Friday, January 27, 2012

Remnant magnetism hints at once-active lunar core

A piece of lunar sample 10020, a rock that appears to carry
the signature of a past magnetic field on the moon [NASA].
John Matson
Scientific American

The moon of today is a static orb with little to no internal activity; for all intents and purposes it appears to be a dead, dusty pebble of a world. But billions of years ago the moon may have been a place of far more dynamism—literally.

A new study of a lunar rock scooped up by Neil Armstrong and Buzz Aldrin during their Apollo 11 mission indicates that the ancient moon long sustained a dynamo—a convecting fluid core, much like Earth's, that produces a global magnetic field. The age of the rock implies that the lunar dynamo was still going some 3.7 billion years ago, about 800 million years after the moon's formation.

That is longer than would be expected if the lunar dynamo were powered primarily by the natural churning of a cooling molten interior, as is the case on Earth. The moon's small core should have cooled off rather quickly and put an end to any dynamo-generated magnetic field within a few hundred million years. So researchers may have to explore alternate explanations for how a dynamo could be sustained—explanations that depart from thinking of the lunar interior in terms of Earthly geophysics.

A standard-issue, Earth-like dynamo "would have died out on the moon much, much before 3.7 billion years ago," says Erin Shea, a graduate student in geology at the Massachusetts Institute of Technology and lead author on a study in the January 27 issue of Science. "We have to start thinking outside the box about what generates a lunar dynamo."
A lunar sample collected by Apollo astronauts suggests that other-Earthly geophysics drove the moon's churning interior
Using a high-resolution magnetometer, the researchers found that the lunar sample indeed formed in the presence of a magnetic field, perhaps even one as strong as Earth's magnetic field today. "What this sample tells us is that at some point the moon did have a dynamo," Shea says. "This magnetic field lasted much longer than we had considered before."

A similar paleomagnetic study in 2009 by some of Shea's co-authors demonstrated the presence of a lunar dynamo some 4.2 billion years ago. That is just at the cusp of what would be possible with an Earth-like dynamo driven by a cooling interior alone. "Even then it's not trivial," says Ian Garrick-Bethell, a planetary scientist at the University of California, Santa Cruz (U.C.S.C.), who was the lead author of the 2009 study.
Read the full online article HERE.

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