The Oddly Magnetic Moon

There’s a problem with the Moon. Rocks from Earth’s natural satellite show evidence of magnetic fields existing too recently in lunar history to fit the theory of how the Moon’s magnetic field is created. Now, two papers in Nature offer different mechanisms for how a lunar dynamo could have been maintained long after it should have been dead.

“These are extremely important papers, because we suspected that there were relatively late magnetic fields on the Moon for a long time,” says Benjamin Weiss (Massachusetts Institute of Technology), who wasn’t involved with either study. “These provide a mechanism for doing it.”

Earth creates its global magnetic field through the convection of its metallic liquid core, “lava-lamp style,” explains Weiss. The convection is basically driven by the planet’s gradual cooling over time. Liquid metal (like solid metal) conducts electricity, and when an electrical conductor moves in the presence of a weak field electric current is generated inside the conductor — which creates more magnetic field, in a runaway process like the one used in an artificial, self-sustaining dynamo. But for small bodies like the Moon, cool-off should have come pretty fast — so fast that convection would soon stop and a core dynamo would cease to exist. For the Moon the cutoff date was around 4.2 billion years ago, according to models of the Moon’s evolution. In fact, Weiss and his collaborators confirmed that there was a field on the Moon 4.2 billion years ago from studies of a lunar rock.

Yet magnetic hints pop up in lunar rocks that are hundreds of millions of years younger than that. Rocks encode magnetic fields that prevailed at the time they solidified. Certain atoms align with a background field like little bar magnets when they are free to move, as they are in lava. As the rock solidifies these mini magnets become locked in place, preserving a record of the ancient field.

A magnetic field doesn’t have to come from a core dynamo. Impact-created plasmas could create local, short-lived fields lasting about a day. Work is ongoing to determine how many lunar samples gained their magnetization by a long-lived dynamo as opposed to more transient processes like impacts, says Weiss. But there are so many magnetized lunar rocks of various ages, he adds, that “it’d be hard to believe that they’re all from an impact.”

The two papers suggest different ways that a dead dynamo could have restarted inside the Moon to create a late, long-lasting field.

• In the first, a difference between the spin axes of the core and mantle is the culprit. In this model, the core’s spin axis once pointed perpendicularly to the ecliptic plane, while the mantle’s axis was slightly off from that and precessing around the core’s axis. In the case of Earth, the core and mantle are locked together, so this process can’t work. But in the Moon this precession — driven by Earth as the Moon circled farther and farther out in its orbit over time — created a stirring mechanism.

“It’s sort of analogous to a laundry machine,” Weiss explains. If the chamber precesses as it spins, the water inside is stirred even though there’s no propeller in the water moving it around.

Once the Moon receded far enough from Earth, about 48 Earth radii — which would have happened 2.7 billion years ago, Christina Dwyer (University of California, Santa Cruz) and her colleagues predict in the paper — the dynamo would have shut off from insufficient power.

• The second theory stirs the core by moving the mantle in a totally different way: by smacking it with a huge impact big enough to jerk the Moon out of synchronous rotation. A team of French and Belgian researchers looked at six lunar craters that contain magnetic anomalies, places where magnetic fields are preserved in the crust from bygone days. The researchers suggest that the melt rocks in these basins, all from around 4 billion year ago, probably formed their anomalies as they cooled in the presence of a magnetic field.

“The large impacts that we need in our model to make a dynamo were present around 4 billion years ago, which is exactly the time when the Moon’s dynamo is expected,” explains coauthor Michael Le Bars (IRPHE, CNRS and Aix-Marseille Université, France). The hits came all within about 100 million years of each other, he notes, and each could have created a temporary dynamo lasting 2,000 to 8,000 years. If the hit caused longitudinal oscillations in the Moon, the effect could last a bit longer, maybe 10,000 years.

“That’s really, really short,” Weiss says. “I mean, the Moon’s billions of years old.”

Still, the theory is a good one. “This is an elegant and carefully thought-out idea that creates a dynamo just long enough to magnetize cooling, molten rocks that formed in the very same crater event,” says Ian Garrick-Bethell (University of California, Santa Cruz), who worked on the 2009 study.

Both theories predict surface magnetic fields of around 1 microtesla, matching previous predictions. Earth’s field at its surface is about 50 times greater.

Distinguishing between these theories will depend in part on figuring out which rocks were magnetized when. Big bull’s-eyes happened pretty rarely in lunar history. If an impact created a dynamo, any molten surface rock around the time of the crash —such as lava created by the hit itself — would record the magnetic field created. But lava that erupted on the surface between these infrequent events wouldn’t. If most lunar rocks everywhere were magnetized during a particular time period, including rocks not made by impacts, that would sway the balance toward the precession argument, Weiss says. If impact melts are always associated with a magnetic field, the balance swings the other way.

But the mechanisms aren’t exclusive, either. Both could have happened at different times in the Moon’s history, or together.

“The problem with the lunar magnetic record is that it is very confusing,” Garrick-Bethell explains. “The patterns of magnetism you see in the Moon's crust are nothing like what you see on the Earth. The magnetism in its rocks may have been magnetized by exotic shock processes that don't operate on the Earth.”

For now, the mystery stands, albeit less darkly.

The studies affect more than our understanding of the Moon’s magnetization. The question of whether the Moon even has a core, instead of being a “pile of primordial space dust” like an asteroid, as Weiss puts it, has recently been clarified. That both theories depend on the Moon having a liquid-metal core is “one of the major reasons for caring about this,” Weiss says. “If the Moon generated a magnetic field in a core, by definition it has a core.”

And because the Moon is a half-step between planet and asteroid, the models might explain how asteroids could have magnetic fields. “This is interesting from the perspective of understanding the Moon,” Weiss says. “It’s also interesting from the perspective of just understanding the physics of how magnetic fields are generated by planets.”