Our Moon could be a chip off the old block, according to a study that shows it is mostly made of material that originated on the early Earth.
As well as untangling how the Moon formed, researchers also found evidence of how water came to our planet.
The prevailing theory is that our Moon was created when a Mars-sized protoplanet, named Theia, collided with the early Earth.
The Moon then coalesced out of the combined debris.
New evidence for the Theia impact
The study looked at 14 lunar rock samples, comparing them to similar minerals found on Earth by measuring levels of oxygen-17, a rare isotope of oxygen.
Both lunar and terrestrial samples had very similar levels, suggesting they shared a common source.
This would mean that most of the material which formed the Moon was thrown off the early Earth during this impact, with little contribution from protoplanet Theia.
"One explanation is that Theia lost its rocky mantle in earlier collisions and then slammed into the early Earth like a metallic cannonball," says Andreas Pack from Göttingen University, who took part in the study.
"If this were the case, Theia would be part of Earth’s core today, and the Moon would have formed from ejected material from Earth’s mantle.
"This would explain the similarity in the composition between the Earth and the Moon.”
Earth and its water
The measurements have also narrowed down another of our planet’s great mysteries: where did its water come from?
Our current best theory here is what’s known as the ‘late veneer event’ – water was brought to the surface by asteroid impacts after the Moon was created.
The similar levels of oxygen-17 found in the study allowed scientists to home in on the type of meteorite that brought water to Earth, a class called enstatite chondrites, since their composition matches the study’s findings most closely.
