Polar hydrogen deposits show that the Moon once had a different axis to the one it spins on today, according to a paper published this week in Nature. The study suggests that this change in the pole of rotation, known as true polar wander, was caused by changes within the Moon’s interior structure billions of years ago.
The locations of hydrogen-containing deposits near the Moon’s poles, which are probably water ice and which were first detected in the 1990s, are inconsistent with where ice would be expected given the current thermal environment of the Moon.
Matthew Siegler and colleagues find that the Moon’s polar hydrogen deposits are antipodal - that is, directly opposite each other, so that a line drawn from one to the other would pass through the centre of the Moon - and are located equal distances from their respective poles, but in opposite directions. The authors suggest that this evidence shows that the current spin axis of the Moon has shifted by about six degrees and, on the basis of the direction and magnitude of the reorientation, they argue that this shift was caused by a low-density thermal anomaly beneath the Procellarum region of the Moon. The Procellarum region was most geologically active early in the Moon’s history and so the authors conclude that the polar wander occurred billions of years ago and that a large portion of the measured polar hydrogen is ancient, suggesting that water was present early on in the inner Solar System.