About 4.5 billion years ago, the solar system was a very different and much more dangerous place. The planets were still forming, with gravity pulling collections of material together. The orbits of the various planets weren’t completely defined yet and collisions between planets, comets and asteroids weren’t uncommon.

The leading theory on how the moon was formed states that during this tumultuous time, the Earth and another planet called Theia, named after a goddess in Greek mythology, collided. The impact sent large amounts of material into space and the largest collection of this material became the moon.

Until now, no one has been able to test samples to prove the theory. Recently, a team of German researchers, led by Dr. Daniel Herwartz, were able to test oxygen isotopes in moon rock. The samples were collected during the Apollo missions and provided by NASA.

Although the differences were small, the researchers claim that they are significant enough to support the Theia theory. Their work will be published in the June 6th edition of the journal Science and presented to the Goldschmidt geochemistry conference in California on June 11.

Previously, the team attempted to test lunar samples collected from meteorites, but the samples had exchanged isotopes with water from Earth and could not provide useful results. The NASA samples, however, showed higher levels of 17O/16O than Earth samples.

“The differences are small and difficult to detect, but they are there. This means two things; firstly we can now be reasonably sure that the Giant collision took place. Secondly, it gives us an idea of the geochemistry of Theia. Theia seems to have been similar to what we call E-type chondrites. If this is true, we can now predict the geochemical and isotopic composition of the moon, because the present moon is a mixture of Theia and the early Earth. The next goal is to find out how much material of Theia is in the Moon,” said Dr. Herwartz in a statement.

Most models estimate that the moon is composed of between 70 and 90 percent material from Theia, with the other 10 to 30 percent of material coming from Earth. According to Herwartz, the data indicates a mixture closer to 50-50.

Researchers used an “an advanced sample preparation technique” before measuring the samples with stable isotope ratio mass spectrometry. The results showed a 12 parts per million (± 3 ppm) difference in the 17O/16O ratio in the moon rocks compared to those found on Earth.

However, some scientists are skeptical of the findings. Meteorite samples from Mars and points further out in the solar system show vastly different ratios. The similarities between the Earth and moon rocks were surprising to many.

“What you are looking for is a much bigger difference, because that is what the rest of the Solar System looks like based on meteorite measurements,” Professor Alex Halliday of Oxford University told the BBC.

However, Halliday concedes that planets formed in close proximity to one another might share similar characteristics. To date, no samples from Mercury or Venus have been tested. Before anything can be known for certain, additional samples of rock from other parts of the moon will have to be collected for testing.