The moon may never actually have had a strong magnetic field

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Despite being made of the same stuff as our planet, the moon may never have had a strong magnetic field like Earth’s

Michael Osadciw/U. Rochester

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 Earth 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 strong as Earth’s, but initial analysis of moon rocks in the 1970s suggested that its magnetic field would have been as strong as Earth’s between 3.6 and 3.9 billion years ago. The assumption was that it had long since disappeared.

John Tarduno at the University of Rochester says that discovering evidence of a strong magnetic field in rocks after the Apollo missions was a surprise, because the moon was not 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 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.

His team analysed a glass-like sample of moon rock that was formed two million years ago by an asteroid impact and found that it had evidence of a strong magnetic field being present when it cooled and solidified. But by that time, the strong magnetic field of the moon should have waned. They say this proves 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.

They also tested samples dating back to between 3.9 billion and 3.2 billion years, the period of time when the moon should have had a strong magnetic field, and found no evidence of it. Examinations revealed that the samples contained minerals that would have been able to record any such field during its cooling and formation, were it present.

Tarduno says that, together, these results show that the moon did not have a long-lived strong magnetic field as was previously thought. He concedes that in the first 100 million years after the moon was formed there may have been a magnetic field, before it cooled and stabilised. But there are no rocks on the surface of the moon that date back that long, 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 the Earth’s atmosphere as the moon would have passed through Earth’s magnetosphere and picked up trace materials, he says. Such an experiment would not be possible if the moon also had a strong magnetosphere.

Journal reference: Science Advances, DOI: 10.1126/sciadv.abi7647

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