Explanation for Ancient Lunar Magnetic Field

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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. 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.
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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. Michael Le Bars of the French National Center for Scientific Research and the Université Aix-Marseille in France, along with his colleagues, propose another explanation for the ancient lunar magnetic field. Le Bars’ team also suggests that the moon’s mantle might have stirred up the liquid in its core. However, they offer a different impetus for this stirring. Instead of tidal interactions between the Earth and the moon, the researchers posit that impacts by large space rocks slamming into the moon have changed its rotation rate, causing differential motion between the mantle and the core.