Ancient lunar dynamo may explain magnetized moon rocks

Nov 09, 2011
Moon. Photo courtesy of NASA

The presence of magnetized rocks on the surface of the moon, which has no global magnetic field, has been a mystery since the days of the Apollo program. Now a team of scientists has proposed a novel mechanism that could have generated a magnetic field on the moon early in its history.

The "geodynamo" that generates Earth's magnetic field is powered by heat from the inner core, which drives complex fluid motions in the molten iron of the outer core. But the is too small to support that type of dynamo, according to Christina Dwyer, a graduate student in Earth and planetary sciences at the University of California, Santa Cruz. In the Nov. 10 issue of Nature, Dwyer and her coauthors -- planetary scientists Francis Nimmo at UC Santa Cruz and David Stevenson at the California Institute of Technology -- describe how an ancient lunar dynamo could have arisen from stirring of the moon's driven by the motion of the solid mantle above it.

"This is a very different way of powering a dynamo that involves physical stirring, like stirring a bowl with a giant spoon," Dwyer said.

Dwyer and her coauthors calculated the effects of differential motion between the moon's core and mantle. Early in its history, the moon orbited the Earth at a much closer distance than it does today, and it continues to gradually recede from the Earth. At close distances, tidal interactions between the Earth and the moon caused the moon's mantle to rotate slightly differently than the core. This differential motion of the mantle relative to the core stirred the liquid core, creating fluid motions that, in theory, could give rise to a magnetic dynamo.

"The moon wobbles a bit as it spins--that's called precession--but the core is liquid, and it doesn't do exactly the same precession. So the mantle is moving back and forth across the core, and that stirs up the core, " explained Nimmo, a professor of Earth and planetary sciences at UCSC.

The researchers found that a lunar dynamo could have operated in this way for at least a billion years. Eventually, however, it would have stopped working as the moon got farther away from the Earth. "The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off," Dwyer said.

Rocks can become magnetized from the shock of an impact, a mechanism some scientists have proposed to explain the magnetization of lunar samples. But recent paleomagnetic analyses of moon rocks, as well as orbital measurements of the magnetization of the lunar crust, suggest that there was a strong, long-lived magnetic field on the moon early in its history.

"One of the nice things about our model is that it explains how a lunar dynamo could have lasted for a billion years," Nimmo said. "It also makes predictions about how the strength of the field should have changed over the years, and that's potentially testable with enough paleomagnetic observations."

More detailed analysis is needed, however, to show that stirring of the core by the mantle would create the right kind of fluid motions to generate a magnetic field. "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."

A working model of a lunar dynamo, combined with more detailed paleomagnetic analysis of , could give scientists a powerful tool for investigating the history of the moon, Dwyer said. In addition, the study presents a novel mechanism for generating a not only on the moon, but also on other small bodies, including large asteroids.

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User comments : 9

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Isaacsname
4 / 5 (1) Nov 09, 2011
Would something like that have had noticeable effects on Earth which could be found using magnetostratigraphic dating ?
SincerelyTwo
5 / 5 (3) Nov 09, 2011
Would something like that have had noticeable effects on Earth which could be found using magnetostratigraphic dating ?


My guess is no, because earth's magnetic field would have been much stronger on the earths surface than the moons, easily erasing clear evidence of the moon's slight influence on the earth's forming rocks. I'm also taking into consideration that billions of years ago the earth's magnetic field was even stronger than it is today.

If we could detect the moon's 'signature' in rock formations I'd wager it would require prohibitively, maybe impossibly, precise measurements and especially over a very long period of time, in order to detect some type of long-lasting gradient in the measurements.

Consider also that the earth has frequently and chaotically gone through many magnetic reversals over billions of years, with only a handful of clear stretches. Separating out a very subtle smooth signal from that 'noise' is probably far from practical ...
SincerelyTwo
not rated yet Nov 09, 2011
made this comment on the wrong story.
omatumr
1 / 5 (11) Nov 09, 2011
Magnetic Moon rocks were probably magnetized by:

a.) The Earth's magnetic core before lunar material was ejected, or

b.) The far more powerful field of the Sun's pulsar core before it became shielded by waste products.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo
http://myprofile....m/manuel
Ethelred
3 / 5 (2) Nov 10, 2011
Oliver, Neutron Repulsion stops a pulsar from forming

You insist there is such a thing as neutron repulsion. You insist it is strong enough to stop the formation of black holes, not just stellar black holes but ALL black holes no matter what the size. Also it you claim it is long ranged enough to sunder galaxies. Though you refuse to answer any question about its actual strength or range those claims make it clear that it MUST be more powerful than gravity per unit of mass even if the mass is mostly hydrogen atoms as we can see makes up most the mass in the in the Universe, based on your denial of Dark Matter that is.

It really doesn't require a great deal of effort to notice that there is a severe problem with that set of claims. They make galaxies, stars, even neutron stars, planets and pretty much everything held together by gravity impossible.>>

Please explain this contradiction of reality that is an inevitable conclusion based on your own claims for Neutron Repulsion.

Ethelred
DavidMcC
not rated yet Nov 10, 2011
The article says nothing about how the NASA photo was taken. It doesn't look like the near side of the moon in unpolarized light, so what is it? Anyone?
yyz
5 / 5 (2) Nov 10, 2011
"The article says nothing about how the NASA photo was taken. It doesn't look like the near side of the moon in unpolarized light, so what is it? Anyone?"

The photo appears to be a (true?)color image of the lunar farside centered on the Orientale Basin and taken by the outbound Galileo probe in 1996: http://wanderings...luna.jpg

Notice bright Aristarchus at the 2 o'clock position.
pres68y
1 / 5 (4) Nov 10, 2011
With the trust NASA does not deserve I doubt the rocks are from a lunar source.
(more likely just picked up from earth)
DavidMcC
5 / 5 (1) Nov 11, 2011
Thanks, yyz.

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