The moon was born a mere 60 million years after the formation of the solar system, according to lunar samples brought back by Apollo 14 astronauts.
Melanie Barboni from the University of California, Los Angeles and colleagues analysed the chemistry of eight zircon grains in rock and soil, picked up during the 1971 moon mission, and found they suggest the moon was mostly solidified around 4.51 billion years ago.
The moon’s birthdate, which is tens of millions of years earlier than some other estimates, was published in Science Advances.
With the odd exception, planetary scientists think the moon was produced when the early Earth collided with another protoplanet, dubbed Theia, in the first couple of hundred million years of the solar system.
The giant plume of material blasted off by the impact settled into a hot disc around Earth, which eventually clumped together to become the moon.
Back then, the moon (and Earth) were both hot balls of melted rock. But nailing down exactly when the moon solidified from the so-called lunar magma ocean has been tricky.
One way has been to date the products of the lunar magma ocean such as crystals that took shape as the hot rock cooled and hardened.
(It’s not just a moon phenomenon either – zircons have been used to find the Earth’s magnetic field 4.2 billion years ago.)
Lunar zircons are thought to form in the lunar magma ocean’s potassium, rare-Earth elements and phosphorus (KREEP) reservoir. And because this reservoir formed towards the end of the magma ocean’s hardening, they provide handy objects to determine when solidification took place.
Using a technique called isotope dilution thermal ionisation mass spectrometry – an incredibly sensitive technique that can accurately measure minute amounts of elements such as lead and uranium – Barboni and her crew found the zircon fragments crystallised between 50 and 60 million years after the birth of the solar system.
“Our data unambiguously show that the moon was […] mostly solidified by 4.51 [billion years ago],” they write, with an uncertainty of around 10 million years.