Scientists believe that this lunar field, like Earth’s, was generated by a powerful dynamo — the churning of the moon’s core. At some point, this dynamo, and the magnetic field it generated, petered out.
Now scientists from MIT and elsewhere have pinned down the timing of the lunar dynamo’s end to around 1 billion years ago.
The new timing rules out some theories for what drove the lunar dynamo in its later stages and favours one particular mechanism: core crystallisation.
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As the moon’s inner iron core crystallised, the liquid core’s electrically charged fluid was buoyantly stirred, producing the dynamo.
Benjamin Weiss, professor of earth, atmospheric, and planetary sciences at MIT, said, "The magnetic field is this nebulous thing that pervades space, like an invisible force field.
"We’ve shown that the dynamo that produced the moon’s magnetic field died somewhere between 1.5 and 1 billion years ago, and seems to have been powered in an Earth-like way."
Over the past few years, Weiss’ group and others have discovered signs of a strong magnetic field, of around 100 microteslas, in lunar rocks as old as 4 billion years. For comparison, Earth’s magnetic field today is around 50 microteslas.
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In 2017, Weiss’s group studied a sample collected from NASA’s Apollo project, and found traces of a much weaker magnetic field, below 10 microteslas, in a moon rock they determined to be about 2.5 billion years old.
Their thinking at the time was that perhaps two mechanisms for the lunar dynamo were at play: The first could have generated a much stronger, earlier magnetic field around 4 billion years ago, before being replaced by a second, more long-lived mechanism that sustained a much weaker field, through to at least 2.5 billion years ago.
Weiss added, "It turns out all these power sources have different lifetimes. So if you could figure out when the dynamo turned off, then you could distinguish between the mechanisms that have been proposed for the lunar dynamo. That was the purpose of this new paper
"It turns out all these power sources have different lifetimes. So if you could figure out when the dynamo turned off, then you could distinguish between the mechanisms that have been proposed for the lunar dynamo. That was the purpose of this new paper."
Most of the magnetic studies lunar samples from the Apollo missions have been from ancient rocks, dating to about 3 billion to 4 billion years old.
These are rocks that originally spewed out as lava onto a very young lunar surface, and as they cooled, their microscopic grains aligned in the direction of the moon’s magnetic field. Much of the moon’s surface is covered in such rocks, which have remained unchanged since, preserving a record of the ancient magnetic field.
The new findings line up with the predicted lifetime of core crystallisation, a proposed mechanism for the lunar dynamo that could have generated a weak and long-lived magnetic field in the later part of the moon’s history.
Weiss said that prior to core crystallisation, a mechanism known as precession may have powered a much stronger though shorter-lived dynamo.
Precession is a phenomenon by which the solid outer shell of a body such as the moon, in close proximity to a much larger body such as the Earth, wobbles in response to the Earth’s gravity. This wobbling stirs up the fluid in the core, the way swishing a cup of coffee stirs up the liquid inside.
"It turns out all these power sources have different lifetimes. So if you could figure out when the dynamo turned off, then you could distinguish between the mechanisms that have been proposed for the lunar dynamo. That was the purpose of this new paper," Weiss explained.
Around 4 billion years ago, the infant moon was likely much closer to the Earth than it is today, and much more susceptible to the planet’s gravitational effects. As the moon moved slowly away from the Earth, the effect of precession decreased, weakening the dynamo and the magnetic field in turn.
Weiss said it is likely that around 2.5 billion years ago, core crystallisation became the dominant mechanism by which the lunar dynamo continued, producing a weaker magnetic field that continued to dissipate as the moon’s core eventually fully crystallised.
The group is looking next to measure the direction of the moon’s ancient magnetic field in hopes of gleaning more information about the moon’s evolution.
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