The moon has joined an exclusive club of theoretical celestial wanderers.
Publishing in Nature, a group of scientists reported that more than 3 billion years ago the Earth’s moon shifted from its original axis by around 125 miles over the course of a billion years.
The discovery was made after scientists glimpsed hydrogen ice data from NASA’s Lunar Prospector and Lunar Reconnaissance Orbiter.
“These deposits (which are probably composed of water ice) survive only if they remain in permanent shadow,” the researchers wrote in their study. “If the orientation of the moon has changed, then the locations of the shadowed regions will also have changed.”
And that’s exactly what the researchers found when they pored through the data.
The phenomenon, known as “true polar wander,” is thought to have occurred on four other celestial bodies within the solar system: Earth, Mars, the Saturn moon Enceladus, and the Jupiter moon Europa.
While polar wander is spurred by varying types of geological activity, on the moon, the shift started internally.
According to the researchers, a portion of the moon’s mantle melted and bubbled upwards to form a radioactive crust, which eventually became the moon’s dark splotches, basaltic plains known as Procellarum. The melted mantle was lighter than the cold mantle.
“Radiogenic heating within this region resulted in the bulk of lunar mare volcanism and altered the density structure of the moon, changing its moments of inertia,” according to the researchers. “This resulted in true polar wander consistent with the observed remnant polar hydrogen.”
Using data from NASA’s Lunar Prospector, the team measured the amount of leftover ice by looking at the neutron energy given off from the body’s surface. Low energy neutrons are indicators of hydrogen.
According to researchers, the largest quantity of hydrogen is offset from the moon’s current axis by around 5.5 degrees, and is of a similar magnitude at both poles.
The moon’s hydrogen ice may provide scientists with clues regarding the source of Earth’s water.
“The ice may be a time capsule from the same source that supplied the original water to Earth,” said study co-author Matthew Siegler in a prepared statement, who’s a planetary scientist with Southern Methodist University. “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.”