The moon may be the closest object to us in space, but scientists are still struggling to understand how it got there.
Most planetary researchers think the moon was created as the result of a collision between the Earth and a long-hypothesized protoplanet called Theia about 100 million years after the birth of the solar system.
But whether that impact was a glancing blow or a full, head-on crash is still up for debate.
This week researchers at UCLA provided new evidence in the journal Science that the collision was head-on, and so powerful that materials from both bodies mixed completely before settling into the Earth-moon system we know today.
To come to that conclusion, the researchers analyzed seven lunar rocks collected by the Apollo 12, 15 and 17 missions, as well as six volcanic rocks that include material from Earth's mantle. Specifically, they wanted to see if the ratio of oxygen isotopes in lunar rocks was the same as that in the terrestrial rocks.
More than 99.9% of Earth's oxygen is O-16, which means each atom has eight protons and eight neutrons, but there are also small amounts of the heavier isotopes O-17 and O-18 in our planet's oxygen mix.
"Every rock in the solar system we've ever found has a unique fingerprint of oxygen isotopes," said Edward Young, a geochemist at UCLA and the first author on the study. And yet, his analysis shows that the moon and Earth have the exact same oxygen isotope profile.
The only way to explain this finding is if Earth and the moon are made of the exact same material.
And herein lies the challenge.
The easiest way for computer modelers to explain the physics of the Earth-moon system, such as how fast our planet spins on its axis and how fast the moon orbits Earth, is to have Theia giving Earth a glancing blow strong enough for both bodies to become molten. In this scenario, most of the Theia material mixes with the Earth's, but a small portion of it forms the moon.
If that's the case, the moon and Earth should have a different chemical fingerprint. Though scientists expect Theia and Earth to have a similar oxygen isotope ratio, they would not expect it to be identical, Young said.
"Even if the proto-Earth and Theia were very similar, they couldn't possibly be this similar," he said.
Back in 2012 two groups of scientists presented computer models that show an alternative story of Earth-moon formation. In their simulations, the two bodies collide head-on, allowing for extensive mixing between Earth and Theia. Those models have since been dismissed as "too special," said Sarah T. Stewart of UC Davis, who was a coauthor on one of those studies.
"The odds of getting one of these perfectly mixing impacts is so low that it is probably not the best solution," she said.
But Young thinks the team was on the right track. "The collision must have been very energetic for everything to melt together," he said.
Stewart, who was not involved in the new study, said the work by Young and his colleagues provides the best measurement yet of the oxygen isotope ratio in the two bodies, and provides a challenge to her and her colleagues.
"We have a very big science problem of how did our neighbor form," she said. "The geochemists have told us the answer, but the physicists have not been able to get us there."