Planetary science: Signs of a wandering Moon.

Abstract

The presence of ice at two positions on opposite sides of the Moon suggests that the satellite's orientation was once shifted away from its present spin axis — a finding that has implications for the Moon's volcanic history. See Letter p.480 Matthew Siegler and co-authors show that the polar hydrogen deposits detected on the Moon by orbital neutron spectroscopy pose something of a mystery: the spatial distribution of hydrogen, which is thought to arise from the presence of water ice, does not match that expected from present-day lunar temperatures. The explanation may lie in the phenomenon known as 'true polar wander', in which a reference point on a solid body rotates with respect to its spin axis. Based on an analysis of polar deposit locations and of the lunar figure, the authors suggest that the shift occurred as a result of changes in the Moon's moment of inertia caused by a low-density thermal anomaly beneath the Procellarum region. Procellarum was most geologically active early in lunar history, which would imply that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System.