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Volcanic activity may have shifted the moon’s axis

The poles of the moon may have wandered away. (Yoshikazu Tsuno/AFP)

Lots of folks think of the moon as a dead rock that's orbiting Earth unchanged. That's untrue — the moon is shrinking and pulling away from us a little bit all the time. But a new study suggests that the moon may have had a much more radical change in its ancient history: The moon may once have spun on a different axis.

In a study published Wednesday in Nature, researchers point to hydrogen-filled deposits near the moon’s poles as evidence that the satellite's axis — the imaginary line that crosses through it, around which it rotates — was once in a different spot.

These hydrogen deposits are thought to be ice that formed at the moon's poles. But in addition to the ice expected on the moon's current poles, scientists found deposits in a different spot — and these spots were directly across from each other. In other words, the ice patches seem to be sitting on areas formerly known as lunar poles. If that's the case, the new axis is shifted by 5.5 degrees.

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"It would be as if Earth's axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth," study author Matt Siegler at Southern Methodist University said in a statement.

That's cold, man on the moon. Real cold.

Scientists call this unusual phenomenon a "true polar wander." Earth has experienced it several times, and slight axis shifts are common on our planet after earthquakes.

But why on Earth (or on moon, rather) did those poles wander? Volcanic activity may be to blame. A world's axis is determined by its relative mass, with its lightest regions forming the poles. For the axis to shift so significantly, the mass would have to shift, too. Siegler and his colleagues think that volcanic activity melted some of the moon's mantle about 3.5 billion years ago, creating a giant, hot blob that was lighter than the colder rock around it. As that magma gurgled around, it slowly shifted the moon's axis about 125 miles.

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“It is like taking a football and removing a chunk out of one side — that would change how the football would spin in the air,” study co-author James T. Keane, a graduate student at the University of Arizona, told The Guardian.

The team suspects that the movement occurred at a rate of about an inch every 126 years.

Other experts in the field point out that we aren't sure that the hydrogen deposits are ice, or whether the patches have actually been around for billions of years. But fingers are crossed: If the ancient polar ice theory is correct, it could mean that the moon is home to samples of ice from the early days of our planet's formation. That could help scientists figure out once and for all just where Earth's water came from.

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