IT MAY be grey but it is not as old as you might imagine. A rock thought to date from the moon's formation points to the satellite being about 200 million years younger than previously calculated, suggesting its history may need to be rewritten.
The moon is generally considered to have formed following a collision between Earth and a Mars-sized body in the early solar system. Once the molten debris coalesced into the moon, the theory goes, its crust solidified over several hundred million years. Evidence for an early lunar magma ocean comes from orbital data showing an abundance of plagioclase, a lightweight mineral that crystallises from and floats to the top of magma.
But measuring the age of lunar plagioclase, samples of which were collected by Apollo astronauts, is fraught with uncertainty. The rocks contain only tiny amounts of the lead isotopes normally used to date rocks and are contaminated with lead dust from Earth.
Now Lars Borg at the Lawrence Livermore National Laboratory in California and colleagues have re-dated one of the rocks after washing it with a weak acid to remove the surface layer and any lead contamination. They were surprised to find it was just 4.36 billion years old, meaning it formed about 200 million years after the solar system's first solid materials. The moon was previously thought to have formed within 30 million years or so of the solar system. Dating techniques using samarium and neodymium isotopes pointed to the younger age (Nature, DOI: 10.1038/nature10328).
The team says this hints that the impact that created the moon may have happened later than previously thought or that a magma ocean did not cover the early moon's surface. "The extraordinarily young age of this lunar sample either means that the moon solidified significantly later than previous estimates, or that we need to change our understanding of the moon's geochemical history," says team member Richard Carlson of the Carnegie Institution for Science in Washington DC.
But Clive Neal of the University of Notre Dame in Indiana says some of the plagioclase - including this sample - might simply have melted again after the moon formed. Different minerals solidify at different temperatures, so if a heavy mineral solidified before a lighter one beneath it, it would sink, pushing magma upwards. This could melt the plagioclase and reset its age. "I remain to be convinced that the moon is as young as suggested by this paper," he says.