Moon mystery: How long did its magnetic field last?

The moon as seen by the Galileo spacecraft in 1992, on its way to Jupiter. NASA/JPL/USGS/JPL/USGS

In the pantheon of astronomical bodies, the moon is a puzzle piece that doesn’t quite fit. There are the rocky planets, spheres of a certain heft with a layer-cake-like structure of core, mantle, and crust. Then there are the asteroids, little more than pieces of rubble from the early solar system that have far less structure inside.

The moon is smaller than a planet, bigger than an asteroid.

For decades, scientists have debated whether our closest neighbor in the solar system is more like a planet or a piece of planetary debris. Understanding the moon’s interior structure — including the key question of whether it had a “dynamo,” a swirling core that creates a magnetic field — could help explain its ancient origins.

But such research could also explain how such dynamos form and evolve on planetary bodies — insights that could aid in the hunt for habitable worlds elsewhere in the galaxy.

“Earth’s magnetic field plays an important role in shielding life on earth from harmful solar particles that could induce damage; magnetic fields nurture a planet’s habitability,” said Sonia Tikoo, a postdoctoral researcher at the University of California, Berkeley. “If we can figure out the types of bodies that can generate a long-lived magnetic field, long enough for life to evolve on another planet, then that can tell us something” when looking for other planets, or moons, where life might have arisen.

In a paper published in the journal Science on Thursday, Tikoo and Benjamin Weiss, a professor of planetary sciences at the Massachusetts Institute of Technology, reviewed the growing evidence for the moon’s dynamo and the search for explanations for its surprisingly long-lived magnetic field.

Since the Apollo astronauts collected and brought back the first lunar rocks, it has been known that the rocks on the moon’s surface are magnetized. But that evidence was far from definitive: It could have been the signature of a planet-like process in which the moon once had a churning core dynamo that created a magnetic field. Or the signature might have imparted to the surface rocks by some external event.

New analyses of Apollo-era moon rocks over the past six years have provided strong evidence the moon had a magnetic field caused by an ancient core dynamo. That field was also surprisingly long-lasting, for more than a billion years — until at least 3.5 billion years ago.

That means the scientific debate has now switched from whether the moon had a core dynamo and magnetic field to how it kept going for so long. The leading theory for the origin of the moon is that it resulted when an object about the size of Mars slammed into the early Earth. But the heat from that collision probably wouldn’t have caused the core to keep swirling long enough to sustain the magnetic field.

That means Weiss, Tikoo, and others are working to understand what other mechanisms can power dynamos and keep the magnetic fields going. One theory, for example, is that the gravity of the earth tugged on the mantle of the moon, causing it to rotate differently from the core.

Now that scientists are convinced the moon did have its own dynamo, they have one other question that could help them understand how it worked.

“When did the thing turn off?” Weiss said. The answer to that simple question would allow scientists to begin to better grasp how the magnetic field was powered in the first place. And such insights could help understand how such fields evolve and persist on other small planetary bodies — perhaps aiding in the search for habitable worlds.