A chemical quirk found in lunar soil backs the theory that the moon was born from a collision between earth and a huge space rock, scientists said.
Astronomers proposed at a conference in 1975 that our satellite was created through a smash-up between the infant earth and a Mars-sized body they named Theia, the mother of the moon in Greek mythology. 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 - about a quarter the size of earth - and so near.
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 were condensed more swiftly in the vapour cloud than lighter ones. The tiny difference is called isotopic fractionation.
"The magnitude of the fractionation we measured in lunar rocks is 10 times larger than … in terrestrial and martian rocks," said Frederic Moynier, assistant professor of earth and planetary sciences at Washington University in St Louis, Missouri. "It's an important difference."
The great impact theory could be the key to understanding why earth is 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 James Day of the Scripps Institution of Oceanography in California.