The Moon’s Poles Haven’t Always Been in the Same Place

Recently discovered hydrogen deposits indicate that the Moon once spun on a different axis than it does today. The research, published in Nature this week, shows that over the course of its 4.5 billion year history, changes deep within the Moon's interior caused the poles, and therefore the axis of rotation, to shift about six degrees.

This new finding sheds light on a longstanding mystery about the Moon.

In 1998, NASA launched a special probe called the Lunar Prospector mission to investigate the polar regions of the Moon. Packed with a suite of scientific instruments, the spacecraft spent a year mapping the lunar surface and identifying resources like hydrogen and oxygen, as well as measuring magnetic and gravity fields to better understand how the Moon evolved over its lifetime.

The mission ended in a cloud of regolith as the spacecraft crashed into the lunar surface once its fuel supply ran out. However, before it met its demise, scientists directed it to land in a crater near the south pole—a region thought to be rich in water ice. When there wasn't any water detected in the impact plume, scientists were puzzled as to why.

It turns out that deposits of hydrogen near the lunar poles may provide insight into this mystery. These deposits, which are probably composed of water ice, can only survive in shadowy areas—any exposure to sunlight and they would sublimate.

Since these regions need shadow to survive, researchers explained on Wednesday, if the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed.

"The data shows that the hydrogen deposits are antipodal. Meaning, if you were to draw a line from one to the other, it would pass right through the center of the Moon," said Matthew Siegler, who presented the research with his colleagues at the 47th annual Lunar Planetary Conference on Wednesday. "The deposits are offset from the current poles, but in opposite directions."

This offset means that the current spin axis, or poles, of the Moon has shifted approximately six degrees. Based on the data collected from the Lunar Prospector mission, they believe the shift was caused by a thermal activity below the Procellarum region. This region, located on the nearside of the Moon was geologically active early on in the Moon's history, and is associated with radiogenic elements, and ancient volcanic activity.

The same thermal activity responsible for the shift in poles still exists today. Most likely caused by the heating of radioactive elements, this activity is still affecting the Moon's orientation. Since the Procellarum region was active early on, the researchers also believe that the majority of the hydrogen discovered is ancient, and could be evidence of early water in the inner Solar System.

This type of polar shift may not be exclusive to the Moon, either. "We do see similar evidence with the Earth," said James Tuttle Keane, co-author of the paper. "There's evidence of our planet being a polar wanderer in the magnetic record."