January 25, 2009 | David F. Coppedge

The Moon Has Core Values

Did the moon have a molten core?  There has been “a long-held consensus that objects in the solar system smaller than than [sic] Mars, can’t sustain magnetic fields,” said National Geographic News based on a paper in Science January 16.1  Apollo rock samples seem to indicate the presence of long-lived magnetism.  It suggests a molten core able to sustain a magnetic dynamo for long period – not heat-shocked magnetism from impacts, which would have dissipated within days.  The original paper explains the unexpected finding:

Before the Apollo missions, the Moon was often thought to be a primordial un-differentiated relic of the early solar system that had never formed a core or generated a magnetic dynamo.  Because it was well known that the Moon presently has no global magnetic field, it was a surprise when the Apollo subsatellites and surface magnetometers detected magnetic fields originating from the lunar crust, and paleomagnetic analyses of returned samples identified natural remanent magnetization (NRM).  The magnetization of many samples must have been produced by ancient magnetic fields, but the association of crustal magnetization with impact structures and the identification of NRM in <200-million-year-old>critical for understanding the thermal evolution of the Moon, the limits of dynamo generation in small bodies, and, by implication, the magnetization of asteroids and meteorites.

The authors argued that the NRM was not due to impacts, but probably to a molten core.  They spoke in terms of billions of years, but admitted in the second paragraph that “A further complication is that the precise thermal histories of most lunar rocks are unknown.  Their magnetization ages have often been assumed to be equal to their radiometric ages, even though thermal events that can remagnetize rocks may have no effect on most geochronometers.”
    What are the implications of positing a liquid core for a small body like the Moon?  “The plausibility of a lunar dynamo has been questioned because of the unconfirmed existence of a fluid metallic core, the difficulty of sustaining a dynamo at least 600 million years after accretion, and large paleointensities of ~100 microteslas that are difficult to reconcile with theoretical predictions.”  They said that there is growing evidence that a small, partially molten core persists today.  National Geographic quoted a French geoscience professor who commented on the consensus-breaking idea that the Moon differentiated and retains an internal melt: “With more studies pointing to the moon’s core, in addition to signs that Mercury and satellites of Jupiter and Saturn have cores, experts are being forced to ‘go backward in that direction,’ [Pierre] Rochette said.”

1.  Ian Garrick-Bethell, Benjamin P. Weiss, David L. Shuster and Jennifer Buz, “Reports: Early Lunar Magnetism,” Science, 16 January 2009: Vol. 323. no. 5912, pp. 356-359, DOI: 10.1126/science.1166804.

If findings are undermining a long-held consensus, if observations are difficult to reconcile with theoretical predictions, and if studies are forcing geoscientists to go backward from their assumptions, then it seems only fair that informed observers of the reasoning of the scientific community have a right to question their confidence about the thermal history and evolution of small bodies in the solar system.  Remember, this is not an isolated instance.  The entire history of planetary science is a record of surprise after surprise.  Very little that was assumed to be true about planetary objects before we got “ground truth” from them still holds today.  What other consensi are up for reversal?  (Yes, the plural is consensuses, but consensi sounds better.)
    Many people just cower in awe before the wizards of planetary science, as if they are onto something wonderful when they weave their imaginative scenarios of the history of the solar system, like the new one that claims Mercury and Mars are just leftover scraps from the formation of Earth and Venus (see this latest weird-science story on National Geographic News).  How could they possibly know such things?  It may be fun for a PhD to spin a “radical new theory” about what happened when he can’t go back in time and watch it, but please don’t call it science.  Science is supposed to mean “knowledge.”  Plausibility and knowledge do not necessary track one another.

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Categories: Physics, Solar System

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