The Oddly Magnetic Moon

There’s a problem with the Moon. Rocks from Earth’s natural satellite show evidence of magnetic fields existing too recently in lunar history to fit the theory of how the Moon’s magnetic field is created. Now, two papers in Nature offer different mechanisms for how a lunar dynamo could have been maintained long after it should have been dead.

“These are extremely important papers, because we suspected that there were relatively late magnetic fields on the Moon for a long time,” says Benjamin Weiss (Massachusetts Institute of Technology), who wasn’t involved with either study. “These provide a mechanism for doing it.”

The Moon's mare show up nicely in this color mosaic of images taken by the Galileo spacecraft. Researchers studying three of the basins shown here, Serenitatis, Humboldtianum, and Crisium (middle left, center, and center bottom respectively) suggest that the large impacts that created these basins could have jump-started the Moon's magnetic dynamo.
NASA / JPL / USGS
Earth creates its global magnetic field through the convection of its metallic liquid core, “lava-lamp style,” explains Weiss. The convection is basically driven by the planet’s gradual cooling over time. Liquid metal (like solid metal) conducts electricity, and when an electrical conductor moves in the presence of a weak field electric current is generated inside the conductor — which creates more magnetic field, in a runaway process like the one used in an artificial, self-sustaining dynamo. But for small bodies like the Moon, cool-off should have come pretty fast — so fast that convection would soon stop and a core dynamo would cease to exist. For the Moon the cutoff date was around 4.2 billion years ago, according to models of the Moon’s evolution. In fact, Weiss and his collaborators confirmed that there was a field on the Moon 4.2 billion years ago from studies of a lunar rock.

Yet magnetic hints pop up in lunar rocks that are hundreds of millions of years younger than that. Rocks encode magnetic fields that prevailed at the time they solidified. Certain atoms align with a background field like little bar magnets when they are free to move, as they are in lava. As the rock solidifies these mini magnets become locked in place, preserving a record of the ancient field.

A magnetic field doesn’t have to come from a core dynamo. Impact-created plasmas could create local, short-lived fields lasting about a day. Work is ongoing to determine how many lunar samples gained their magnetization by a long-lived dynamo as opposed to more transient processes like impacts, says Weiss. But there are so many magnetized lunar rocks of various ages, he adds, that “it’d be hard to believe that they’re all from an impact.”

The two papers suggest different ways that a dead dynamo could have restarted inside the Moon to create a late, long-lasting field.

• In the first, a difference between the spin axes of the core and mantle is the culprit. In this model, the core’s spin axis once pointed perpendicularly to the ecliptic plane, while the mantle’s axis was slightly off from that and precessing around the core’s axis. In the case of Earth, the core and mantle are locked together, so this process can’t work. But in the Moon this precession — driven by Earth as the Moon circled farther and farther out in its orbit over time — created a stirring mechanism.

“It’s sort of analogous to a laundry machine,” Weiss explains. If the chamber precesses as it spins, the water inside is stirred even though there’s no propeller in the water moving it around.

Once the Moon receded far enough from Earth, about 48 Earth radii — which would have happened 2.7 billion years ago, Christina Dwyer (University of California, Santa Cruz) and her colleagues predict in the paper — the dynamo would have shut off from insufficient power.

• The second theory stirs the core by moving the mantle in a totally different way: by smacking it with a huge impact big enough to jerk the Moon out of synchronous rotation. A team of French and Belgian researchers looked at six lunar craters that contain magnetic anomalies, places where magnetic fields are preserved in the crust from bygone days. The researchers suggest that the melt rocks in these basins, all from around 4 billion year ago, probably formed their anomalies as they cooled in the presence of a magnetic field.

Humboldtianum Basin
The Humboldtianum Basin, shown here in false color based on altimeter data from the Lunar Reconnaissance Orbiter, spans 650 kilometers and sinks 4.5 km deep. The impact that created the basin may have jolted the Moon out of synchronous rotation with Earth, one study suggests.
NASA / GSFC
“The large impacts that we need in our model to make a dynamo were present around 4 billion years ago, which is exactly the time when the Moon’s dynamo is expected,” explains coauthor Michael Le Bars (IRPHE, CNRS and Aix-Marseille Université, France). The hits came all within about 100 million years of each other, he notes, and each could have created a temporary dynamo lasting 2,000 to 8,000 years. If the hit caused longitudinal oscillations in the Moon, the effect could last a bit longer, maybe 10,000 years.

“That’s really, really short,” Weiss says. “I mean, the Moon’s billions of years old.”

Still, the theory is a good one. “This is an elegant and carefully thought-out idea that creates a dynamo just long enough to magnetize cooling, molten rocks that formed in the very same crater event,” says Ian Garrick-Bethell (University of California, Santa Cruz), who worked on the 2009 study.

Both theories predict surface magnetic fields of around 1 microtesla, matching previous predictions. Earth’s field at its surface is about 50 times greater.

Distinguishing between these theories will depend in part on figuring out which rocks were magnetized when. Big bull’s-eyes happened pretty rarely in lunar history. If an impact created a dynamo, any molten surface rock around the time of the crash —such as lava created by the hit itself — would record the magnetic field created. But lava that erupted on the surface between these infrequent events wouldn’t. If most lunar rocks everywhere were magnetized during a particular time period, including rocks not made by impacts, that would sway the balance toward the precession argument, Weiss says. If impact melts are always associated with a magnetic field, the balance swings the other way.

But the mechanisms aren’t exclusive, either. Both could have happened at different times in the Moon’s history, or together.

“The problem with the lunar magnetic record is that it is very confusing,” Garrick-Bethell explains. “The patterns of magnetism you see in the Moon's crust are nothing like what you see on the Earth. The magnetism in its rocks may have been magnetized by exotic shock processes that don't operate on the Earth.”

For now, the mystery stands, albeit less darkly.

The studies affect more than our understanding of the Moon’s magnetization. The question of whether the Moon even has a core, instead of being a “pile of primordial space dust” like an asteroid, as Weiss puts it, has recently been clarified. That both theories depend on the Moon having a liquid-metal core is “one of the major reasons for caring about this,” Weiss says. “If the Moon generated a magnetic field in a core, by definition it has a core.”

And because the Moon is a half-step between planet and asteroid, the models might explain how asteroids could have magnetic fields. “This is interesting from the perspective of understanding the Moon,” Weiss says. “It’s also interesting from the perspective of just understanding the physics of how magnetic fields are generated by planets.”

9 thoughts on “The Oddly Magnetic Moon

  1. nootaramus

    the moon was probably manufactured and moved into orbit around the earth. it has hollow characteristics.

    odd that if we actually went to the moon in the late 60’s ( with pathetically inept computing power but with a dune buggy that ran flawlessly on electric power still not available today), we have never attempted a return trip. very odd. and no visual proof that we were ever on the moon aside from the doctored photos with numerous light sources- no stars and a breeze blowing the flag.

    the moon is -in my opinion- an observatory for aliens to observe and study humans and our planet.

    one day we will know for sure- at any rate, the moon did not just happen- it is in too precise an orbit and its size is too perfect for it to have been a spun-off accident. no way.

  2. Marshall

    To the commenter who thinks the moon is manufactured…Are you serious? (JK, of course you are). I submit you have ssen too many Star Trek episodes (possibly on ‘shrooms). Please, this is not the site for your psychotic ramblings. If I offend, it is because I was offended. Please sir, refrain from your flat-earth type musings and we will all be much happier!

  3. Alen

    A scientist should always have an open mind, even to other people’s "ramblings". There is no need to be offended if someone show different oppinion

  4. Raul

    US spent nearly a trillion $ and involved half a million people in the project. And people died in the project. No other country could done it. Today is impossible to anyone do it again, in face of the money involved. I’m offended when someone, without any evidence, even think that humankind is incapable of do the amazing. The heroes of the moon era are just old or dead and someone brainwashed want to eliminate they accomplishment!

  5. Tonster

    The magnetohydrodynamicists have a sweet technical conundrum on their hands, trying to account for the Moon’s remnant magnetism. There are bound to be lots more papers to be submitted to Icarus and elsewhere.
    However, I am more fascinated by that Topo of Mare Humboldtianum. Its outer rim is nearly hexagonal in shape, being spoiled only by those two deep craters North Preceding. To top it off, the inner basin is also a hexagon, but twisted with respect to the outer one. Now, what process might have brought that about? Nearby Mare Crisium is much more circular, as are most other large Lunar impacts.
    @ Nootaramus, the Moon is entirely natural. Being so close to us makes it extremely rare; I doubt there would be another twin planet to ours within 30 thousand light years. This makes Alien Visitation extremely unlikely. We Earthlings are most likely all alone in this Galaxy.

  6. uwe

    But, it´s seems that you do not account the fact that the pure friction of the movement , revolution, of the moon can make a magnetik field, as the comb has it by combing,
    I would apreciate your comment about it.

  7. nootaramus

    I stand by my statements.

    That many think a moon type object could not be manufactured is very terrestrial thinking.

    as for the moon missions, maybe they happened but there is considerable documentation that perhaps the moon missions were faked to impress the Russians we had much stronger technical capabilities than they could muster.

    Stanley Kubrick had just made a very realistic movie-2001 A Space Odyssey. he certainly could have assisted in producing a very realistic mission on a soundstage, which I think he did.There are a very credible group who think we did not go to the moon in the late 60’s and early 70’s.

    Guss Grissom was a very vocal in his criticism of NASA and what they were capable of. He died in a fire that was of very mysterious orgin.

    And there is still no tangible evidence on the moon that we were ever there except some very blurry photos of something indistinguishable on the moon’s surface.

    and I did watch what appeared to be the lunar landing in 1969. Flawless transmission with no transmission delay, too perfect. I don’t think we pulled it off. and we sure as hell have not attempted even a robotic mission back to the lunar surface.

  8. Billy

    Oh please. We went to the moon. Its just that simple. The rest of the "resonable doubt" as to the validity of that statement is based soley on ignorance. If people really knew the facts then there would be no questions. It’s like listening to a bunch of guys in a bar who have all answers to financial wisdom. It is not being closed minded by ridiculing those of us who choose to believe it was a faked moon mission. I MUST MENTION THIS: THERE IS NO CREDIBLE GROUP OF PEOPLE THAT BELIVE WE FAKED THE MOON MISSION. Those people are wackos. Ok old timer, if you think that 2001 has realistic special effects, well….. And you know, the moon used to be even closer than it is now. And, the moon is moving farther away every year, by about an inch I think. And you know how we know that. There is a laser system set-up with mirrors on the moon and we can track its movent very accurately. Silly really, so sad.

  9. Warren

    Mr. Nootaramus, the Russians have radio and TV antenneas too. They could certainly tell where those transmissions were coming from, and if we had faked them, THEY would have known and told the world! Certainly they had every motivation to call us on any deception.

    The Mythbusters program, or a similar one, has taken every supposed evidence for faking and, in detail, proved it wrong. And then of course we have the recent photos taken from Lunar orbit, showing definitely non-natural objects and foot paths. Plus there are the astronomers who continue to bounce laser beams from the mirrors we left up there (think all of THEM are lying too??!). Sorry but you’re just barking up the wrong tree.

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