Moon rocks collected by the聽Apollo missions sparked a mystery because they showed signs of having formed under a magnetic field as strong as Earth鈥檚, but it was unclear how a body as small as the moon could have created such a field. Now there is a potential solution.
at Brown University in Rhode Island and at Stanford University in California propose that giant rocks as large as 60 kilometres across once sank through the moon鈥檚 mantle and stirred up the interior, generating sporadic, relatively strong magnetic fields during the first billion years of the moon鈥檚 history.
Early after its formation 4.5 billion years ago, the moon was covered by seas of molten rock. Its surface has gradually cooled and solidified, although it retains a . During that cooling process, Evans and Tikoo say that a layer rich in relatively dense titanium stayed warmer for longer because it contained heat-generating radioactive uranium and thorium. When this layer did eventually cool, its density allowed it to break into chunks 60 kilometres in diameter, which sank through the moon鈥檚 mantle and touched the hot core. This stirred up material, according to the researchers, and created a temporary dynamo 鈥� something that would then have generated a short-lived but strong magnetic field that affected rocks at the moon鈥檚 surface.
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鈥淵ou can think of it a little bit like a drop of water hitting a hot skillet,鈥� said Evans. 鈥淵ou have something really cold that touches the core, and suddenly a lot of heat can flux out. That causes churning in the core to increase, which gives you these intermittently strong magnetic fields.鈥�
But at the University of Rochester in New York, who has published research claiming that the evidence of a strong field is actually due to moon rocks having been magnetised by the shock of asteroid impacts, believes there are problems with the model.
鈥淚t鈥檚 important to recognise that cold blobs moving toward the core do not automatically create a dynamo,鈥� he says. 鈥淒ynamo action producing a strong field requires more factors. In their model, the dynamo would be present for less than 0.1 per cent of lunar history, making it incredibly unlikely that abundant records of a past dynamo would have been recovered from the [rocks collected during the] Apollo missions.鈥�
Nature Astronomy
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