For decades, planetary scientists have debated a fundamental question about the moon’s early history: Did it once generate a powerful or a weak magnetic field? A new study suggests both options are true.
Today, the moon doesn’t have a magnetic field at all. But some rocks — notably, many samples returned from NASA’s Apollo missions — have strong cues of magnetism, indicating the moon once had a magnetic field comparable to, or even stronger than, Earth’s. That interpretation implied the young moon once hosted a vigorous internal dynamo — a molten, convecting core capable of generating a global magnetic field, much like Earth’s today.
But some scientists argued that because the moon is relatively small, it would have struggled to sustain such a powerful field for hundreds of millions of years. An alternative theory proposed the moon’s core only generated a weak magnetic field, suggesting only massive asteroid impacts may have temporarily amplified it.
Now, scientists from the University of Oxford offer a resolution to the debate, reporting the moon could have experienced bursts of extremely strong magnetism long ago, but that these episodes would have been fleeting. For most of its early history — between 3.5 and 4 billion years ago — the study says the lunar magnetic field would have been weak.
To come to this conclusion, the team revisited rock samples collected during NASA’s Apollo program and discovered that the long-running disagreement stemmed from a sampling bias. The six Apollo missions landed in relatively flat, dark plains known as mare regions, which happen to be rich in a specific type of volcanic rock that recorded these magnetic events.
“Our new study suggests that the Apollo samples are biased to extremely rare events that lasted a few thousand years — but up to now, these have been interpreted as representing 0.5 billion years of lunar history,” study lead author Claire Nichols, an associate professor at Oxford, said in a statement. “It now seems that a sampling bias prevented us from realizing how short and rare these strong magnetism events were.”
Analyzing the chemistry of mare basalts, the researchers identified a link between the formation of titanium-rich rocks and lunar magnetism. Samples that recorded strong magnetic fields contained high levels of titanium, while samples that recorded weak magnetic fields had low levels of titanium.
“We now believe that for the vast majority of the moon’s history, its magnetic field has been weak, which is consistent with our understanding of dynamo theory,” says Nichols. “But that for very short periods of time — no more than 5,000 years, but possibly as short as a few decades — melting of titanium-rich rocks at the moon’s core-mantle boundary resulted in the generation of a very strong field.”
Computer models confirm that if scientists had sampled the lunar surface randomly, rather than from only the mare regions, they would have been unlikely to capture evidence of these rare magnetic spikes. That lends support to the idea that strong magnetic episodes were rare exceptions, not the rule.
Understanding the moon’s magnetic past matters because magnetic fields shield planetary surfaces from solar wind and help scientists probe the evolution of planetary interiors. Pinning down when — and how — the moon’s dynamo operated offers clues about how its core cooled, how its mantle evolved, and why its geologic activity faded.
It also provides a key comparison point for understanding why Earth’s dynamo persists while the moon’s shut down. Some researchers have even suggested the moon’s ancient magnetic field may have interacted with Earth’s early magnetosphere, potentially influencing how our planet retained its atmosphere.
With NASA’s upcoming Artemis program set to explore new regions of the moon, researchers hope to test their predictions and further unravel the history of the moon’s vanished magnetic field.
A study about these results was published on Feb. 26 in the journal Nature Geoscience.
