Lunar Polar Wander

The North and South Poles of the Moon haven't always been where they are today. Asteroid impacts both large and small, most of which occurred early in the Moon's history, have changed the Moon's mass distribution. After each impact, the Moon gradually rebalanced itself around its spin axis. This kind of rebalancing, in which a planet or moon reorients itself while the spin axis continues to point in the same direction in space, is called true polar wander.

In a study published in the Planetary Science Journal, David E. Smith, Vishnu Viswanathan, and their coauthors used maps of the Moon's topography and gravity, based on data gathered by NASA's LRO and GRAIL missions, to infer the evolution of the Moon's mass distribution and its effect on the location of the poles. They found that impacts have moved the poles almost 10 degrees in latitude – 300 kilometers or 190 miles – over the roughly 4.25 billion years since the cataclysmic event that created the South Pole-Aitken basin.

The visualization on this page shows the polar wander calculated by the computer simulation created for the study. As seen here, the South Pole arrives in roughly its present position relatively early in the Moon's history, raising the possibility that some of the water ice and other volatiles trapped in permanently shadowed regions near the South Pole may be up to 3.8 billion years old.

See NASA Study: Small Craters Add Up to Wandering Poles on the Moon