The powerful magnetic field that surrounded the fledgling moon billions of years ago probably originated from the roiling lunar interior, not asteroid impacts.
Magnetized lunar material toted back during the Apollo missions established that the moon at some point had a magnetic field. Decades after NASA’s manned moon missions, a flurry of new research shows that this ancient magnetic field lasted for more than a billion years. At one point, it was as least as strong as the one generated by modern Earth.
Reviewing this recent work in the Dec. 4 Science, planetary scientists Benjamin Weiss of MIT and Sonia Tikoo of the University of California, Berkeley conclude that this magnetic field originated from flowing material inside the moon.
“Despite the fact that the moon is only about 1 percent of the Earth’s mass,” Weiss says, “this tiny little thing seemed to have generated whopping magnetic fields lasting for an extremely long time — longer than you might think would be possible.”
When a rock heats up, some of the electrons inside can move freely. In the presence of a magnetic field, these electrons will align with the field, creating regions of rock similar to a bar magnet. As the rock cools, the electrons can no longer alter their orientation and the aligned electrons create their own magnetic field. While the modern moon lacks a global magnetic field, magnetized rocks offer a record of what the lunar magnetic field was like when they cooled down billions of years ago.
The discovery of a long-lasting lunar magnetic field created a problem for researchers, Weiss says. The Earth produces its field through the motion of molten rock sloshing around the outer core, creating a dynamo — a setup where electrically conductive fluid moving inside a magnetic field induces a secondary magnetic field. But the moon’s puny size means that it would cool off relatively quickly. A magnetic field created by the moon’s hot interior alone could not have persisted for hundreds of millions of years.
Eventually two competing explanations emerged: Either some outside force helped stir the lunar interior and drove the dynamo, or the magnetized rocks were created by short-lived fields produced during violent asteroid impacts on the lunar surface (SN: 12/17/11, p. 17). Over the last six years, improvements in measurement techniques and highly detailed lunar orbiter maps of the magnetized lunar crust sparked Weiss, Tikoo and other scientists to reexamine the Apollo rocks for new clues.
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The researchers gauged the age of the rocks by looking at the amount of radioactive potassium inside that had decayed into stable argon. Using this technique they found the most strongly magnetized rock samples dated back to 4.25 to 3.56 billion years ago and that a magnetic field persisted for more than a billion years.
The moon rocks contain crystals, many of them fractions of a millimeter wide, that formed as the rock solidified, the researchers discovered. In lab experiments, scientists found that the longer it took the rock to cool, the larger the resulting crystals, allowing researchers to use crystal size to determine how long a rock was hot and its electrons susceptible to alignment by magnetic fields. Cooling times for the rocks varied from 100 days to 10,000 years, the researchers found.
But superheated plasma created when large asteroids slam into the lunar surface can produce a magnetic field for at most only one or two days. Since the rocks took significantly longer to cool, then scientists could rule out asteroids as the main driver of ancient lunar magnetism. The ancient magnetic field must have originated from a dynamo, not asteroids.
“The moon is in many respects like a planet, rather than something like an asteroid” as many other moons in the solar system are, Weiss says. “The moon is layered, it has a core, it had volcanism and now we know it generated a magnetic field.”
Scientists still don’t know what sustained the moon’s dynamo. One possible explanation is tidal effects caused by Earth’s gravity, says planetary scientist Francis Nimmo of the University of California, Santa Cruz. The solid exterior of the moon rotates as it’s tugged along its orbit, but a liquid core, if present, would resist rotation. At the boundary, Nimmo explains, the outer layer would blend and heat up the liquid interior through friction.
This process could have kept the moon warm enough to support a dynamo for hundreds of millions of years. Over time, as the moon’s orbit moved away from Earth, this effect would have weakened.