Ice near the lunar north and south poles show a shift in the moon's axis
The moon's poles have shifted over the eons, likely as a result of geological activity beneath the lunar crust, a new study suggests.
This finding — which is based on an analysis of the distribution of water ice near the lunar north and south poles — sheds light on the structure and evolution of the moon, and also provides clues about where Earth's water came from, researchers said.
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"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 study lead author Matt Siegler, of the Planetary Science Institute in Tucson, Arizona, and Southern Methodist University in Dallas.
"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 told Space.com via email.
Water on the moon
The moon's rotational axis has shifted by 5.5 degrees over the ages
Observations made by a variety of spacecraft over the past few decades suggest that the moon harbors a lot of water ice in permanently shadowed craters near the poles, which are some of the coldest locales in the solar system.
Siegler and his colleagues studied measurements made by two of these probes: NASA's pioneering Lunar Prospector (LP) spacecraft, which circled the moon from January 1998 through July 1999, and the agency's Lunar Reconnaissance Orbiter (LRO), which is still in operation.
The orbiters' data revealed ice deposits at both poles, as expected. But there was a surprise as well: A large patch of ice exists near each pole, in a spot offset from the true pole by 5.5 degrees. Moreover, these "displaced" deposits are positioned such that a straight line drawn through the center of the moon would connect them.
Siegler and his team have an explanation for this finding, which they reported online March 23 in the journal Nature: The moon's rotational axis has shifted by 5.5 degrees over the ages, and the offset ice patches mark the "paleopoles."
Modeling work suggests these paleopoles were the actual poles about 3 billion years ago, Siegler said.
"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 3 billion years ago," he said.
The lunar poles then shifted by about 125 miles (200 kilometers) over the course of one billion years or so — a rate of 1 inch (2.5 centimeters) every 126 years, the researchers think.
"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," Siegler said in a statement. "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."
