Asteroid impacts added tilt to the Moon

Asteroid impacts on the Moon, which have scarred its surface with craters, are responsible for the poles of the Earth’s satellite moving over time.

A team at NASA’s Goddard Space Flight Center used computer simulations to “erase” thousands of craters from the Moon’s surface, as if turning back the clock 4.25 billion years before the craters formed.

They found that as the Moon was moved back and forth by the effects of asteroid impacts, the location of the poles shifted 10 degrees in latitude (or 186 miles/300 kilometers), the scientists reported in the Planetary Science Journal. .

The geographic north and south poles are located where a celestial body’s axis of rotation intersects its surface. In this case, the Moon’s axis of rotation, the imaginary line through its center and around which it rotates, stayed the same as the Moon’s body moved.

Information about pole wanderers can be helpful in understanding the evolution of the Moon; specifically, the condition of resources, such as water, on its surface. Scientists have found frozen water in shaded regions near the Moon’s poles, but they don’t yet know how much is there. If the Moon had dramatically shifted the location of its poles to a warmer, less shaded region such as the equator, some of the frozen water might have sublimated (changed from a solid to a gaseous state) from the surface, and the new water it would have had less time to accumulate at the new poles.

But, Vishnu Viswanathan, a Goddard scientist who led the study, says in a statement, “Based on the Moon’s cratering history, the polar wander appears to have been moderate enough that water near the poles has remained in shadows and have enjoyed stable conditions for billions of years.

The phenomenon behind the changing poles is known as True Polar Wander, and it is what happens under the laws of physics to an object, in this case the Moon, that tries to keep spinning when faced with obstacles, such as changes in the the way it moves. mass is distributed.

As the asteroid impacts the excavated mass, leaving surface depressions, or pockets of lower mass, the Moon reorients itself to bring those pockets poleward, while higher mass areas push equatorward through the moon. centrifugal force. It is the same force that acts on the dough when a pizza maker throws it and spins it in the air to stretch it.

To determine the degree of the Moon’s polar shift, Viswanathan teamed up with several scientists, including David E. Smith, principal investigator for the Lunar Orbiter Laser Altimeter (LOLA), aboard NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft. . Smith became interested in using gravity data to find out how far the Moon’s poles have drifted after serving as deputy principal investigator for 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.

“If you look at the Moon with all these craters, you can see them in the gravity field data,” said Smith, of the Massachusetts Institute of Technology in Cambridge. “I thought, ‘Why can’t I just take one of those craters and suck it out, remove the signature entirely?'”

Smith, Viswanathan and their team worked with around 5,200 craters, ranging in size from 20 to 1,200 kilometers (12 to 750 miles) across. They designed computer models that took the coordinates and widths of all these craters from topographic maps of the Moon made with LOLA data and then found their corresponding gravitational signatures, or pockets of greater or lesser gravity, on a GRAIL gravity map. The scientists then ran simulations that removed the gravitational signatures of each crater sequentially by age, essentially rewinding the Moon’s evolution and rolling the poles back toward their ancient locations with each removed impact.

While other researchers studying polar wander have removed craters from the record, they’ve only removed a couple dozen of the largest ones. “People assumed that small craters are insignificant,” Viswanathan said. “They are insignificant individually, but collectively they have a big effect.”

Viswanathan said his team is getting closer to determining the true degree of polar wander on the Moon, but scientists still need to refine their estimate. They plan to erase more small craters on the Moon and remove other features, such as volcanic eruptions, that could have contributed to the pole shift.

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