A history of tiny asteroid impacts has forced our moon to “wander” on its axis.

New evidence has shown how asteroids impacting the moon have changed the position of its poles.

Over the past 4.25 billion years, asteroid impacts have caused the lunar body to “wander,” rolling it about 10 degrees with respect to its axis of rotation. This is a relatively small shift, meaning ice hidden in craters at the lunar poles is unlikely to have been significantly affected. This in turn means that future exploration of the moon can proceed accordingly.

“Based on the moon’s crater history,” says planetary scientist Vishnu Viswanathan of NASA’s Goddard Space Flight Center, “polar migration appears to have been moderate enough for water near the poles to remain in shadow and stable conditions for billions of years would have. “

Much of the moon’s history is written in its craters. Earth’s largest natural satellite is riddled with the scars of impacts spanning billions of years that have been carefully mapped and dated by lunar scientists. And those impacts have changed the mass distribution on the moon, a metric directly related to gravity.

Every time a piece of space rock hits the lunar surface, it changes the moon’s gravity profile, albeit slightly. Cumulatively, this can change the way an object moves and orients itself in space over a very long period of time.

The empty spaces excavated by asteroid impacts cause the moon to realign, bringing these lower-mass holes closer to the poles. Meanwhile, higher mass concentrations are being pulled closer to the equator. Think of the way a hammer thrower spins to apply centrifugal force to the hammer to propel it a greater distance.

Thanks to a NASA mission called Gravity Recovery and Interior Laboratory (GRAIL), we have an extremely detailed map of the moon’s gravitational field; so detailed that you can see the effect of the craters. This gave planetary scientist David Smith of the Massachusetts Institute of Technology an idea.

“If you look at the moon with all these craters on it, you can see them in the gravitational field data,” Smith explains. “I thought, ‘Why can’t I just take one of these craters and suck it dry, remove the signature completely?'”

So that was the team’s goal, to extinguish craters larger than 20 kilometers (12 miles) in diameter. They identified nearly 5,200 craters and basins, mapped them using GRAIL’s gravity data, and then worked backwards in time to erase them.

Initially, they worked manually before handing off the job to computers to virtually rewind the moon’s history.

The effect of each individual crater was tiny. But there were plenty of them, and with each subtraction, the lunar poles crept back to where they were billions of years ago. Collectively, the gravitational pull of all these small craters was almost equal to that of the South Pole-Aitken Basin, a colossal impact zone about 2,500 kilometers (1,550 miles) across, almost a quarter of the lunar surface.

“People assumed that small craters were negligible,” says Viswanathan. “They are negligible individually, but together they make a big impact.”

This is important: If the effect was large enough, it could have shifted the moon’s polar regions to places where the craters are illuminated by sunlight. If this happened, any frozen volatiles sheltered in the previously shadowed crater floors would sublime, leaving less (or even no) ice as a permanent record. As scientists look to probe the poles to find these icy patches, it would have implications for future exploration of the moon, including NASA’s upcoming manned Artemis mission.

The team showed that the effect wasn’t large enough for this, which is good. But there is still more to do.

The bottom line of the analysis is fascinating, but it’s not the whole story. There are many craters on the moon that fall outside of the team’s included parameters; they would also have had an effect, albeit perhaps a lesser one. Also, the moon hasn’t always been geologically as calm as it is now. Volcanic activity may also have altered its gravitational profile over time.

However, previous work has only focused on craters larger than 200 kilometers (125 miles) in diameter. This work, say the team, shows that every little seems to count.

“There are a few things we haven’t considered yet,” says planetary scientist Sander Goossens of NASA’s Goddard Space Flight Center, “but one thing we wanted to point out is these little craters that people have neglected, they don’t really matter, so that’s the main point here.”

The research was published in the Planetary Science Journal.