New tests have shown how asteroids that collide with the Moon have changed the position of its poles.
Over the past 4.25 billion years, asteroid impacts have caused the Moon’s body to “wander,” rolling it about 10 degrees relative to its axis of rotation. This is a relatively small change, meaning that any ice hidden in the lunar pole craters is unlikely to have been significantly affected. In turn, this means that future lunar exploration can continue accordingly.
“Based on the history of cratering on the Moon,” says planetary scientist Vishnu Viswanathan of NASA’s Goddard Space Flight Center, “the polar wander appears to have been moderate enough that water near the poles has remained in the shade and enjoy stable conditions for billions of years.”
Much of the Moon’s history is written in its craters. Earth’s largest natural satellite is scarred 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 every time a piece of space rock hits the lunar surface, it alters the lunar gravitational profile, even just a little. Cumulatively, over a long time, this can alter the way an object moves and orients itself in space.
Hollow spaces carved out by asteroid impacts cause the Moon to reorient, bringing the lower-mass holes closer to the poles. Meanwhile, the highest concentrations of mass approach the equator. Think of how a hammer thrower spins to exert a centrifugal force on the hammer, to throw it a greater distance.
We have, thanks to a NASA mission called the Gravity Recovery and Interior Laboratory (GRAIL), an extremely detailed map of the Moon’s gravity field; so detailed that you can distinguish the effect of the craters. That gave planetary scientist David Smith of the Massachusetts Institute of Technology an idea.
A map of the Moon showing how the GRAIL data lines up with the distribution of impact craters. (Vishnu Viswanathan)
“If you look at the Moon with all these craters, you can see them in the gravity field data,” explains Smith. “I thought, ‘Why can’t I just take one of these craters and suck it up, get the signature out completely?'”
So that’s what the team set out to do, looking to erase craters more than 20 kilometers (12 miles) in diameter. They identified nearly 5,200 craters and basins, mapping them with GRAIL gravity data and then working backwards in time to erase them.
Initially, they worked by hand, before handing over the work to computers to virtually rewind the history of the Moon.
The effect of each individual crater was minuscule. But there were many, and with each remnant the lunar poles drifted back to the position they were in billions of years ago. Combined, the gravitational effect of all these small craters was nearly equal to that of the South Pole-Aitken Basin, a colossal impact zone about 2,500 kilometers (1,550 miles) in diameter, nearly a quarter of the surface of the Moon
This plot shows the lunar wanderer over time, superimposed on the current lunar topology. Current poles are in the center; the starting point is the red circle. (Smith et al., Planet. Sci. J., 2022)
“People assumed that small craters are insignificant,” says Viswanathan. “They’re individually insignificant, but collectively they have a big effect.”
This is important: If the effect was large enough, it could have pushed the Moon’s polar regions into places where the craters are illuminated by sunlight. If this were to happen, any frozen volatiles protected in the previously shadowed crater floors would sublimate, leaving less (or even no) ice as an enduring record. Since scientists want to probe the poles to find these icy patches, this would have implications for future lunar exploration, including NASA’s upcoming manned Artemis mission.
The team showed that the effect has not been large enough for this, which is good. But there is more work to be done.
The end result of the analysis is fascinating, but it’s not the whole story. There are many craters on the Moon that are outside the parameters the team included; they would also have had an effect, although perhaps smaller. Also, the Moon has not 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) in diameter. This work, the team says, shows that every little bit seems to count.
“There are some 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 those little craters that people have been neglecting, actually. it matters, so that’s the main point here.”
The research has been published in the Planetary Science Journal.