A SPECK of the mineral zircon that’s older than any yet found on Earth has been recovered from a rock sample brought back by Apollo 17 astronauts. The grain has helped pinpoint the age at which the molten moon solidified.
Lunar zircons were not studied at the time of the Apollo missions because the technology to date them did not exist, says geologist Clive Neal of the University of Notre Dame, Indiana. “It’s serendipitous to find this, and really emphasises the [value] of sample returns,” he says.
Until now, the zircon found in lunar rocks was between 3.90 and 4.35 billion years old, the same as the oldest zircon found on Earth. But many of these lunar grains came from low-lying areas on the moon, where the crust had been resurfaced after being melted by meteorite impacts.
The new sample, found by Alexander Nemchin at Curtin University of Technology in Perth, Australia, and colleagues, is 4.42 billion years old, and came from the lunar highlands. That means it crystallised after the crust first solidified, within 100 million years of the moon’s formation (Nature Geoscience, DOI: 10.1038/NGEO417).
The grain sets limits on the moon’s age, says Dianne Taylor of the University of California, Los Angeles, who has studied similar samples. The moon is thought to have formed from debris ejected by a giant impact between Earth and a smaller body between 10 and 100 million years after the formation of the solar system, 4.57 billion years ago. Taylor reckons the lunar crust formed within 90 million years of the impact, which tallies well with the age of the zircon.
Zircons from Earth tell the story of a fast-cooling planet that developed a solid crust within 200 million years of formation from the solar nebula, says John Valley at the University of Wisconsin-Madison, whose group dated the oldest terrestrial samples. “It’s reasonable that there would be something older from the moon than on Earth,” he says, because the smaller moon cooled more quickly after the colossal impact.
More on these topics: