Ancient Moon Had Earthlike Core?

Christine Dell'Amore
National Geographic News
January 15, 2009
The mystery of the moon's magnetic field may finally be explained by the presence of an Earthlike core, a new study says.

Magnetic moon rocks picked up on the Apollo missions in the late 1960s and early 1970s surprised scientists, who thought no such field existed on the moon.

Since then, two competing theories have emerged for the moon's magnetism: shockwaves locking in magnetic fields generated by meteors slamming into the heavily cratered surface, or the movement of heat inside a molten metallic core.

New analysis of the oldest known "unshocked" Apollo sample—a rock never affected by a meteor impact—favors the iron core theory, researchers say. (See photos of moon explorers.)

That's because the 4.2-billion-year-old rock has a history of longer, slower cooling periods, a discovery more consistent with the core's influence than occasional meteor impacts.

"There's growing evidence for the internal structure of the moon having a core, and this study supports those theories," said study lead author Ian Garrick-Bethell, a Ph.D. student from Massachusetts Institute of Technology.

(Related: "Moon Has Iron Core, Lunar-Rock Study Says" [January 11, 2007].)

Relic Thinking

Before the Apollo missions, lunar scientists thought the moon was just a rock—a relic of the solar system's formation that never formed a core.

Detecting what's at the moon's heart is tricky, with only ambiguous data available, said Garrick-Bethell, whose work appears tomorrow in the journal Science.

Even Earth's core is only known through seismology, the study of earthquakes and the seismic waves that pass through the planet, he added.

This uncertainty has pushed lunar scientists to adopt creative approaches, such as paleomagnetism—tracing the moon's ancient magnetic field.

In the new study, Garrick-Bethell and colleagues built on paleomagnetism with another technique of studying moon rocks: Looking at their heat—or thermal—histories.

The team was able to determine that the rock heated and then cooled only twice in 4.2 billion years.

These cooling events were more drawn out than what would be expected if a barrage of meteors had magnetized the rock.

Magnetic fields created by meteor impacts usually last a day at most, Garrick-Bethell said.

Furthermore, the rock dates to an era during which the moon would have most likely had an active core, the study authors say.

Going Backward

Pierre Rochette, a geoscience professor at Université d'Aix-Marseille 3 in France, said the result "makes again a strong case for both the presence of a metallic core in the moon and … more firm magnetic evidence."

The research could also reveal more about the wider solar system, added Rochette, who was not part of the study.

For instance, there is a long-held consensus that objects in the solar system smaller than than Mars, can't sustain magnetic fields.

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," Rochette said.

Likewise, the new moon research can also tell scientists more about the moon itself, such as when its dark features known as "mare plains" formed, study lead author Garrick-Bethell said.

The large basaltic areas were likely caused by ancient volcanic eruptions.

"This is related to features everyone looks up and sees, these dark plains," Garrick-Bethell said.

"If we can do this study for lots of other rocks, we have the potential to unravel the internal thermal history and evolution of the moon."

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