New research shows that the moon’s poles have changed as a result of asteroid collisions over billions of years.
Astronomers have long used lunar craters to plot the history of both the moon and all Solar System, because the distribution of devastation caused by asteroid impacts paints a picture of the violent conditions present in the young solar system. The new research turns the tables around those studies by simulating the removal of thousands of craters and also by looking at the effects of smaller craters, thus returning 4.25 billion years of the moon’s history.
The researchers, based at NASA’s Goddard Space Flight Center in Maryland, found that while the moon was swinging asteroid Striking, its north and south poles wander about 10 degrees in latitude — the equivalent of about 186 miles (300 kilometers).
The Moon’s geographical poles are located where its axis of rotation – the imaginary line that revolves around it – intersects with the surface of the Moon. Simulations showed that during the movement of the lunar body, the axis of rotation remained constant.
The discovery could shed light on how Earth’s natural satellite evolved and could help researchers locate water and other resources that could be used for future manned space missions.
Scientists found frozen water in Cold dark areas at the poles of the moon, but how much water there is a mystery. By understanding how and where the poles shift, researchers can work out how much frozen water directly transformed from solid ice into gas — a process called sublimation. An extreme shift in the positions of the Moon’s poles—particularly toward warmer, less shaded regions on the Moon—could have caused water to sublimate quickly and lose it to space, giving new water less time to accumulate at the poles.
“Based on the history of lunar craters, it appears that polar wandering was moderate enough for water near the poles to remain in the shadows and have stable conditions over billions of years,” Vishnu Viswanathan, a research scientist at NASA Goddard who led the study on the moon’s wandering poles, said in a statement (Opens in a new tab).
The displacement of the poles is caused by a phenomenon called “true polar wander,” which occurs when a rotating object encounters obstacles, such as a change in its mass distribution. In the case of the Moon, this occurred when asteroid collisions created a deep depression in the Moon’s surface, redistributing mass and regions of lower mass.
The moon reorientated itself, turning these “pockets” of low mass toward the poles. When this happened, centrifugal force—the same force that flattens and stretches dough at the base of a pizza—moved regions of higher mass toward the lunar equator.
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“If you look at the moon with all these craters, you can see the ones in the gravitational field data,” said David E. Smith, a research scientist at MIT and co-author of the new research. “I thought, ‘Why can’t I just take one of those pits and suck it in, and completely remove the signature?’ “
Smith is the principal investigator for the Lunar Orbiter Laser Altimeter (LOLA) instrument aboard the Lunar Reconnaissance Orbiter and has experience using the gravity Data to assess the shift at the lunar poles. Smith, Viswanathan and their team used LOLA data to design computer models that took the coordinates and widths of 5,200 lunar craters ranging from 12 to 746 miles (19 to 1,200 kilometers) in diameter.
Next, the team matched the impact craters to pockets of higher or lower gravity found on a lunar gravity map created using data from NASA’s Gravitational and Interior Recovery Laboratory. They ran these simulations backwards, removing these high and low gravity pockets and thus sequentially erasing the pits according to their age. This undoing of the Moon’s evolution returned the poles to the positions they occupied billions of years ago.
Researchers have attempted a similar process previously, but by focusing only on the largest lunar craters, those efforts did not take into account the net effect of smaller impacts on the moon’s poles.
“People assumed the little pits were negligible,” Viswanathan said. “They are neglected individually, but collectively, they have a huge impact.”
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 the moon’s polar wander.
The results were published on September 19 in Planetary Science Journal (Opens in a new tab).