A new study reveals that the geological activity underneath the lunar crust of the moon has led to the shift of its poles over the eons. The team revealed the surprising finding during their analysis of the distribution of water ice near the lunar north and south poles, which not only advances our understanding of the structure and evolution of the moon but provides more clues as to how water made its way onto Earth.
"The ice at the poles of the moon records the interior evolution of the moon, which seems crazy - that is the last place you would think to look," said Matt Siegler, lead author of the study. "Also, that means the ice has to be really old, and therefore may record the ancient delivery of ice to the inner solar system."
Siegler and his team studied measurements made by NASA's Lunar Prospector spacecraft and Lunar Reconnaissance Orbiter, both of which have previously found evidence that the moon contains water ice in the craters near its poles.
The results confirmed ice deposits at both poles, but also revealed large patches of ice at both poles in locations offset from the true poles by 5.5 degrees. Furthermore, these deposits are positioned in a way that they are connected to the center of the moon by a straight line, leading the team to suggest that the moon's rotational axis has shifted by 5.5 degrees over time.
The shift likely stemmed from volcanic activity in the Procellarum, a region that includes the majority of the moon's dark spots that can be seen from the Earth. The geologic activity generated from these activities made these areas warmer and lighter than the rest of the moon, producing a drop in density that could have created a wobble enough to create the two new locations, called "paleopoles."
Modeling techniques suggest that the paleopoles were likely the moon's actual poles approximately three billion years ago, before the rotational axis shifted and they moved around 200 kilometers over the course of one billion years.
"Models are models, so you can make the migration happen any time between 1.5-4.5 billion years ago depending on how you tweak parameters (such as the past rigidity of the lunar crust), but it most likely was around three billion years ago," Siegler said. "This was such a surprising discovery. We tend to think that objects in the sky have always been the way we view them, but in this case the face that is so familiar to us - the Man on the Moon - changed."
"It would be as if Earth's axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth," he concluded.
The findings were published in the March 23 issue of Nature.
