A chemical quirk found in lunar soil backs a 37-year-old theory that the Moon was born from an apocalyptic collision between Earth and a huge space rock, scientists said on Wednesday.

Way back in 1975, astronomers proposed at a conference that billions of years ago, our satellite was created through a smashup between the infant Earth and a Mars-sized body they named Theia, in Greek mythology the mother of the moon, Selene.

The collision melted and vaporised Theia and much of Earth's nascent mantle, and the rock vapour condensed to form the Moon.

This would explain why the Moon is so big -- it is about a quarter the size of Earth and the fifth biggest satellite in the Solar System -- and so near to us.

For years, the "Giant Impact Theory" lingered in the margins until computer simulations showed that it could be true.

Sifting through precious grains of lunar soil brought back by the Apollo missions, researchers say they have now found chemical proof to validate the concept.

It lies in a minute excess in a heavier isotope, or atomic variant, of the element zinc.

This enrichment would have happened because heavier zinc atoms would have been condensed swiftly in the vapour cloud rather than lighter ones.

The tiny but telltale difference is called isotopic fractionation.

"The magnitude of the fractionation we measured in lunar rocks is 10 times larger than what see in terrestrial and martian rocks," said Frederic Moynier, an assistant professor of Earth and Planetary Sciences at Washington University in St. Louis, Missouri.

"It's an important difference."

The fractionation was sought in 20 samples of lunar rocks from four Apollo missions, which explored different areas of the Moon, and from one lunar meteorite.

These were matched against 10 meteorites that have been identified as being martian in origin, including one that was in collection at the Vatican, and against rocks found on Earth.

Analysis by a mass spectrometer -- in which light from a vaporised sample points to the elements in it -- showed that zinc in general was severely depleted on the Moon, but bore the signatures of heavier isotopes.

Large-scale evaporation of the zinc points to a mega-event like the collision, rather than localised volcanic activity, the researchers contend.

"You require some kind of wholesale melting event of the Moon to provide the heat necessary to evaporate the zinc," said James Day of the Scripps Institution of Oceanography in California.

With this success, say the researchers, the Great Impact Theory could be the key to understanding another mystery: why is Earth so endowed with water but the Moon so dry?

"This is a very important question, because if we are looking for life on other planets, we have to recognise that similar conditions are probably required," said Day.

"So understanding how planets obtain such conditions is critical for understanding how life ultimate occurs on a planet."

The study appears in the British journal Nature.