If you’re an avid watcher of the night sky — or a Pink Floyd fan — you probably know that we Earthlings always see the same side of the moon. We always see the bright side, while the far side (the dark side) hides from our view. That’s because the moon spins slowly on its axis, completing one rotation in the same amount of time it takes to orbit the Earth and producing a kind of lunar optical illusion.
Now, a new study in the journal Nature suggests that the moon’s axis of rotation has changed over time. Researchers found evidence for the shift by looking at maps of the moon’s hydrogen deposits, which are thought to represent pockets of water ice. These maps were generated fifteen years ago by NASA spacecraft that measured the composition of the moon’s surface. They did so by studying the flux of neutrons produced by cosmic ray impacts — which varies depending on the surface material.
Scientists think that ice can only accumulate in the cold, shadowy craters at the moon’s poles, but that’s not the only place the maps showed hydrogen deposits. When the researchers reanalyzed NASA’s data, they discovered that two large deposits of hydrogen sit about 6 degrees off of the current polar axis on exact opposite sides of the moon. The scientists say this suggests the deposits did form at the poles, but that the moon’s axis has since changed.
“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,” said Matt Seigler, a planetary scientist at Southern Methodist University and the lead author of the study, in a statement. The scientists calculated that when the axis shifted, the pole migrated about 200 kilometers. And as it did, “the Man on the Moon turned his nose up at the Earth,” Seigler said.
Several processes can reorient the axis around which a planet — or a moon — rotates, including meteorite impacts and changes inside a planetary body that shift around its mass. The researchers first studied the impact craters on the moon’s surface, looking for events that could have kicked the moon out of alignment. But they didn’t find any that were the right size or in the right spot to explain the change in orientation implied by the hydrogen deposits.
Instead, they say the most likely cause of the shift was a change in the internal structure of the moon. Specifically, they linked the change in lunar orientation to a well-known region known as the Oceanus Procellarum. This vast plain of smooth, basaltic rock is clearly visible from Earth as a large dark blotch on the moon’s face.
Scientists originally thought the region formed as the result of an asteroid impact that unleashed massive lava flows. However, recent research suggests the region represents a part of the moon’s crust that is enriched in radioactive elements. Over time, these elements would have given off heat that melted the mantle “like an oven broiler,” Seigler said, fueling Precellarum’s eruptions.
These elements also could have caused the axis of rotation to shift by changing the moon’s internal structure. That’s because hot material is less dense that cold material. And as the Procellarum region heated up, the internal structure of the moon changed, altering its orientation so that the less dense region moved closer to the pole.
The researchers say this discovery could shed light on more than just the moon’s history — it could help scientists answer one of the major outstanding questions about Earth: where did our water come from? For many years, scientists have thought that Earth and the other rocky planets were too hot to retain any water when they formed, and that water came later, delivered by comet or meteorite impacts. Other researchers have suggested that water may have existed throughout the solar system from the very beginning.
The moon’s ice might provide clues about what really happened because it probably hasn’t much changed over time — like water in Earth’s active hydrologic cycle has — and because it could be very old. The positions of the moon’s hydrogen deposits suggest they formed before the axis shifted more than 3 billion years ago. However, the researchers say that they won’t know more until scientists do more work, like sending a lander to measure the moon’s water directly or even bring samples back to Earth.
“The ice may be a time capsule from the same source that supplied the original water to Earth,” said Seigler in the statement. “Ancient ice from the moon could provide answers to this deep mystery.”
Learn more about Earth’s hydrologic cycle in our module.
Learn more from PBS about why we always see the same side of the moon.
Learn more from Science News about where Earth’s water came from.
Written by Julia Rosen
Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelors degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her Master's in backpacking around the world and skiing. Julia is a periodic contributor to Oregon States research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.