Fresh analysis of moon rocks collected by Apollo missions reveals that our only natural satellite may never have had a strong magnetic field, as has been believed since geological samples were first returned to Earth.
The moon formed around 4.5 billion years ago in a collision that sent a chunk of our planet out into orbit, so it has a similar iron core to Earth’s. Currently, the moon’s magnetic field is less than one-thousandth as powerful as Earth’s, but initial analysis of moon rocks in the 1970s suggested that this field would have been as strong as Earth’s between 3.9 billion and 3.6 billion years ago. The assumption was that it had long since disappeared.
John Tarduno at the University of Rochester in New York says that discovering evidence of a strong magnetic field in rocks after the Apollo missions was a surprise, because the moon wasn’t large enough to power it. “No one’s been able to solve that paradox,” he says. “How do you have a magnetic field if you have no way to power it? The answer is that you didn’t have a magnetic field.”
He and his colleagues believe that the evidence of that strong field is actually due to moon rocks having been magnetised by the shock of asteroid impacts. They found that other Apollo-era rock samples from different lunar locations show no sign of such a field.
The researchers analysed a glass-like sample of moon rock that was formed 2 million years ago by an asteroid impact and found that it showed evidence of a strong magnetic field being present when it cooled and solidified. But by that time, the moon’s strong magnetic field should have waned. They say this indicates that the impact caused the magnetisation – and that the same could be true of much older samples, erroneously leading previous researchers to conclude that the moon had an active magnetic field.
The team also tested samples dating back to between 3.9 billion and 3.2 billion years ago, and found no evidence of a strong magnetic field. The samples contained minerals that would have been able to record any such field during their cooling and formation, were it present.
Tarduno says that, together, these results show that the moon didn’t have a long-lived strong magnetic field as was previously thought. He concedes that there may have been a magnetic field in the first 100 million years after the moon was formed, before it cooled and stabilised. But no rocks on the lunar surface date back that far, as the landscape is constantly pummelled and churned by asteroid impacts.
Were we able to drill down and discover such rocks, perhaps as part of NASA’s upcoming Artemis programme, they could provide insight into the early composition of Earth’s atmosphere because the moon would have passed through our planet’s magnetic field and picked up trace materials, says Tarduno. Such an experiment wouldn’t be possible if the moon also had a strong magnetic field.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abi7647
Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday
More on these topics:
Read more at New Scientist