The craters on the moon hold billions of years of history. By examining the makeup, dimensions, and distribution of these ancient lunar surface craters caused by asteroid collisions, scientists have gained knowledge about the circumstances of our early solar system.
Instead of directly examining these holes’ characteristics, a team from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, chose to take a different approach. They “erased” thousands of craters from the surface of the moon using computer simulations, as if turning back time 4.25 billion years to a period before the craters formed.
They discovered that over this time the north and south poles of the moon moved slightly.
Frozen Moon Water
GRAIL gravity model GRGM1200B (left), and GRGM1200B with 5197 crater gravity anomalies removed (right). Credit: The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac8c39
The poles “wandeered” 10 degrees in latitude (or 186 miles/300 kilometres) as the moon moved in all directions as a result of asteroid impacts, according to scientists. Where a celestial body’s rotational axis crosses its surface is where its geographic north and south poles are located.
The imaginary line that circles the moon’s centre and around which it rotates in this instance remained constant as the moon’s body moved.
Understanding the evolution of the moon, specifically the state of resources like water on its surface, can be aided by knowledge of wandering poles. Although scientists have discovered frozen water near the poles of the moon, they are unsure of its exact volume.
Some frozen water may have sublimated (changed from a solid to a gaseous state) off the surface if the moon’s poles were abruptly moved toward a warmer, less-shadowed location, like the equator. This would give new water less time to accumulate at the new poles.
True Polar Wander
But according to the study’s lead author, NASA Goddard scientist Vishnu Viswanathan,
“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.”
True Polar Wander, the phenomenon responsible for the shifting poles, is what happens when an object, in this case the moon, tries to maintain its rotation in the face of challenges like modifications to the distribution of its mass.
The moon changed its orientation to move areas of higher mass out toward the equator through centrifugal force as asteroid impacts extracted mass, leaving potholes in the surface — pockets of lower mass. When a pizza maker spins and tosses dough in the air to stretch it out, the same force is at work.
Sucking Out Craters
Viswanathan collaborated with a number of researchers, including David E. Smith, the head of NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft’s Lunar Orbiter Laser Altimeter (LOLA), to ascertain the extent of the moon’s polar wander. After working as the NASA Gravity Recovery and Interior Laboratory (GRAIL) mission’s deputy principal investigator, Smith developed an interest in using gravity data to determine how far the moon’s poles have moved.
Before the mission’s conclusion in 2012, GRAIL meticulously mapped the gravity field of the moon.
“If you look at the moon with all these craters on it, you can see those in the gravity field data,” said Smith. “I thought, ‘Why can’t I just take one of those craters and suck it out, remove the signature completely?'”
About 5,200 craters, ranging in size from 12 miles (20 kilometres) to 746 miles (1,200 kilometres), were the subject of Smith, Viswanathan, and their team’s work. They created computer models that used LOLA data to create topographical maps of the moon and used those coordinates and widths to find the corresponding gravitational signatures—or areas of higher or lower gravity—on a GRAIL gravity map.
The scientists then used simulations to erase the gravitational traces left by each crater in order of age, effectively reversing the moon’s evolution and moving the poles backwards toward their prehistoric positions with each impact that was erased.
Small Craters Add Up
While other polar wander researchers have removed craters from the record, they only did so for a small number of the largest ones — about a dozen.
“People assumed that small craters are negligible,” said Viswanathan. “They’re negligible individually, but collectively they have a large effect.”
The true extent of the moon’s polar wander is becoming more apparent, according to Viswanathan, but the researchers still need to improve their estimate. More minor craters will be removed from the moon, and other features like volcanic eruptions that may have assisted in shifting the poles will also be eliminated.
“There are a few things that we haven’t taken into account yet, but one thing we wanted to point out is those small craters that people have been neglecting, they actually do matter, so that is the main point here,”
said Goddard planetary scientist Sander Goossens, who also took part in the study.
Reference: David E. Smith et al, The Contribution of Small Impact Craters to Lunar Polar Wander, The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac8c39
