Asteroid impacts ‘made the moon’s poles shift’

Closeup of moon surface texture
Asteroids wobbled the moon so much its poles have moved. (Getty)

A study that looked at billions of years worth of asteroid craters on the moon’s surface has concluded that space rock impacts wobbled the moon so much its poles moved.

Over the past 4.25 billion years, the moon’s poles have ‘wandered’ by 186 miles or 10 degrees in latitude.

The study could be important to finding water on the moon, as NASA returns to the satellite with manned missions in coming years.

Vishnu Viswanathan, a NASA Goddard scientist who led the study, said: “Based on the moon’s cratering history, polar wander appears to have been moderate enough for water near the poles to have remained in the shadows and enjoyed stable conditions over billions of years.”

A team based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland used computer simulations to “erased” thousands of craters from the moon’s surface, as if turning back the clock 4.25 billion years to a time before the craters formed.

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They found that the locations of the moon’s north and south Poles moved slightly over this time period.

Information about wandering poles can be useful for understanding the evolution of the moon; specifically, the condition of resources, such as water, on its surface.

Scientists have found frozen water in shadowed regions near the moon’s poles, but they don’t yet know how much there is.

If the moon had drastically shifted the locations of its poles toward a warmer, less shadowed region, such as the equator, some frozen water could have sublimated (changed from a solid state to a gaseous state) off the surface, with new water having had less time to accumulate at the new poles.

The phenomenon behind the shifting poles is known as True Polar Wander, and it’s what happens under the laws of physics to an object, in this case the moon, that’s trying to keep itself spinning when faced with obstacles, such as changes to the way its mass is distributed.

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As asteroid impacts excavated mass, leaving depressions in the surface — or pockets of lower mass — the moon reoriented itself to bring those pockets toward the poles, while bringing areas of higher mass out toward the equator via centrifugal force.

It’s the same force that acts on dough when a pizza maker tosses and spins it in the air to stretch it out.

The researchers became interested in using gravity data to figure out how far the moon’s poles have wandered after serving as deputy principal investigator of NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission.

GRAIL mapped the moon’s gravity field in great detail before the mission ended in 2012.

David E Smith, principal investigator for the Lunar Orbiter Laser Altimeter (LOLA), aboard NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft, said: “If you look at the moon with all these craters on it, you can see those in the gravity field data.

“I thought, ‘Why can’t I just take one of those craters and suck it out, remove the signature completely?’”

Viswanathan said his team is getting closer to figuring out the true degree of polar wander on the moon, but the scientists still need to refine their estimate.

They plan to erase more small craters from the moon and to remove other features, such as volcanic eruptions, that could have contributed to shifting the poles.

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