Earth's moon wandered off axis billions of years ago, study finds

March 23, 2016, Southern Methodist University
A new study published today in Nature reports Earth's moon wandered off its original axis roughly 3 billion years ago. Ancient lunar ice indicates the moon's axis slowly shifted location 125 miles, or 6 degrees, over 1 billion years. Credit: James Keane, U. of Arizona

A new study published today in Nature reports discovery of a rare event—that Earth's moon slowly moved from its original axis roughly 3 billion years ago.

Planetary scientist Matt Siegler at Southern Methodist University, Dallas, and colleagues made the discovery while examining NASA data known to indicate lunar polar hydrogen. The hydrogen, detected by orbital instruments, is presumed to be in the form of hidden from the sun in craters surrounding the moon's north and south poles. Exposure to direct sunlight causes ice to boil off into space, so this ice—perhaps billions of years old—is a very sensitive marker of the moon's past orientation.

An odd offset of the ice from the moon's current north and south poles was a tell-tale indicator to Siegler and prompted him to assemble a team of experts to take a closer look at the data from NASA's Lunar Prospector and Lunar Reconnaissance Orbiter missions. Statistical analysis and modeling revealed the ice is offset at each pole by the same distance, but in exactly opposite directions.

This precise opposition indicates the moon's axis—the imaginary pole that runs north to south through it's middle, and around which the moon rotates—shifted at least six degrees, likely over the course of 1 billion years, said Siegler.

Discovery of lunar polar wander gains the moon entry into an extremely exclusive club. The only other planetary bodies theorized to have permanently shifted location of their axis are Earth, Mars, Saturn's moon Enceladus and Jupiter's moon Europa. What sets the moon apart is its polar ice, which appears to effectively "paint out" the path along which its poles moved. Credit: James Keane, U of Arizona

"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 Siegler, who also is a scientist at the Planetary Science Institute, Tucson, Ariz.

"Billions of years ago, heating within the Moon's interior caused the face we see to shift upward as the pole physically changed positions," he said. "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 discovery is reported today in an article in the scientific journal Nature, "Lunar true polar wander inferred from polar hydrogen." Siegler's primary co-authors are astrophysicist Richard S. Miller, a professor at the University of Alabama Huntsville, and planetary dynamicist James T. Keane, a graduate student at the University of Arizona.

Very few planetary bodies known to permanently shift their axis

Planetary bodies settle into their axis based on their mass: A planet's heavier spots lean it toward its equator, lighter spots toward the pole. On the rare occasion mass shifts and causes a planet to relocate on its axis, scientists refer to the phenomenon as "true polar wander."

Discovery of lunar polar wander gains the moon entry into an extremely exclusive club. The only other planetary bodies theorized to have permanently shifted location of their axis are Earth, Mars, Saturn's moon Enceladus and Jupiter's moon Europa.

What sets the moon apart is its polar ice, which appears to effectively "paint out" the path along which its poles moved.

Planetary bodies settle into their axis based on their mass: A planet's heavier spots lean it toward its equator, lighter spots toward the pole. On the rare occasion mass shifts and causes a planet to relocate on its axis, scientists refer to the phenomenon as "true polar wander."

Billions of years ago, the moon's change in mass was internal -- the shift of a large, single mantle "plume." Ancient volcanic activity some 3.5 billion years ago melted a portion of the moon's mantle, causing it to bubble up toward its surface, like goo drifting in a lava lamp. Credit: James Keane, U of Arizona

Moon's axis likely started relocating about 3 billion years ago

On Earth, polar wander is believed to have happened due to movement of the continental plates. Polar wander on Mars resulted from a heavy volcanic region. The moon's change in mass was internal—the shift of a large, single mantle "plume." Ancient volcanic activity some 3.5 billion years ago melted a portion of the moon's mantle, causing it to bubble up toward its surface, like goo drifting upward in a lava lamp.

"The moon has a single region of the crust, a large basaltic plain called Procellarum, where radioactive elements ended up as the moon was forming," Siegler said. "This radioactive crust acted like an oven broiler heating the mantle below."

Some of the material melted, forming the dark patches we see at night, which are ancient lava, he said.

"This giant blob of hot mantle was lighter than cold mantle elsewhere," Siegler said. "This change in mass caused Procellarum—and the whole moon—to move."

The moon likely relocated its axis starting about 3 billion years ago or more, slowly moving over the course of a billion years, Siegler said, etching a path in its ice.

Over time, the axis shifted 125 miles or 200 kilometers—about half the distance from Dallas to Houston, or equal the distance from Washington D.C. to Philadelphia.

A cross-section through the Moon, highlighting the antipodal nature of lunar polar volatiles (in purple), and how they trace an ancient spin pole. The reorientation from that ancient spin pole (red arrow) to the present-day spin pole (blue arrow) was driven by the formation and evolution of the Procellarum—a region on the nearside of the Moon associated with a high abundance of radiogenic heat producing elements (green), high heat flow, and ancient volcanic activity. Credit: James Tuttle Keane, University of Arizona

Moving 125 miles over a billion years means the moon would have wandered at a rate of one inch every 126 years, or one centimeter every 50 years. That distance is a big deal on the moon, which is only about a quarter the diameter of Earth, Siegler said.

Neutrons can indicate the presence of water or ice

Polar wander explains why the moon appears to have lost much of its ice.

Siegler compares true polar wander to holding a glass filled with water. Most planets are like a steady hand holding a glass, their axis doesn't shift and the water stays put. A planet whose mass is changing is like a wobbly hand, causing its axis to shift and the water to spill out. Similarly, as Earth's moon changed its axis, much of its ice ceased to be hidden from the sun and was lost.

Co-author Richard Miller mapped the moon's remaining ice by using data from NASA's Lunar Prospector mission, which orbited the moon from 1998 to 1999. The presence of ice is inferred by measuring the energy of neutrons emitted from the lunar surface. Instruments on NASA's satellite, including a neutron spectrometer, measured neutrons liberated from the moon by a rain of stellar particles scientists call cosmic rays. Low energy neutrons indicate the presence of hydrogen, the dominant molecule in water and ice.

"The maps show four key features," said Siegler and his colleagues. "First, the largest quantity of hydrogen is offset from the current rotation of the moon by roughly 5.5 degrees. Second, the hydrogen enhancements are of similar magnitude at both poles. Third, the asymmetric enhancements do not correlate with expectations from the current thermal or permanently shadowed environment. And lastly, and most significantly, the spatial distributions of polar hydrogen appear to be nearly antipodal."

Lunar ice is ancient time capsule; may hold answers to deep mysteries

Siegler's discovery opens the door to further discoveries around an even deeper question—the mystery of why there is water on the moon and on Earth. Scientific theory surrounding the formation of the solar system postulates water could not have formed much closer to the sun than Jupiter, Siegel said.

"We don't know where the Earth's water came from. It appears to have come from the outer solar system well after the Earth and moon formed," he said. "Ice on other bodies, like the moon or Mercury, might give us a clue to its origin."

The fact lunar ice correlates so well with true polar wander implies that it predates this motion, Siegler said, making the ice very ancient.

"The ice may be a time capsule from the same source that supplied the original water to Earth," he said. "This is a record we don't have on Earth. Earth has reworked itself so many times, there's nothing that old left here. Ancient ice from the could provide answers to this deep mystery."

Other co-authors on the scientific paper include Matthieu Laneuville, David A. Paige, Isamu Matsuyama, David J. Lawrence, Arlin Crotts and Michael J. Poston.

Explore further: Rare full moon on Christmas Day

More information: M. A. Siegler et al. Lunar true polar wander inferred from polar hydrogen, Nature (2016). DOI: 10.1038/nature17166

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15 comments

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Jeffhans1
5 / 5 (3) Mar 23, 2016
From the article:
"Very few planetary bodies known to permanently shift their axis"
Other than the Earth, Moon and Mars. Have we really studied other bodies long enough to make that statement?
wduckss
2 / 5 (8) Mar 23, 2016
Before the The moon was closer to earth. It completely rules out the possibility of rotation and each is speculation on this subject is wrong and not scientific.
See database of satellites and our system: http://www.svemir...#Natural
Paulw789
not rated yet Mar 23, 2016
I thought the Moon has been tidally locked to the Earth at least as far back as the late heavy bombardment just based on the degree of impact structures.
torbjorn_b_g_larsson
4.5 / 5 (8) Mar 23, 2016
@Jeffhans: The rotation of the gas giants are more or less aligned with the orbit and Sun's rotation, likely inherited from the protoplanetary disk. (See for example Jupiter: https://en.wikipe.../Jupiter )

However in that case they forgot Uranus.

And yes, it was sort of a blanket statement on the most likely default as per above.

@wd: It was about axial tilt, not about the rotation which nowadays are roughly 1/30 days. (The Moon is tidally locked to Earth; Earth's orbit is 10 times slower so the Sun coordinate estimate holds.)

You shouldn't discuss 'scientific', based on your earlier comment history.

@Paul: That is one interpretation of the nearside/farside dichotomy, but these results would reject it if fact. Procellarum looks to be a single mantle diapir from the Moon likely hot (liquid) core, see the updated Apollo measurements. It has also been hinted from a similar magnetic anomaly wandering, so I am happy to see confirmation.
barakn
3.9 / 5 (11) Mar 23, 2016
Paul, check out this video from NASA on lunar libration. Think of the Moon as being only loosely locked. https://www.youtu...21N3wcX8
wduckss
2 / 5 (4) Mar 24, 2016
torbjorn_b_g_larsson

Rotation speed is not in connection with the rotation of the satellite (Pluto day is 6.4, Charon is locked).

There is a big difference between an independent body rotation and rotation locked body.

See data for other satellites in our solar system, http://www.svemir...#Natural

Science begins by observing the data, not with speculation.
obama_socks
1.6 / 5 (5) Mar 25, 2016
torbjorn_b_g_larsson

Science begins by observing the data, not with speculation.
- wduckss
Theoretical scientists do a little of both. First they observe past data; then they try to fit that data to their speculation as to which part of the data their speculation would fit best. Then they enter a mix of the past data along with what they speculate and some complicated math equations and a little bit of mumbo jumbo into a super computer. The computer then spits out a model with a graph and some numbers. They look at the results, check it twice, three times. Then call up some friends aka colleagues. They all get together over some cocktails and do a lot of nodding toward each other in agreement.
One of them calls up their favorite 'zine to request a space since their new data has been already 'peer reviewed'. 'Zine editor says "sure, plenty of room". The author of the paper and his colleagues get their pictures taken and the headline says: "Climate Change Confirmed".
Guy_Underbridge
2.6 / 5 (5) Mar 25, 2016
The author of the paper and his colleagues get their pictures taken and the headline says: "Climate Change Confirmed".
I disagree, but it was pretty damn funny. So I'll give you a 2 instead of the usual 1.
obama_socks
1.8 / 5 (5) Mar 25, 2016
And I gave you a FIVE because I'm feeling charitable toward the needy.
Guy_Underbridge
2.3 / 5 (6) Mar 25, 2016
Thanks buddy, I'm going to take that 5 and get me some cheap wine and a hooker
obama_socks
1 / 5 (2) Mar 25, 2016
Don't forget your Viagra, old timer.
wduckss
1 / 5 (2) Mar 26, 2016
obama_socks

Many computer is blind to input errors. When it would be not fallible lotto would not make sense.
As long we observe the generation of the body and the universe as the bible (4445 years before the world was created or 13.8 or 4.5 billion years) as believers adhering to the Scriptures (hypotéza) instead of evidence, any computer can not help us.
We discuss in the comments the text, not discussing the authors, they must write in order to survive and receive a salary, we do not have that burden.
viko_mx
1 / 5 (2) Mar 31, 2016
The moon is moving away from the Earth at constant speed and there is no way this process to continue fictional bilion of years because the moon would be far from the Earth and would have not a stabilizing effect on the Earth's axis of rotation.
This world will perish due to deficiency of love
Da Schneib
5 / 5 (2) Mar 31, 2016
Seriously, the klimate kooks now want to argue about the axis of the Moon?

I mean, really?

Wait whut that doesn't I don't even
Captain Stumpy
5 / 5 (2) Mar 31, 2016
The moon ... there is no way ...fictional bilion of years because blah blah blah religious fundie horsesh*t blah
@viko

ah, so basic math isn't part of your cultural education program either, then????

thanks for that
maybe that is why you have such a hard time comprehending the studies which refute your claims?

http://theness.co...o-close/

LMFAO

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