Billions of years ago, a volcanic eruption shifted the axis of the Moon a few degrees. And some ancient ice leftover from that time is telling the story of that mysterious change and raising more questions.
The man who noticed evidence of this ancient shift in the Moon’s axis is planetary scientist Matt Siegler. While looking at data from the Lunar Prospector gathered in the late 1990s, he noticed something a little off, United Press International reported.
Siegler was looking at the distribution of hydrogen ice on the Moon’s poles, which is thought to collect in the pits of its shadowed craters. The deposits didn’t line up with the Moon’s north and south poles but was off-kilter a bit and in opposite directions.
What he’d found was proof that the Moon’s axis had shifted long ago by about six degrees. The axis changed very gradually over a billion years.
“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.”
In other words, the off-kilter ice deposits mark “palaeopoles” — or former north and south poles with the ice collected billions of years ago, the Guardian added.
This phenomenon is fairly rare, but the Earth, Mars, Saturn’s moon Enceladus, and Jupiter’s moon Europa have all experienced a shift in axis.
Siegler described the axis shift by continuing to use the Man in the Moon as a metaphor.
“Billions of years ago, heating within the Moon’s interior caused the face we see to shift upward as the pole physically changed positions. 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.”
The culprit is the volcanic activity that occurred 3.5 billion years ago, a time when volcanoes on the near-side of the Moon erupted and lava flowed across its surface. Evidence of those flows can be seen today as dark expanses of basalt.
The axis shift was likely caused by an internal change of mass. Ancient volcanoes actually heated up a bit of the Moon’s surface, and the mantle below melted. As a result, the whole Moon moved.
“It is like taking a football and removing a chunk out of one side — that would change how the football would spin in the air,” said researcher James Keane.
Then, the volcanic activity stopped.
“The moon cools very, very slowly so even though there is not enough heat to cause volcanism today there is still excess heat in the region. Presumably if the moon had an infinite amount of time to cool off we would predict it would go back to that original pole.”
As it cooled, the Moon’s axis drifted to its current tilt of 1.54 degrees. These once-shadowed craters moved into the sunlight, and the ice hiding inside melted away. Enough of it still remains to provide evidence of the axis shift billions of years ago.
With the finding comes more questions. Dr. Ian Garrick-Bethell, who was not involved in the study, said scientists don’t really know much about those hydrogen deposits, noting that “we think it is ice — we don’t have any idea what kind.”
“There’s a whole bunch of mysteries there. Are these ice deposits really billions of years old? How could they have survived that long? It’s amazing.”
What’s even more amazing is that it could be water, not hydrogen, and, therefore, solves the enduring mystery of where water on Earth came from. Siegler called it a time capsule.
“This is a record we don’t have on Earth,” he said. “Earth has reworked itself so many times, there’s nothing that old left here. Ancient ice from the moon could provide answers to this deep mystery.”
[Photo by Dan Kitwood/Getty Images]