New evidence has shown how asteroids slamming into the Moon have changed the positions of its poles.
Over the past 4.25 billion years, asteroid impacts have caused the Moon's body to 'wander', rolling it by around 10 degrees in relation to its rotational axis. This is a relatively small shift, which means any ice tucked away in craters at the lunar poles is unlikely to have been significantly affected. In turn, this means that future lunar exploration can continue accordingly.
"Based on the Moon's cratering history," says planetary scientist Vishnu Viswanathan of NASA's Goddard Space Flight Center, "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."
A lot of the Moon's history is written in its craters. Earth's largest natural satellite is speckled with the scars of impacts that have taken place over billions of years, painstakingly mapped and dated by lunar scientists. And these impacts have changed the distribution of mass on the Moon, a metric directly tied to gravity.
So, each time a chunk of space rock slams into the lunar surface, it alters the lunar gravitational profile, even if just by a little bit. Cumulatively, over a very long time, this can alter the way an object moves and orients itself in space.
The empty spaces excavated by asteroid impacts causes the Moon to reorient, bringing those lower-mass holes closer to the poles. Meanwhile, higher concentrations of mass are pulled closer to the equator. Think of the way a hammer thrower spins to exert a centrifugal force on the hammer, to hurl it a greater distance.
We have, thanks to a NASA mission called Gravity Recovery and Interior Laboratory (GRAIL), an extremely detailed map of the Moon's gravity field; so detailed that the effect of the craters can be made out. 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 those in the gravity field data," Smith explains. "I thought, 'Why can't I just take one of those craters and suck it out, remove the signature completely?'"
So that is what the team set out to do, looking to erase craters wider than 20 kilometers (12 miles) across. They identified nearly 5,200 craters and basins, mapping them to the gravitational data from GRAIL and then working backwards in time to erase them.
Initially, they worked manually, before handing the job to computers to virtually rewind the Moon's history.
The effect of each individual crater was miniscule. But there were a lot of them, and with each subtraction the lunar poles crept back towards the position they were in billions of years ago. All together, the gravitational effect of all these small craters was almost equal to that of the South Pole-Aitken Basin, a colossal impact zone around 2,500 kilometers (1,550 miles) across, nearly a quarter of the surface of the Moon.
"People assumed that small craters are negligible," Viswanathan says. "They're negligible individually, but collectively they have a large effect."
This is important: if the effect was large enough, it could have pushed the polar regions of the Moon into places where the craters are illuminated by the sunlight. If this were to occur, any frozen volatiles sheltered in the previously shadowed crater floors would sublimate, leaving less (or even no) ice as an enduring record. Since scientists want to investigate the poles to find these icy patches, this would have implications for future lunar exploration, including NASA's upcoming crewed Artemis mission.
The team showed that the effect has not been large enough for this, which is good. But there's more work to be done.
The final result of the analysis is fascinating, but it's not quite the whole story. There are a lot of craters on the Moon that are outside the parameters the team included; they would have had an effect too, although perhaps a smaller one. In addition, the Moon hasn't always been as geologically quiet as it is now. Volcanic activity could also have altered its gravitational profile over time.
However, previous work has focused only on craters larger than 200 kilometers (125 miles) across. This work, the team says, shows that every little bit does seem to count.
"There are a few things that we haven't taken into account yet," says planetary scientist Sander Goossens of NASA's Goddard Space Flight Center, "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."
The research has been published in the Planetary Science Journal.