theia shaped the moon, as the hypothesis goes, with Theia contributing significantly more material. But until now, this has been hard to prove because scientists haven't been able to find evidence of anything other than earthly debris in the moon's makeup.

Most planets in the solar system have their own unique composition that can be discerned by looking at isotopes, or variants of chemical elements like oxygen in cosmic samples. For the Giant Impact Hypothesis to hold true — implying a unique cosmic body slammed into Earth, and its collision-born debris contributed significantly to making the moon — the Earth and the moon also should contain different ratios of elemental isotopes.

Now, a group of German researchers led by Herwartz has found something distinct in lunar samples: an isotopic signature dissimilar from that of Earth's. To make this discovery, Herwartz and colleagues first looked at ratios of oxygen isotopes in lunar meteorites that had fallen to Earth. But, the samples were so weathered that it was very difficult to detect differences in the isotope ratios in these lunar samples and earthly ones.

The researchers analyzed fresher lunar samples, provided by NASA from three Apollo landing sites. They also applied a more sensitive isotope analysis technique, which guaranteed the separation of the samples' oxygen gases from other trace gases, Herwartz explained.

Applying this advanced analytical technique, Herwartz and his team were able to detect a slight but distinctly higher composition of a particular oxygen isotope in the lunar material. "The differences are small and difficult to detect, but they are there," Herwartz explained. "This means we can now be reasonably sure that the giant collision took place."

The isotopic evidence also gives the researchers an idea of Theia's geochemistry, which appears to be similar to that found in rare forms of stony, non-melted meteorites called enstatite chondrites.

The team's next goal is to find out how much material from Theia is in the moon. Though most models estimate that the moon is composed of around 70-90% material from Theia, some models argue for as little as 8% material from Theia. Herwartz said that the new data indicate that a 50/50 mixture seems possible, but this will need to be confirmed in future studies.