Apollo-era samples suggest that any lunar magnetosphere endured no more than 500 million years
A lunar magnetic field may not have just been short-lived; it may have persisted for only a blip in geologic time, a new study finds.
Shortly after the moon formed about 4.5 billion years ago, it may have begun generating a magnetic field, a protective sheath that can deflect away charged particles from the sun (SN: 1/11/17). Now, analyses of moon rocks suggest that any lunar magnetic field was gone by at least 4 billion years ago, researchers report August 4 in Science Advances.
Magnetized lunar rocks brought back by Apollo astronauts decades ago were the first indication that the moon may have once had an internal dynamo, in which molten, iron-rich rock swirls inside the core of a celestial body, giving rise to a magnetic field (SN: 11/9/11). But how long such a lunar dynamo may have lasted has been unclear.
The moon’s core is “really small,” says John Tarduno, a geophysicist at the University of Rochester in New York, and it’s not clear how that core could have sustained a dynamo for long before cooling.
In the new study, Tarduno and colleagues examined the magnetization of a handful of Apollo rock samples. Analyzing the magnetism of tiny shards of metal trapped in crystals in rock dating to 3.9 billion, 3.6 billion, 3.3 billion and 3.2 billion years ago showed that those rocks were barely magnetized at all.
But the analyses also found that a piece of lunar glass formed during a meteorite impact about 2 million years ago “had a strong magnetic field — just a little weaker than Earth’s today,” Tarduno says. That’s odd, because “everyone agrees there isn’t a magnetic field on the moon now, and there wasn’t one 2 million years ago,” he says. “How does this happen?”
(Except for the headline, this story has not been edited by The Technology Express staff and is published from a syndicated feed)
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