New questions have been raised about the birth of the Moon after a new study found samples from Earth and the lunar surface to be virtually identical.
The study reported in the journal Nature Geoscience contradicts the theory that the Moon formed after the impact of a Mars-sized object named Theia with the early Earth about 4.5 billion years ago.
At the time both the Earth and Theia were still partially molten, the impact causing Theia's core to sink into the Earth's core, while lighter ejecta and debris was thrown into space eventually coalescing to form the Moon.
The new study by Junjun Zhang, of the University of Chicago, and colleagues compared isotopic ratios of titanium in lunar and terrestrial samples.
Zhang and colleagues found the ratio of titanium isotopes on both Earth and the Moon to be identical to within about four parts per million.
Numbers don't add up
Because the Mars-sized impactor is expected to have been isotopically different, the measurements suggest the Moon is either made entirely of material from Earth, or intense mixing occurred on both bodies after impact.
Zhang and colleagues say the similarity can't be explained by both bodies forming in the same part of the solar system because meteorite samples show extensive diversity in titanium isotopic ratios.
They conclude the isotopes are far more likely to have come from Earth rather than another planet.
Devil is in the details
Planetary scientist Dr Brad Carter from the University of Southern Queensland says while it's unlikely to have two planets with the same chemical composition, it's not impossible.
"Despite what the paper says, a planet forming very close to the early Earth could have a similar composition resulting in similar isotopic ratios," says Carter.
"It's also possible that Theia was essentially made of ice, something from the Kuiper belt in the outer solar system."
"This would have provided the energy of impact, but with the ice evaporating away leaving Earth material to eject into space and form the moon."
Another option suggested by Zhang and colleagues involved the proto-Earth spinning much faster than previously thought, allowing a greater degree of mixing.
"However we don't really think Earth had spun rapidly enough for that to happen," says Carter.
"And then you have the problem of slowing down the Earth's rotation afterwards."
"Zhang and colleagues suggest a gravitational resonance effect between the Earth, Moon and Sun may have provided the forces needed to slow down the Earth's spin rate," says Carter.
Despite the problems, Carter believes the paper makes a good point about the need for revision of the current theory.
"The simple idea of Theia slamming into Earth remains sound," says Carter.
"But we need to refine current theories that don't explain the incredible similarities of isotopic ratios between the Earth and the Moon."