The most accepted hypothesis for the formation of the moon involves a giant collision between the Earth and another Mars-sized planetary body about 4.5 billion years ago. The debris that resulted from this impact coalesced into the moon. While this sounded like the most reasonable scenario and there were many clues suggesting an impact, we've never been able to detect any evidence of that other planetary body on the moon. Now, three lunar samples that were returned in the Apollo missions have now revealed traces of material that is believed to have come from that other planetary body, known as Theia. The results of the study were published in Science.

Though computer simulations had shown that the Giant Impact Hypothesis would result in a moon made up primarily from Theia’s material, all prior analyses had indicated that the moon was completely made up of material from Earth. However, a recent, more sensitive round of testing has shown that there are key differences between the moon and the Earth, which the researchers say supports the impact hypothesis.

Some are skeptical, and believe that the differences between the moon and Earth stem from changes in materials on Earth after the moon’s formation. The differences are also quite subtle, as they identified small differences in oxygen isotopes within the rock samples. As meteorites indicate that the solar system is riddled with things of completely different compositions, it almost doesn’t make sense that Theia, which is believed to have been the size of Mars, would only have such a minute difference.

However, it could be that Theia and Earth were formed in close proximity, and would therefore be made out of the same material. If that is true, it challenges the idea that planetary bodies have unique “fingerprints” that allow them to be easily identified. This concept is based on the idea that meteorites originating from Mars have a highly different composition, though we don’t have any samples from Mercury or Venus to confirm the composition of all planets in the inner solar system. 

Alternative hypotheses for the moon’s formation include that Theia was larger than Mars and closer to the size of Earth. At impact, the majority of the debris would have come from Earth. Another one states that a nuclear explosion deep below the surface ejected the material out, which then formed the moon.

While the preliminary results are fairly exciting, their implications are pretty staggering, which is why researchers need to be very cautious moving forward. More samples will need to be analyzed in order to confirm these findings. Ideally, future manned missions to the moon would collect samples below the surface that haven’t been weathered by solar wind and impacts from space debris. While manned lunar landings have been proposed by the space programs India, Japan, Europe, China, Iran and Russia, none of them have been funded as of yet. 

[Hat tip: Pallab Ghosh, BBC]