How the moon got its magnetism

Earth’s tug or asteroid impacts may have generated the ancient lunar magnetic field

External forces beating up the ancient moon may explain how it once maintained a magnetic field for more than 400 million years — longer than scientists had thought such a small object could be magnetized.

Either wobbly rotation produced by Earth’s gravitational tug or asteroids smacking into the lunar surface may have triggered enough turbulence in the moon’s molten core to generate a long-lasting magnetic field, report two teams of scientists in the Nov. 10 Nature.

“This has been a very fundamental question for 40 years,” says study author Christina Dwyer, a graduate student at the University of California, Santa Cruz. Though absent today, this ancient field is recorded in rocks retrieved from the moon’s surface and in magnetized patches of crust spied by orbiting spacecraft. “The moon was magnetized. We don’t know how.”

Normally, heat escaping from a planet’s interior causes fluid in the core to slosh around, creating a magnetic field. But this explanation doesn’t work for the moon, which would have cooled off too quickly for the sloshing to be maintained. Instead, each research team points to a different spoon that would have mechanically stirred the early moon’s innards to create magnetism.

Dwyer’s team suggests that a slight, Earth-driven wobble in the moon’s axis of rotation mixed the liquid core. “The Earth is tugging on the moon and that tug — even though it’s quite small — keeps the moon wobbling,” says planetary scientist Francis Nimmo of UC Santa Cruz. The wobble is greater when the moon is closer to Earth. As the moon moves farther away — as it has, over the last four billion years or so — the wobble decreases. The magnetic field decreases, too, disappearing completely around 2.7 billion years ago, the team reports. If this model is right, says Nimmo, “younger [lunar] rocks ought to be magnetized, but they ought to be magnetized less strongly than older rocks.”

The second team proposes that large asteroids smacking into the moon messed up its rotation rate and perturbed the liquid core. Six large, ancient lunar impact basins with magnetized rocks at their centers support this idea, says study author and fluid dynamicist Michael Le Bars of the French national research agency and the University of Aix-Marseille. “A reasonably large impact can generate a magnetic field for about 10,000 years,” he says. If so, then lunar rocks might show spike after spike of magnetism as impacts pummeled the early moon.

Though different, the two theories are not mutually exclusive and together comprise “a major, major advance in our understanding,” says Benjamin Weiss, a planetary scientist at MIT. Studying the magnetic record preserved in lunar rocks would be a good way to test the theories, he notes, since tracing magnetic intensities over time should produce a pattern that might match one or both of the predictions.  Right now, scientists are limited to working with the lunar rocks already on Earth — some of which have already revealed magnetic fields, though the measurements are decades old. “It would be nice if somebody goes back and redoes those measurements with the modern techniques,” Nimmo says.

Ian Garrick-Bethell, a planetary scientist at UC Santa Cruz, says both papers are “clever and elegant,” but that more mathematical simulations need to be done. “They’ve done their homework,” he says. “But some of the detailed mechanics may not entirely be understood.”

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