Small craters add up to the wandering poles on the Moon

Moon craters preserve billions of years of history. Scientists have learned about the conditions of our early solar system by studying the composition, size and distribution of these holes on the moon’s surface, which were created long ago by collisions with asteroids.

But instead of studying the properties of these holes directly, a team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, decided to try something different. Using computer simulations, they “erase” thousands of craters from the surface of the Moon, as if turning back the clock 4.25 billion years before the craters formed. They found that the positions of the north and south poles of the moon moved quite a bit during this time period.

19 in the Journal of Planetary Science, scientists reported that as the Moon shifted in this way from the effects of asteroid collisions, the position of the poles “wandered” by 10 degrees in latitude (or 186 miles/300 kilometers). The north and south geographic poles are located where the axis of rotation of a celestial body intersects with its surface. In this case, the Moon’s axis of rotation, the imaginary line that passes through its center and revolves around it, remained the same as with the shift of the Moon’s body.

Information about the wanderings of the poles can be useful for understanding the evolution of the moon; Specifically, the state of a resource, such as water, at its surface. Scientists have found frozen water in shaded areas near the moon’s poles, but they don’t yet know how much water is there. If the Moon shifted significantly the positions of its poles toward a warmer, less shaded region, such as the equator, some of the frozen water might have risen (changed from a solid to a gaseous state) from the surface, with new water having less time to accumulate at the new poles.

But, says Vishnu Viswanathan, a NASA Goddard scientist who led the study, “Based on the history of craters, polar wandering appears to have been moderate enough for water near the poles to remain in the shadows and enjoy stable conditions over billions of years.”

The phenomenon behind the changing poles is known as the True Polar Wander, which is what happens under the laws of physics for an object, in this case the Moon, which tries to maintain its rotation when encountering obstacles, such as changes in the way its mass is distributed.

When the asteroid hit the cratered mass, leaving depressions in the surface — or pockets of low mass — the moon reorientated itself to bring those pockets toward the poles, with regions of higher mass brought toward the equator by centrifugal force. It’s the same force that acts on the dough when the pizza maker throws it and spins it in the air to stretch it.

To determine the degree of lunar wandering, Viswanathan partnered 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 work out the distance traveled by the lunar poles after serving as deputy principal investigator for the NASA Gravity Recovery and Interior Laboratory (GRAIL) mission. Grill mapped the moon’s gravitational field in great detail before the mission ended in 2012.

“If you look at the moon with all these craters, you can see the ones in the gravitational field data,” said Smith, of the Massachusetts Institute of Technology in Cambridge. “I thought, ‘Why can’t I just take one of those pits and suck it in, and completely remove the signature?’ “

Smith, Viswanathan and their team worked with about 5,200 craters — ranging in size from 12 miles (20 kilometers) to 746 miles (1,200 kilometers) wide. They made computer models that took coordinates and displayed all these craters from lunar topographic maps made with LOLA data and then found the corresponding gravity signatures – or pockets of higher or lower gravity – on the GRAIL gravity map. Then the scientists ran simulations that removed the gravitational signatures of each crater sequentially by age, essentially recreating the moon’s evolution and slowly returning the poles toward their ancient positions with each impact removed.

While other researchers studying polar wandering have removed craters from the record, they have removed only a few dozen of the largest. “People assumed the little pits were negligible,” Viswanathan said. “They are hardly mentioned individually, but collectively they have a huge impact.”

Viswanathan said his team is getting close to figuring out the true degree of polar wandering on the moon, but scientists still need to revise their estimates. They plan to erase more small craters from the moon and remove other features, such as volcanic eruptions, that could have contributed to changing the poles.

“There are a few things we haven’t considered yet, but one thing we wanted to point out is those little pits that people have been ignoring, they really matter, so that’s the main point here,” Sander said. Goossens, a Goddard planetary scientist who took part in the study.