The mystery of Moon’s lost magnetism solved?

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One of the enduring mysteries of our moon is why it apparently once had a magnetic field. Now, two teams of scientists have offered two separate, but potentially complementary, explanations.

When Apollo astronauts brought back samples of moon rock from their moon landing missions in the 1960s and 1970s, some of them shocked scientists by being magnetic. This means that individual rocks can have a north and south magnetic pole and a small magnetic field of their own.

This can happen to rocks with the right minerals inside, if they cool in the presence of a magnetic field. The problem is that scientists had no idea the moon ever had a magnetic field and were unable to explain how it could have happened.

A magnetic field is generated by what is called a dynamo, which is caused by the fluid motion of a conductive material, such as liquid iron. In the case of the Earth’s magnetic field, this movement occurs in the outer core of the planet and is caused by the convection of heat.

But the moon is not big enough for convection to take place. Scientists were unable to explain what else could generate the required liquid movement of iron inside the moon, until now. [Photos: Our Changing Moon]

stir it up

In a new proposal, Christina Dwyer of the University of California, Santa Cruz and her colleagues suggest that the moon’s middle layer of solid rock, called its mantle, stirs its liquid iron core. Researchers believe this happens because the moon’s core and its mantle rotate around slightly different axes, and the boundary between them isn’t quite spherical, so their relative motion causes the moon to mix. fluid.

The strength of this agitation is determined by the angle between the core and the mantle, and the distance between the Earth and the moon, because the gravitational tug of the Earth’s tide rotates the moon’s mantle differently from the core.

This model would explain why the moon had a magnetic field, but no longer has one. That’s because the angle between the mantle and the core has narrowed over time, while the distance between the moon and Earth has widened, causing tidal forces to steadily decrease. While these forces were sufficient to generate a dynamo inside the moon, they are no longer so.

Based on their calculations, the researchers estimate that the lunar magnetic field may have lasted around a billion years, around 2.7 billion to 4.2 billion years ago.

“Based on what we know about agitation and everything we know about fluid motion, we can’t find any reason why it shouldn’t work,” Dwyer told SPACE.com. “All the flags are gone, and now this has to go to the next level to be tested.”

Dwyer said his research team had studied the base case scenario, but hopes scientists who specialize in modeling the complex physics of dynamos will now step in to determine if it could explain what happened on the moon.

The researchers reported their theory in the November 10 issue of the journal Nature.

Violent impacts

But this is not the only possible solution to the mystery of the moon.

In the same issue of Nature, Michael Le Bars of the French National Center for Scientific Research and Aix-Marseille University in France, along with his colleagues, offer an alternative explanation for the ancient lunar magnetic field.

Le Bars’ team also suggests that the moon’s mantle may have stirred up liquid in its core. However, they offer another boost to this brew. Instead of tidal interactions between Earth and the Moon, the researchers posit that impacts from large space rocks hitting the Moon changed its rate of rotation, causing differential motion between the mantle and the core.

While the first scenario would cause constant shaking as the Earth and moon were the right distance apart, the second image would induce brief periods of particularly strong core shaking, creating magnetic field spikes on the moon.

While either option could be correct, it’s also possible that both mechanisms played a role in creating an ancient magnetic field on the moon.

“Both studies are thought-provoking and can be complementary,” wrote Dominique Jault, a researcher at ETH Zürich in Switzerland and the Université Joseph-Fourier in France, who was not involved in either of the new studies, in a Accompanying essay in the same matter of Nature. “Future paleomagnetic experiments on very old lunar rock samples will test their theories.”

Editor’s note: This story has been corrected to reflect that the distance between Earth and the Moon has grown, not shrunk, over time.

You can follow SPACE.com Deputy Editor Clara Moskowitz on Twitter @ClaraMoskowitz. Follow SPACE.com for the latest space science and exploration news on Twitter @Spacedotcom and on Facebook.

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