Earth’s moon wandered off its original axis about 3 billion years ago, a new study led by Southern Methodist University planetary researcher Matt Siegler reports.
Paleo- and present-day lunar poles. According to scientists, planetary bodies settle into their axis based on their mass: a planet’s heavier spots lean it toward its equator, lighter spots toward the pole. On the rare occasion mass shifts and causes a planet to relocate on its axis, researchers refer to the phenomenon as ‘true polar wander.’ Discovery of lunar polar wander gains the Moon entry into an extremely exclusive club. Image credit: James Keane, University of Arizona.
Dr. Siegler and his colleagues from the United States and Japan discovered antipodal water deposits on the Moon that may indicate its poles have shifted over time.
An antipodal water distribution pattern means that water deposits at the north and south lunar poles point in opposite directions.
“This is unexpected, since if the water is ‘recent’ it should be distributed approximately uniformly around the poles. This led us to postulate that the off-axis and antipodal nature of the water deposits might hint at an ancient pole,” said co-author Dr. Richard Miller from the University of Alabama in Huntsville.
“If the water distribution is indicative of ancient lunar poles — called paleo-poles — it suggests the Moon tipped over, or re-oriented, sometime in the past.”
That would mean that billions of years ago the Moon was oriented differently, with a different set of poles, and then tipped over to its current orientation.
“The Moon likely relocated its axis starting about 3 billion years ago or more, slowly moving over the course of a billion years, etching a path in its ice,” Dr. Siegler said.
“Over time, the axis shifted 125 miles (200 km), or 6 degrees.”
The scientists made the discovery while examining data from NASA’s Lunar Prospector and Lunar Reconnaissance Orbiter missions known to indicate lunar polar hydrogen.
“This was such a surprising discovery,” Dr. Siegler said. “We tend to think that objects in the sky have always been the way we view them, but in this case the face that is so familiar to us — the Man on the Moon — changed.”
“Billions of years ago, heating within the Moon’s interior caused the face we see to shift upward as the pole physically changed positions.”
“It would be as if Earth’s axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth.”
The team’s discovery, published this week in the journal Nature, opens the door to further discoveries around an even deeper question — the mystery of why there is water on the Moon and on Earth.
“Scientific theory surrounding the formation of the Solar System postulates water could not have formed much closer to the Sun than Jupiter,” Dr. Siegel said.
“We don’t know where the Earth’s water came from. It appears to have come from the outer Solar System well after the Earth and Moon formed. Ice on other bodies, like the Moon or Mercury, might give us a clue to its origin.”
“The fact lunar ice correlates so well with true polar wander implies that it predates this motion, making the ice very ancient.”
“The ice may be a time capsule from the same source that supplied the original water to Earth.”
“This is a record we don’t have on Earth. Our planet has reworked itself so many times, there’s nothing that old left here. Ancient ice from the Moon could provide answers to this deep mystery.”
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M.A. Siegler et al. 2016. Lunar true polar wander inferred from polar hydrogen. Nature 531, 480-484; doi: 10.1038/nature17166
