Moon magnetism mystery solved

We thought the moon was magnetic. Now we know it's not. So why are some moon rocks magnetic?...
10 August 2021

Interview with 

John Tarduno, University of Rochester

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Earth's moon

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Walk almost anywhere on Earth armed with a compass, and the magnetic field issuing from inside the planet will point you in the direction of the North Pole. But what would happen if you went for a similar walk on the Moon? Does it have a magnetic field? Some claimed that - at least at one point - it did, others were doubtful. Now Rochester University’s John Tarduno has analysed samples of Moon rock brought back by the Apollo missions and confirmed that the Moon never did have a magnetic field. Instead, he’s shown, rocks around craters can become magnetised by meteor impacts, and it’s this that tripped up earlier researchers, as he told Chris Smith...

John - Since the return of the rocks from the Apollo missions there has been this idea that the moon once had a magnetic field, just like the earth, but it lost its magnetic field, but it only lost the magnetic field relatively recently. That's a paradox because the moon doesn't really have a power source to drive a magnetic field. The Earth's magnetic field is created in the earth's core, the moon has a core, but the core is really small. So how could the moon's core have created a magnetic field? That was a question that we're really after.

Chris - What did people think it did have one, then?

John - The Apollo astronauts, part of the missions of course were to return lunar rocks. And when these rocks were analysed in the 1970s, it was found that some of them had strong magnetisations. And on the basis of that scientists concluded that the moon once had a magnetic field as strong as the earth's.

Chris - So what would you speculate caused those magnetisations in the bits that the Apollo astronauts brought back then?

John - What we found is that a very young sample - young on the moon means 2 million years old - collected by the Apollo astronauts from an impact crater had a really strong magnetisation. So actually impacts themselves can create magnetic fields, and if there are rocks nearby, they can be magnetised.

Chris - You've mentioned that you did this by taking samples that had been brought back to earth by various Apollo missions and so on. How did you analyse the samples you got?

John - We have a magnetics laboratory and we have a special magnetometer - a device to measure magnetic fields - but we can measure the trace magnetic fields, and we also use a special laser to heat the samples. What we're trying to do is we're trying to reproduce the process where, during its formation, the rock actually acquired magnetisation. And by reproducing that process in the laboratory, we can then gauge the nature of the magnitisation contained by the samples. Using these techniques we were able to demonstrate that most of the samples that we measured, the ones that were not associated with impacts, had no magnetisation, but most importantly, they have the ability to acquire a magnetisation if the magnetisation had been in place. So essentially what we're doing is we're evaluating the reliability of these ancient rocks as tape recorders. So we were able to demonstrate that they were actually are good tape recorders and they're recording no magnetic field.

Chris - And what do you think the implications are then? How does this rewrite the story we have assembled so far about where the moon came from?

John - Well, first of all, it resets the idea of the interior evolution of the moon. This has been a paradox for decades and that problem goes away now. But I think the bigger importance here really is the history of the surface of the moon. Without this magnetic shield, this means that solar wind elements, things like helium three, which is really interesting and could be a potential power source for exploration, and hydrogen - hydrogen can actually combine with other elements in the lunar soil to form water. So we could have helium three and water, much more of it than we might've thought in the past, in the lunar soils useful for future exploration.

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