Ancient Lunar Ice Reveals the Moon’s Poles Aren’t Where They Used to Be

March 29, 2016 | Joanne Kennell

Diagram showing the old and new poles of the moon
Photo credit: James Keane/University of Arizona

This ice could be a clue to how Earth got its water.

A rare event occurred on the moon roughly three billion years ago — it moved from its original axis.

Published in the journal Nature, planetary scientist Matt Siegler at Southern Methodist University, Dallas, and colleagues Richard S. Miller, a professor at the University of Alabama Huntsville, and planetary dynamicist James T. Keane, a graduate student at the University of Arizona, reported that while examining NASA data known to indicate lunar polar hydrogen — a form of ice hidden from the sun in craters surrounding the moon’s north and south poles — they discovered an odd offset of the ice from the moon’s current north and south poles.

NASA’s Lunar Prospector and Lunar Reconnaissance Orbiter missions detect lunar polar hydrogen, which is very sensitive to direct sunlight because it causes the ice to boil off into space, meaning it is also very sensitive to the moon’s orientation.

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Statistical analysis and modeling of the data revealed that the ice is offset at each pole by the same distance, but in exactly opposite directions. This indicates that the moon’s axis — the imaginary pole that runs north to south through it’s middle, and around which the moon rotates — shifted at least six degrees, likely over the course of one billion years, according to Siegler.

“This was such a surprising discovery. 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,” said Siegler in a press release.

“Billions of years ago, heating within the Moon's interior caused the face we see to shift upward as the pole physically changed positions,” Siegler continued. “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.”

Planetary bodies settle into their axis based on their mass: A planet’s heavier spots lean it towards its equator, while the lighter spots towards the pole, and on rare occasions, this mass shifts and causes the planet to relocate on its axis, a phenomenon known as “true polar wander.”

This puts the moon into an extremely exclusive club. The only other planets theorized to have shifted on their axis are Earth, Mars, Saturn’s moon Enceladus, and Jupiter’s moon Europa. On Earth, polar wander is believed to have happened due to the movement of the continental plates, while on Mars it resulted from a heavy volcanic region. However, the moon’s change was an internal shift of a large portion of the moon’s mantle.

The moon likely began relocating its axis around three billion years ago, slowly moving over the course of a billion years to where it is now. The axis shifted 200 kilometers (125 miles) — the distance between Washington D.C. and Philadelphia.

Photo credit: James Keane/University of Arizona

It is this polar wander that explains why the moon appears to have lost most of its ice. Siegler compares true polar wander to holding a glass filled with water: Most planets are like a steady hand holding a glass — the axis doesn’t shift and the water stays put. However, a planet whose mass is changing is like a wobbly hand, causing the axis to shift and the water to spill out.

The hope is that this discovery will help solve the mystery of where Earth’s water came from.

“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,” he said. “Ice on other bodies, like the moon or Mercury, might give us a clue to its origin.”

“The ice may be a time capsule from the same source that supplied the original water to Earth,” Siegler explained. “This is a record we don't have on Earth. Earth 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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