Spots on opposite sides of Moon suggest its axis tilted


A new study hints that the Moon’s poles may have shifted over billions of years, with geological activity beneath its crust being the most likely reason behind this.
“The ice at the poles of the Moon records the interior evolution of the moon, which seems crazy — that is the last place you would think to look,” said Planetary Science Institute and Southern Methodist University researcher and study lead Matt Siegler. “Also, that means the ice has to be really old, and therefore may record the ancient delivery of ice to the inner solar system.”
Various observations from space missions over the years hints that there is a lot of water ice in the Moon, particularly in dark craters located near the poles. Keeping this in mind, Siegler’s team took data from two spacecraft – the Lunar Prospector, which went around the Moon from 1998 to 1999, and the still-active Lunar Reconnaissance Orbiter (LRO).
While there was nothing shocking about the ice deposits at both of the Moon’s poles, Siegler and colleagues discovered a large ice patch near each of the poles. These were “displaced” from the actual pole by about 5.5 degrees, and located in such a way that they can be connected by an imaginary straight line through the center of the Moon. According to Siegler, that means the moon’s axis has shifted by 5.5 degrees over time.
“Models are models, so you can make the migration happen any time between 1.5-4.5 billion years ago depending on how you tweak parameters (such as the past rigidity of the lunar crust), but it most likely was around 3 billion years ago,” Siegler observed. He added that the poles eventually shifted by approximately 125 miles over the next one billion years, or about one inch per 126 years.
The researchers believe this axis shift took place because of changes in how lunar mass was distributed internally.
“Planets can change their orientation if their internal mass distribution changes. Pockets of dense material tend to be close to the equator to minimize the planet’s spin energy,” said University of California-Santa Cruz’s Ian Garrick-Bethell in a Nature editorial accompanying the study. He cited the example of New York’s latitude shifting slightly southward “if a huge pile of lead weights” appeared in the city, and shifting northward if the city’s density went down.

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