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Our Moon’s Poles Moved 300 Kilometers Due to Asteroid Impacts

The two completely different sides of the Moon. Credit: NASA/JPL-Caltech/LRO

Scientists have found that the poles of the Moon moved 10 degrees in latitude, which is equivalent to a distance to a distance of 186 miles / 300 kilometers, due to the effects of asteroid impacts.

Throughout history, asteroids impacting the Moon have caused its surface to be scarred with craters, which cause its poles to move. Earth’s natural, faithful satellite is a time capsule of a billion-year-old history that is preserved by the Moon’s craters. And this so-called time capsule is of great importance to science. By studying the size, composition, and distribution of these scars on the Moon’s surface caused by asteroids colliding long ago, scientists have gained insight into our early solar system.

Scientists at NASA’s Goddard Space Flight Center, however, took a different approach instead of studying the characteristics of these impact holes directly. By simulating how the Moon looked 4.25 billion years ago, researchers erased thousands of craters from its surface as if they had never occurred. They found that the North and South Pole locations of the Moon had moved slightly over this period of time; this was quite unexpected.

Scientists reported in the Planetary Science Journal that the poles moved 10 degrees in latitude, which is equivalent to a distance to a distance of 186 miles / 300 kilometers, due to the effects of asteroid impacts. The geographic north and south poles are those points on an object’s surface where the rotational axis intersects. As the Moon shifted, its rotational axis, or the imaginary line passing through its center, remained the same.

Wandering poles can provide useful information about the evolution of the Moon, including the condition of its resources, such as water. In a time when mankind is about to return to the Moon and even attempt to build lunar colonies, this is of vital importance. The amount of frozen water near the Moon’s poles is not yet known, but scientists found it in shadowed areas. In the event that the Moon’s poles had been moved to a warmer, less shadowed region, like the equator, some frozen water would have sublimated (changed from a solid to a gaseous state) from the surface, resulting in less time for water to accumulate at the new poles.

Based on the history of cratering on the Moon, NASA Goddard scientist Vishnu Viswanathan explained that polar wander was moderate enough to allow water near the poles to remain hidden for billions of years and enjoy durable conditions.

As it turns out, the shifting poles are caused by a phenomenon known as True Polar Wander, which occurs under the laws of physics when an object, in this case, the Moon, tries to keep itself spinning despite obstacles, such as changes in the distribution of its mass.

By centrifugal force, the Moon reoriented itself as asteroid impacts excavated mass and left depressions on its surface. These depressions were brought toward the poles, while higher masses were pushed toward the equator. NASA explains that this is the same force at work when a pizza maker spins and throws dough in the air to stretch it.

The Moon’s polar drift was measured by Viswanathan in collaboration with several scientists, including David E. Smith, principal investigator for NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft and principal investigator for the Lunar Orbiter Laser Altimeter (LOLA). The gravity data that Smith collected while serving as a deputy principal investigator on NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission inspired him to figure out how far the Moon’s poles have wandered. Prior to its end in 2012, GRAIL provided an in-depth map of the Moon’s gravity field.

According to Smith, a researcher at the Massachusetts Institute of Technology in Cambridge, you can see the craters on the moon 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?’”

There were approximately 5,200 craters studied by Smith, Viswanathan, and their team that ranged in size from 12 miles (20 kilometers) to 746 miles (1,200 kilometers). By using topographical maps of the Moon made using LOLA data, they calculated the coordinates and widths of all these craters and then compared them to GRAIL gravity maps in order to find their gravitational signature – or pockets of higher or lower gravity. Scientists were able to rewind the evolution of the Moon by removing the gravitational signatures of each crater sequentially in order to inch its poles closer to their ancient positions by running simulations.

Only a handful of the largest craters have been removed from the record by other researchers studying polar wander. According to Viswanathan, “people tend to think that small craters are negligible.” “They’re negligible individually, but collectively they have a large effect.”

Scientists are getting closer to figuring out the extent of lunar polar wander, but Viswanathan said their estimates need to be refined. In addition to removing craters, other features such as volcanic eruptions that could have affected the poles will be removed from the Moon in new simulations.

In spite of the fact that we haven’t yet taken everything into account, Sander Goossens, a Goddard planetary scientist who participated in the study, pointed out that those small craters that have been ignored actually matter, which is what the study emphasizes.

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