A history of the small asteroid impacts that forced our moon to ‘roam’ around its axis: ScienceAlert

Show new evidence of how asteroids collide the moon It changed the positions of its poles.

Over the past 4.25 billion years, asteroid The collisions caused the moon’s body to “wander”, rotating it by about 10 degrees relative to its axis of rotation. This is a relatively small shift, which means that any ice hidden in craters at the Moon’s poles is unlikely to be significantly affected. This, in turn, means that future lunar exploration can continue accordingly.

“Based on the history of craters of the moon,” Planetary scientist Vishnu Viswanathan says From NASA’s Goddard Space Flight Center, “Polar wandering appears to have been moderate enough for water near the poles to remain in the shade and have stable conditions over billions of years.”

Much of the moon’s history is written in its craters. Earth’s largest natural satellite is riddled with the scars of impacts over billions of years, painstakingly mapped and dated by lunar scientists. These effects have altered the distribution of mass on the Moon, a measure directly related to gravity.

Therefore, every time a piece of space rock hits the lunar surface, it changes the lunar gravitational profile, even slightly. Cumulatively, over a very long period, this can change the way an object moves and orients itself in space.

Empty spaces dug by asteroid impacts cause the moon to reorient, bringing those low-mass holes closer to the poles. Meanwhile, higher concentrations of mass are being pulled closer to the equator. Think of a way hammer thrower It rotates to exert centrifugal force on the hammer, to fling it a greater distance.

We have, thanks to a NASA mission called Gravity Recovery and Interior Laboratory (Grill), a highly detailed map of the gravitational field on the Moon; Very detailed so that the effect of drilling can be illustrated. This gave an idea to planetary scientist David Smith of the Massachusetts Institute of Technology.

A map of the moon showing how the GRAIL data correspond to the distribution of impact craters. (Vishnu Viswanathan)

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

That’s what the team set out to do, looking to erase craters more than 20 kilometers (12 miles) wide. They identified nearly 5,200 craters and basins, plotted them on gravity data from GRAIL and then worked in time to erase them.

At first, they worked manually, before handing over the mission to computers to roughly return the date of the moon.

The effect of each individual hole was minimal. But there were a lot of them, and with each subtraction, the lunar pole crept into the position where it was billions of years ago. Altogether, the gravitational effect of all these small craters was roughly equal to that of the Antarctic-Aitken Basin, a huge impact area about 2,500 kilometers (1,550 miles) in diameter, roughly Quarter of the moon.

Histogram plotting polar wander over time due to the contribution of craters
This plot shows lunar wanderings over time, superimposed on the current lunar structure. The current electrodes are in the center; The starting point is the red circle. (Smith et al., Planet. Sci. J., 2022)

“People assumed the little pits were negligible,” Viswanathan says. “They are hardly mentioned individually, but collectively they have a huge impact.”

This is important: If the impact was large enough, it could have pushed the moon’s polar regions to places where their craters are illuminated by sunlight. If this happens, any frozen volatiles protected in the previously shaded crater floors will harden, leaving little (or even no) ice as a permanent record. As scientists want to explore the poles to find these ice patches, this will have implications for future lunar exploration, including for NASA. The next manned mission of Artemis.

The team showed that the effect wasn’t big enough for this, which is a good thing. But there is more work to be done.

The end result of the analysis is great, but it’s not the whole story. There are a lot of craters on the Moon outside of the criteria the team included; It will also have an effect, although perhaps a smaller one. In addition, the Moon has not always been geologically calm as it is now. Volcanic activity could also have changed its gravitational appearance over time.

However, previous work has focused only on craters greater than 200 km (125 mi) in diameter. The team says this work shows that every little bit matters.

“There are some things we haven’t taken into consideration yet,” Planetary scientist Sander Goossens says: From NASA’s Goddard Space Flight Center, “But one thing we wanted to point out is those little craters that people have been overlooking, they really matter, so that’s the main point here.”

The search was published in Planetary Science Journal.