May 02, 2016

The same face of the Moon has not always pointed towards the Earth. The spin axis of the Moon has moved by at least six degrees and that motion is recorded in ancient lunar ice deposits, reports PSI’s Matthew Siegler and his colleagues in a new paper in the journal Nature.

This motion is believed to have resulted from a warm, low-density region of the lunar mantle below the dark patches known as the lunar maria. The same heat source that caused the volcanic maria to form also warmed the mantle. This is the first physical evidence that the Moon underwent such a dramatic change in orientation and implies that the ice on the Moon is billions of years old.

Siegler is lead author on the Nature paper “Lunar True Polar Wander Inferred from Polar Hydrogen.” The new findings help explain the earliest dynamical and thermal history of the Moon and shed light on the origin of lunar water.

“We found that the polar shift required to explain the distribution of ice matches perfectly with the existence of a fossilized mantle plume below the lunar mare,” said Siegler. “So, the same thing that caused the dark lavas that make up the face of the Man on the Moon also caused the axis of the Moon to move – and it is recorded in the polar ice.”

“This ice distribution tells us the near side of the Moon shifted towards the North Pole – so the Man on the Moon is sort of turning his nose up at the Earth. This gives us a way to model exactly where the ice should be, which tells us about its origin and where astronauts might find a drink on future missions to the Moon.”

Above, this illustration shows a cross-section through the Moon, highlighting the antipodal nature of lunar polar volatiles (in cyan), and how they trace an ancient spin pole (green arrow). The reorientation from that ancient spin pole (green arrow) to the present-day spin pole (blue arrow) was driven by the formation and evolution of the Procellarum—a region on the nearside of the Moon associated with a high abundance of radiogenic heat producing elements, high heat flow, and ancient volcanic activity.