The moon’s poles shifted following asteroid impacts over billions of years, new research has found.
Astronomers have long used lunar craters to chart the history of moon and the whole solar system, because the pattern of destruction left behind by asteroid impacts paints a picture of the violent conditions found in the young solar system. The new research turns the tables on those studies by simulating the removal of thousands of craters and also accounting for the impacts of smaller craters, rewinding 4.25 billion years of lunar history.
The researchers, based at NASA’s Goddard Space Flight Center in Maryland, found that when the moon was shaken by asteroid strikes, its north and south poles have wandered about 10 degrees of latitude, which equates to about 186 miles (300 kilometers).
Related: The moon has many (many) more craters than we thought
The geographic poles of the moon are located where its axis of rotation – the imaginary line around which it revolves – intersects the lunar surface. The simulation showed that while the moon’s body moved, the axis of rotation remained fixed.
The discovery could shed light on the evolution of Earth’s natural satellite and could help researchers locate water and other resources that could be used for future crewed space missions.
Scientists have found frozen water in cold and dark regions at the poles of the moon, but how much water was there a mystery. By understanding how and where the poles moved, researchers could find out how much frozen water was transformed from solid ice directly into gas – a process called sublimation. An extreme shift in the location of the lunar poles – particularly to hotter, less dark regions of the moon – would have resulted in rapid water sublimation and loss to space, also giving the Earth less time to new water to accumulate at the poles.
“Based on the history of the cratering of the Moon, polar wander appears to have been moderate enough that water near the poles remained in shadow and enjoyed stable conditions for billions of years. “said Vishnu Viswanathan, a NASA Goddard researcher who led the study. on the wandering poles of the moon, said in a statement (opens in a new tab).
Pole shift is caused by a phenomenon called “true polar drift,” which occurs when a rotating object encounters obstacles, such as a change in its mass distribution. In the case of the moon, this happened when asteroid impacts carved deep depressions in the lunar surface, which redistributed mass and left behind regions of lower mass.
The moon has reoriented, moving these low-mass “pockets” toward the poles. When that happened, centrifugal force — the same force that flattens and stretches dough in a pizza base — moved regions of high mass toward the lunar equator.
(opens in a new tab)
“If you look at the Moon with all of these craters on it, you can see the ones in the gravity field data,” David E. Smith, a Massachusetts Institute of Technology researcher and co-author of the new research, said in the statement. . “I was like, ‘Why can’t I just take one of these craters and suck it out, completely remove the signature?'”
Smith is the principal investigator of the Lunar Orbiter Laser Altimeter (LOLA) instrument aboard NASA’s Lunar Reconnaissance Orbiter and has experience operating gravity data to evaluate the displacement of the lunar poles. Smith, Viswanathan and their team used the LOLA data to design computer models that took the coordinates and widths of 5,200 lunar craters between 12 and 746 miles (19 to 1,200 km) in diameter.
Next, the team matched impact craters with pockets of higher or lower gravity found on a gravity map of the moon created with data from NASA’s Gravity Recovery and Interior Laboratory. They ran these simulations in reverse, removing these pockets of high and low gravity and thus erasing the craters in sequence according to their age. This rewind of the moon’s evolution has brought the poles back to the positions they held billions of years ago.
The researchers had previously attempted a similar process, but by focusing only on the largest lunar craters, those efforts failed to account for the net effect of smaller impacts on the moon’s poles.
“People have assumed that small craters are negligible,” Viswanathan said. “They are negligible individually, but collectively they have a big effect.”
The researchers will continue to simulate the removal of small craters from the lunar surface and plan to remove features caused by volcanic eruptions in the moon’s history. The team hopes these additional steps will help paint a more complete picture of polar wandering on the moon.
The results were published on September 19 in The Journal of Planetary Science (opens in a new tab).
Follow us on Twitter @Spacedotcom (opens in a new tab) and on Facebook (opens in a new tab).