About three billion years ago, inside the moon’s interior, a massive single mantle “plume” shifted. Volcanic activity melted part of the moon’s mantle and caused it to move toward the surface. Researchers liken it to “goo drifting upward in a lava lamp.”
“The moon has a single region of the crust, a large basaltic plain called Procellarum, where radioactive elements ended up as the moon was forming,” Siegler said. “This radioactive crust acted like an oven broiler heating the mantle below.”
He added, “this giant blob of hot mantle was lighter than cold mantle elsewhere. This change in mass caused Procellarum – and the whole moon – to move.”
Did you know: Oceanus Procellarum is Latin for ‘Ocean of Storms.’ It’s the only lunar maria (basaltic plains) with the Oceanus name. That’s because of how big it is. It stretches more 1,600 miles across and covers an area measuring roughly 1.5 million square miles.
This activity led to the moon’s axis shifting 125 miles over the course of a billion years. “It would be as if Earth’s axis relocated from Antarctica to Australia,” said Siegler. “As the pole moved, the Man on the Moon turned his nose up at the Earth.”
Here’s an animation showing the shift.
Ice gave it away
Discovery of the moon’s axis shift came while scientists examined a NASA data set on lunar polar hydrogen. This hydrogen is believed to be in the form of ice and sits in craters around the moon’s north and south poles. Because any exposure to direct sunlight causes the ice to sublimate (turn instantly to gas), the ice offers ancient evidence of the moon’s orientation.
Siegler noticed a weird offset of ice from the moon’s current north and south poles. Using analysis and modeling, Siegler and other scientists showed this offset at each pole was the same distance, but in opposite directions.
The moon’s polar ice appears to ‘paint out’ the path where the poles shifted.
While the moon’s axis shifting 125 miles might not seem like huge news, the ramifications could be huge. Because lunar ice matches up so well with the axis shift, the ice could predate the 125-mile shift. That means the ice could be old, very old.
“The ice may be a time capsule from the same source that supplied the original water to Earth,” says Siegler. “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.”