The Moon hasn't always been spinning at the same angle as it does today, a new study suggests. The Moon's axis likely shifted sometime around three billion years ago, due to the work of ancient lunar volcanoes. These volcanoes heated up one side of the lunar surface, changing the mass and structure of the space rock's insides and causing the Moon to tip over onto its current axis.
Researchers uncovered this angle adjustment by studying the position of water-ice at the Moon's poles. They used the distribution of the ice to show that the Moon's axis shifted by about six degrees. "It'd be like the Earth's North Pole going to Greenland," said study author Matt Siegler, a researcher at the Planetary Science Institute. His team's work is published today in the journal Nature.
The distribution of the ice shows the Moon's axis shifted by about six degrees
The researchers say their work proves that a large portion of water-ice at the lunar poles formed sometime before the Moon's orientation changed. That means that some of the Moon's ice is from the early years of the Solar System, which is thought to be 4.5 billion years old. "If we were to send astronauts to the poles, they'd be sampling primordial water," said James Keane, a planetary scientist at the University of Arizona.
Scientists first suspected that the Moon held water-ice back in the late 1990s, after hydrogen was discovered on the lunar surface by NASA's Lunar Prospector probe. The existence of water-ice has been inferred by these hydrogen measurements, which indicates the ice is lurking inside craters at the lunar poles — areas that are in constant shadow.
But when studying these polar regions, Siegler and his team noticed that water-ice seemed to be missing from some of the shadowed craters. That was odd, since ice should have accumulated in all the darkened regions over time, not just a few. This suggested that some of the craters may have been exposed to sunlight sometime in the past, causing the water-ice to disappear. "The ice is like a vampire; as soon as it gets hit by sunlight, it poofs into smoke," Siegler said.
The distribution of ice at the poles also pointed toward some kind of lunar shift. The team noticed that water-ice formed trails away from each pole. These two trails are mirror opposites of each other and located at equal distances away from each pole. The researchers agreed that true polar wander — the shifting of the north and south pole — was responsible for this ice placement. The ice trails essentially provide a path from the ancient pole's location to where today's pole is found. "The paths we're seeing are the small parts that have always remained in shadow, even through the Moon's motion," said Siegler. "It's the true permanent shadow on the Moon."
The Moon's water-ice is just as old as the volcanic eruptions, if not older
The culprit of this axis shift was likely ancient volcanic activity on the near side of the Moon — the region permanently facing Earth. Over three billion years ago, volcanism caused this area to get hotter than the rest of the Moon's surface, making the region less dense. This change in mass caused the Moon to tip a little.
The researchers say their work shows the Moon's water-ice is just as old as the volcanic eruptions, if not older. Much of the water-ice seen at the poles is what has survived the Moon's tilt. "The paradigm up until this was that ice was continually being formed on the Moon," said Keane. "We show that some fraction has been preserved since the beginning of the Solar System." Keane hopes their discovery will provide more incentive to explore the Moon's polar ice caps in the future, as it could tell us more about how water-ice came to be on the lunar surface billions of years ago.