When the Apollo astronauts returned from the Moon, they brought a puzzle back with them. Some of the rocks they collected were so strongly magnetic, it implied that the Moon’s magnetic field must have been stronger than the Earth’s when the rocks formed 3.9‒3.5 billion years ago. “That doesn’t make any sense with the physics that we understand about how planets generate magnetic fields,” says Claire Nichols, a planetary geologist at the University of Oxford, UK.

Nichols and her Oxford colleagues Jon Wade and Simon N Stephenson have now identified a possible explanation. The key, they say, lies in the rocks’ composition, which happens to provide ideal spacecraft landing sites, leading to sampling bias. “It was a proper kind of Eureka moment,” Nichols says.

The lunar dynamo

The magnetic fields of planets and moons stem from convective currents in their largely iron cores. Scientists expect that objects with smaller cores, such as the Moon, will have lower magnetic field strengths. But measurements of the Apollo samples suggested that the magnetic field strength might, in some cases, have exceeded 100 μT – higher than the typical value of 40μT on the surface of the Earth. It’s as if an AA battery were somehow powering a fridge.

“The dynamo modelling community have been trying to come up with all sorts of mechanisms to give you these really strong fields,” Nichols tells Physics World.

When Nichols mentioned this problem to Wade, a petrologist, his response intrigued her. “He said, kind of as a throwaway comment, ‘Have you looked to see if there’s any link between the composition and the intensities?’”

Upon inspecting the data, Nichols realized that Wade could be onto something. While all the lunar basalt samples with high magnetization contained large quantities of titanium, samples with low magnetization contained little.

A possible mechanism

Other researchers had previously suggested a process that could have supercharged the Moon’s dynamo, boosting the magnetization of titanium-bearing basalt in the process. When the Moon formed, an ocean of molten magma developed that gradually crystallized into today’s lunar mantle. The last material to solidify was a titanium-rich mineral called ilmenite. Solid ilmenite is incredibly dense, so once it solidified, it sank towards the Moon’s magnetic core.

According to the hypothesis, heat transfer across the core-mantle boundary then pushed the ilmenite to its melting point and increased the local temperature gradient, thereby boosting convection and, by extension, magnetic field strength. This means that the ilmenite-bearing rocks supercharged the dynamo behind the Moon’s magnetic field and became unusually highly magnetized in the process. Eventually, volcanic activity brought the rocks to the lunar surface, where the Apollo astronauts collected them.

The problem with this explanation, Nichols says, is that the heat flux at the boundary would only be raised for brief periods, meaning that by this mechanism, only two in every thousand Apollo samples would be strongly magnetized. The real figure is roughly half.

A further role for heat transfer?

Nichols and her colleagues therefore dug deeper into the process. They realized that while the period of melting was brief, it played a crucial role in creating the samples the Apollo astronauts found. “Those samples are all being erupted only at the times where the heat flux is high,” Nichols tells Physics World. And when they eventually made their way to the lunar surface, they did so as part of basaltic flows, which happen to make perfect landing sites for spacecraft.

Case solved? Not quite. According to widely accepted theories of convection in the lunar mantle, the ilmenite lumps could not have got as far as the boundary between the core and mantle, because if they did, they would have lacked the buoyancy to rise again. Still, John Tarduno, whose research at the University of Rochester, US, centres on the origins of Earth’s dynamo, describes Nichols and colleagues’ ideas as “intriguing and certainly worth further consideration through data collection and modelling”.

Tarduno, who was not involved in this work, adds that he isn’t sure that core heat flux alone would ensure that the lunar core once had an intermittent strong dynamo. “The work should motivate numerical dynamo simulations as well as modelling of mantle evolution to test the authors’ ideas,” he says.

Nichols is up for the challenge. By studying additional Apollo samples, together with new ones from the Artemis and Chang’e missions to other parts of the Moon, she aims to determine whether magnetization intensity really does correlate with titanium content, and thereby lay the mystery to rest.

The study appears in Nature Geoscience.

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