How the Moon Became Magnetized

by Jason Major on November 10, 2011

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Apollo 17 astronaut Harrison "Jack" Schmitt at Tracy Rock on the lunar surface. Credit: NASA.

It’s been a mystery ever since the Apollo astronauts brought back samples of lunar rocks in the early 1970s. Some of the rocks had magnetic properties, especially one collected by geologist Harrison “Jack” Schmitt. But how could this happen? The Moon has no magnetosphere, and most previously accepted theories state that it never did. Yet here we have these moon rocks with undeniable magnetic properties… there was definitely something missing in our understanding of Earth’s satellite.

Now a team of researchers at the University of California, Santa Cruz thinks they may have cracked this enigmatic magnetic mystery.

In order for a world to have a magnetic field, it needs to have a molten core. Earth has a multi-layered molten core, in which heat from the interior layer drives motion within the iron-rich outer layer, creating a magnetic field that extends far out into space. Without a magnetosphere Earth would have been left exposed to the solar wind and life as we know it could may never have developed.

Apollo 17 lunar rock sample

Simply put, Earth’s magnetic field is crucial to life… and it can imbue rocks with magnetic properties that are sensitive to the planet-wide field.

But the Moon is much smaller than Earth, and has no molten core, at least not anymore… or so it was once believed. Research of data from the seismic instruments left on the lunar surface during Apollo EVAs recently revealed that the Moon may in fact still have a partially-liquid core, and based on a paper published in the November 10 issue of Nature by Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz, and her co-authors Francis Nimmo at UCSC and David Stevenson at the California Institute of Technology, this small liquid core may once have been able to produce a lunar magnetic field after all.

The Moon orbits on its axis at such a rate that the same side always faces Earth, but it also has a slight wobble in the alignment of its axis (as does Earth.) This wobble is called precession. Precession was stronger due to tidal forces when the Moon was closer to Earth early in its history. Dwyer et al. suggest that the Moon’s precession could have literally “stirred” its liquid core, since the surrounding solid mantle would have moved at a different rate.

This stirring effect – arising from the mechanical motions of the Moon’s rotation and precession, not internal convection – could have created a dynamo effect, resulting in a magnetic field.

This field may have persisted for some time but it couldn’t last forever, the team said. As the Moon gradually moved further away from Earth the precession rate slowed, bringing the stirring process – and the dynamo – to a halt.

“The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off,” said Christina Dwyer.

Still, the team’s model provides a basis for how such a dynamo could have existed, possibly for as long as a billion years. This would have been long enough to form rocks that would still exhibit some magnetic properties to this day.

The team admits that more paleomagnetic research is needed to know for sure if their proposed core/mantle interaction would have created the right kind of movements within the liquid core to create a lunar dynamo.

“Only certain types of fluid motions give rise to magnetic dynamos,” Dwyer said. “We calculated the power that’s available to drive the dynamo and the magnetic field strengths that could be generated. But we really need the dynamo experts to take this model to the next level of detail and see if it works.”

In other words, they’re still working towards a theory of lunar magnetism that really sticks.

 

Read the article by Tim Stephens on the UCSC website.

 

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

CJ Wijtmans November 10, 2011 at 12:39 PM

The sun maybe? Also it could have retained some magnetism when the moon cooled off.

Torbjörn Larsson November 10, 2011 at 1:09 PM

The Moon was rotating with respect to the Sun, even after tidal lock with Earth, so remnant fields would average out.

Even if not, the carried field from the Sun, the interplanetary magnetic field, is but a few nT. Even if the plasma keeps it as such high strengths (order of magnitude higher than a naive r^-3 dipole field would give), todays remaining Moon field can be 1-2 order of magnitude higher, ~ 100 nT.

Add that here we are talking of, presumably, 1-2 order of magnitude yet stronger original fields on the order of microtesla. (Earth’s field is some ~ 10-100 microtesla at the surface, see Wikipedia.) The Sun IMF is simply too weak to do this.

Kev Girard November 10, 2011 at 1:55 PM

Has the possibility of other bodies that carried magnetic rocks been ruled out yet? It is possible that other planetary bodies have had collisions in the past in which resulted in magnetic rocks flying and hitting the moon.

Lights in the Dark November 10, 2011 at 2:45 PM

While shocks via impacts can magnetize rocks, paleomagnetic studies of the Moon show that there was more likely a stronger, long-term magnetic field in play at one point.

IVAN3MAN_AT_LARGE November 10, 2011 at 7:33 PM

Yo Jason, at the third paragraph, in the second line, you have a t missing in multi-layered.

Torbjörn Larsson November 10, 2011 at 1:33 PM

An early Moon magnetic field is interesting for early Earth history, since it could affect our own early atmosphere escape. See the ref below.

The early field ties in with models predicting the far side local maximum and other asymmetries of the Moon near vs far face. With a liquid core you can expect a convective mantle. If the first mantle diapir (rising convection bulge) freezes before it can kick-start plate tectonics as in our case, you get the pretty 1st harmonic magnetic field observed here and at the correct field strength.

Some of the other high magnetic field strength areas is fitted with the mismatch of diapir vs crust rotation, i.e. the diapir “painted” the crust with remnant field in a wobbly but strengthening track as it rose. You can probably pick that up in the linked image, starting on the near side! “Thermal core-mantle coupling in an early lunar dynamo: Implications for a global magnetic field and magnetosphere of the early moon”, Takahasi & Tsunakawa, Geo Phys Res Ltrs 2009; it is one of those “predicts so much” results you see now and then, and personally I found it interesting albeit somewhat technical in places.

The Bobs November 10, 2011 at 5:04 PM

“Simply put, Earth’s magnetic field is crucial to life…”

I doubt this is true. When the polarity of the earth’s field reverses, there is a period of a few to several thousand years when there is no field. The is no evidence that anything dramatic happens to life during these times. The atmosphere provides sufficient protection.

Lights in the Dark November 10, 2011 at 5:29 PM

The magnetosphere does’t disappear during pole reversals, it actually becomes more complex and unpredictable. But it’s still there, shielding Earth from the solar wind, cosmic rays and electromagnetic storms. Thankfully.

squidgeny November 10, 2011 at 8:02 PM

Wouldn’t deep-sea life (as around the hydrothermal vents, far from the surface) still be able to exist without the magnetic field though? I know that’s a picky point, if true, but still ;)

Lights in the Dark November 10, 2011 at 8:49 PM

I’m probably guilty of being species-centric. I would wager that a magnetic field is crucial to our life. I’m sure there’s some critters around that wouldn’t miss it one bit.

Torbjörn Larsson November 10, 2011 at 8:03 PM

I don’t know how to say this without being offending, but the last part of the analysis can be a religious type of argument. Even if true vs field strength, claiming it is necessary at that point, which you may or may not claim, is the same as saying as long as I pray nothing bad will happen.

You have to assert the shielding claim anyway, and yes it shields from some of that (but not all, see above), but is it crucial for life? Likely not.

Lights in the Dark November 10, 2011 at 8:48 PM

Well I didn’t mean “thankfully” as in “thank you Lord for the lovely magnetic field you-really-shouldn’t-have”, more as in as a living breathing person on Earth right now I’m sure glad it’s there.

What about all the notions about Mars, lacking such a magnetic field, has had most of its atmosphere “stripped away” by the solar wind? There seems to be a sense of causality there, I’d say.

Torbjörn Larsson November 11, 2011 at 1:04 PM

Maybe we are talking cross purpose, I was referring to the logic of a part of the argument and not its wording. Ah, well.

Yes, I think Mars was likely stripped as it is small (but I haven’t seen the science). That would fit under “the existence of planetary dipole fields increase habitability potential of planets immensely” from my other comments. Next sentence went: “But for Earth, we could perhaps made it to life anyway.”

Here is a possible model: super-Earth’s habitability is scarcely affected by not having intrinsic magnetic fields, unless they started out with little atmosphere and/or an unruly star. When you get down to Earth size presumably some planets like ours may squeeze by potentially without having magnetic fields as a requisite. Mars analogs needs all the help they can get!

In the light of possible controversy*, I think your initial claim was too strong. I see now you have revised it, and the result looks valid to me. Thanks for the response, by the way!

—————–
* There is controversy over how magnetic fields originate too. IIRC there is a paper that claims super-Earths will have convectively generated fields regardless of any tidal lock to stars. So they can be close to M stars and yet be better protected from such stars more unruly behavior.

Torbjörn Larsson November 11, 2011 at 1:04 PM

Maybe we are talking cross purpose, I was referring to the logic of a part of the argument and not its wording. Ah, well.

Yes, I think Mars was likely stripped as it is small (but I haven’t seen the science). That would fit under “the existence of planetary dipole fields increase habitability potential of planets immensely” from my other comments. Next sentence went: “But for Earth, we could perhaps made it to life anyway.”

Here is a possible model: super-Earth’s habitability is scarcely affected by not having intrinsic magnetic fields, unless they started out with little atmosphere and/or an unruly star. When you get down to Earth size presumably some planets like ours may squeeze by potentially without having magnetic fields as a requisite. Mars analogs needs all the help they can get!

In the light of possible controversy*, I think your initial claim was too strong. I see now you have revised it, and the result looks valid to me. Thanks for the response, by the way!

—————–
* There is controversy over how magnetic fields originate too. IIRC there is a paper that claims super-Earths will have convectively generated fields regardless of any tidal lock to stars. So they can be close to M stars and yet be better protected from such stars more unruly behavior.

Torbjörn Larsson November 10, 2011 at 7:57 PM

Ha, I didn’t see that one. It is one of my main itches to scratch.

Your assessment is, to my knowledge, mainly correct with respect to its conclusion. What shields us from cosmic rays are mainly the heliosphere (~ 90 % IIRC) and our own atmosphere. What would shield us from solar wind and mass ejection effects in the absence of magnetic field is the atmosphere.

The magnetic field protects the atmosphere from mainly CME attrition, but CMEs stands for “only” ~ 1/3 of the attrition IIRC. Presumably, we would have ~ 2/3 of our atmosphere without it.

That simply means the oxygen content would have been ~ 40 % (liberated from water) instead of ~ 20 %, assuming the amount is regulated by the tendency to set vegetation on fire. Also, birds would be short distance travelers (huge effort).

I can understand why the magnetic field is fetishized, who wouldn’t want an invisible force field overhead which existence can be verified by playing with magnets. But it isn’t rational or borne out by the facts, AFAIK.

What seems correct is that the existence of planetary dipole fields increase habitability potential of planets immensely. But for Earth, we could perhaps made it to life anyway.

Fun story: I was at an astrophysicist seminar where the same analysis was raised. (Yes, there may be scientists supporting the “not so important as you think” result.) Boy, did the discussion heat up! So it is one of these “we all know this” claims which floats around and which you can’t get to the source facts to because “we all know this”. (O.o)

Anonymous November 11, 2011 at 9:54 PM

I like the following statement.. lots: “Research of data from the seismic instruments left on the lunar surface during Apollo EVAs recently revealed that the Moon may in fact still have a partially-liquid core…”

How goes the GRAIL mission? any news? http://moon.mit.edu/

Lights in the Dark November 10, 2011 at 8:53 PM

Thank you…

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