Around 4.5 billion years ago, something the size of Mars collided with a newly formed Earth, to colossal effect. This object is not only thought to have fused with Earth and primed it for life, it also broke off a large chunk that went on to become the Moon.
This story is known as the giant-impact hypothesis; the Mars-sized object is called Theia; and now, for the first time, scientists believe they've found traces of Theia in the Moon.
The giant-impact hypothesis has been the favoured model for explaining the formation of the Moon for years.
"This model was capable of accounting for the then-recent observations from samples returned by the Apollo missions, which included the Moon's low iron content relative to Earth, depletion in volatiles and enrichment in refractory elements, while avoiding most of the pitfalls of previous lunar origin theories," researchers from the University of New Mexico wrote in their paper.
But there was one big spanner stuck in the works.
Models predicted that around 70 to 90 percent of the Moon should have been made up of mooshed and reformed Theia. However, oxygen isotopes in lunar samples collected by Apollo astronauts were very similar to terrestrial oxygen isotopes - and very different from oxygen isotopes on other Solar System objects.
One possible explanation is that Earth and Theia had similar compositions to start with. Another is that everything got completely mixed during the impact, which, according to simulations, isn't very likely.
Furthermore, the odds of Theia having a similar composition to Earth - as far as oxygen isotopes go - are actually extremely small. Which means that, if the Moon is mostly Theia, its oxygen isotopes should be different from Earth's oxygen isotopes.
This close similarity has been a major pain in the proverbial butt for the giant-impactor hypothesis. Over the years, researchers have published several papers trying to explain it.
That's where the idea that Theia fused with Earth originated. Another paper proposed that the impact created a cloud of dust that went on to become Earth and Moon. Another suggested that perhaps Theia and Earth formed really close to each other. And others have sought to rewrite the history entirely.
Planetary scientist Erick Cano and colleagues went a different route: a careful reanalysis of the lunar samples.
They acquired a range of samples from different rock types gathered on the Moon - both high- and low-titanium basalts from the lunar maria; anorthosites from the highlands, and norites from the depths, brought upwards during a process called lunar mantle overturn; and volcanic glass.
For the new analysis, the research team modified a standard isotope analysis technique to produce high-precision oxygen isotope measurements. And they found something new indeed: that oxygen isotopic composition varied depending on the type of rock tested.
"We show," they wrote in their paper, "that the method of averaging together lunar isotope data while ignoring lithological differences does not give an accurate picture of the differences between the Earth and Moon."
In fact, the deeper the rock sample's origins, the researchers found, the heavier the oxygen isotopes, compared to Earth's.
This difference could be explained if only the outer surface of the Moon was pulverised and mixed during the impact, resulting in the similarity with Earth. But deep inside the Moon, the Theia chunk remained relatively intact, and its oxygen isotopes were left closer to their original state.
The study claims that this is a pretty neat bit of evidence showing Theia could have formed farther out in the Solar System, and moved inwards before the big bada-Moon-making-boom.
Importantly, these results could also tidily clean up that messy problem with the giant-impactor hypothesis.
"Clearly, Theia's distinct oxygen isotope composition was not completely lost through homogenisation during the giant impact," the researchers concluded.
"This result thereby eliminates the necessity for giant-impact models to include a mechanism for complete oxygen isotope homogenisation between the two bodies and provides a foundation for future modelling of the impact and lunar formation."
Humans have not set foot on the Moon since 1972, thus precious Moon rocks available for analysis are in short supply, and replicating these results may be a little tricky for now.
However, within the next few years we might finally see crewed missions execute a long-awaited return to the lunar surface, and can hope for a real boom in Moon science - including further research around the giant-impact hypothesis.
The research has been published in Nature Geoscience.