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Have scientists finally solved the mystery of the magnetic moon?

Why did the moon once have a magnetic field? A new proposal might hold the answer to a mystery that has baffled scientists for decades.

By Clara MoskowitzSPACE.com / November 9, 2011

A leaf in New Jersey's Eagle Rock Reservation is silhouetted by the rising moon Tuesday.

Julio Cortez/AP

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One of the abiding 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.

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When Apollo astronauts brought back samples of moon rocks from their lunar landing missions in the 1960s and '70s, some of them shocked scientists by being magnetic. That means that individual rocks might have a magnetic north and south pole and a small magnetic field of their own.

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

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

But the moon isn't large enough for convection to take place. Scientists were at a loss to explain what else might generate the required liquid motion of iron inside the moon, until now. [Photos: Our Changing Moon]

Stirring it up

In one new proposal, Christina Dwyer of the University of California, Santa Cruz, and her colleagues suggest that the moon's solid-rock middle layer, called its mantle, stirs up its liquid iron core. The researchers think this happens because the moon's core and its mantle rotate around slightly different axes, and the boundary between them is not quite spherical, so their relative motion causes the fluid to mix around.

The strength of this stirring is determined by the angle between the core and the mantle, and the distance between the Earth and the moon, because the tidal gravitational tug from the Earth causes the moon's mantle to rotate differently than the core.

This model would explain why the moon used to have a magnetic field, but no longer does. That's because the angle between the mantle and the core has narrowed over time, while the distance between the moon and the Earth has widened, causing the tidal forces to steadily decrease. While these forces used to be enough to generate a dynamo inside the moon, they aren't anymore.

Based on their calculations, the researchers estimate the lunar magnetic field might have lasted for about a billion years, somewhere between around 2.7 billion and 4.2 billion years ago.

"Based on what we know about stirring, and everything we know about fluid motion, we can find no reason that this would not work," Dwyer told SPACE.com. "All the flags are go, and now this needs to be taken to the next level to get tested."

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