Scientists at the University of Oxford’s Department of Earth Sciences have settled a decades long argument over the strength of the Moon’s magnetic field. For years, researchers have questioned whether the Moon generated a powerful magnetic field or only a weak one during its early history (3.5 — 4 billion years ago). A new study published February 26 in Nature Geoscience concludes that both views were partly right.
By reexamining rock samples returned by the Apollo missions, the team found evidence that the Moon did experience periods of extremely strong magnetism, at times even surpassing Earth’s. However, these intense phases were rare and brief. For most of its history, the Moon’s magnetic field was relatively weak.
The disagreement persisted largely because all Apollo missions collected samples from the same general region of the lunar surface. That area happened to contain rocks that recorded these unusual bursts of strong magnetism, giving the impression that such conditions lasted far longer than they actually did.
Lead author Associate Professor Claire Nichols (Department of Earth Sciences, University of Oxford) explained: “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. It now seems that a sampling bias prevented us from realizing how short and rare these strong magnetism events were.”
Titanium Rich Rocks and Lunar Magnetism
Although many Apollo samples showed strong magnetism, some scientists argued that the Moon’s small core, which measures only about 1/7th of its radius, would have struggled to sustain a powerful magnetic field. The new research offers an explanation for how the Moon could briefly generate and preserve such intense magnetism.
The researchers examined the chemistry of Mare basalts, a type of volcanic rock found on the Moon. They discovered a clear relationship between a rock’s titanium content and the strength of its recorded magnetism. Every sample that showed signs of a strong magnetic field contained high levels of titanium. In contrast, rocks with less than 6 wt.% titanium consistently showed evidence of a weak field.
This pattern suggests that the formation of titanium rich rocks is directly connected to the creation of strong magnetic episodes. The team proposes that melting of titanium rich material deep inside the Moon temporarily triggered unusually powerful magnetic fields.
Professor Nichols said: “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. 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.”
Apollo Sampling Bias and Future Artemis Missions
The Mare basalts provided relatively smooth terrain, making them ideal landing sites for Apollo astronauts. As a result, astronauts collected a disproportionately large number of titanium rich rocks that preserved evidence of strong magnetism. Scientists later analyzed many of these samples on Earth and concluded that the Moon’s magnetic field must have been strong for extended periods.
Computer models developed in the new study support the idea that this was a sampling bias. If researchers had examined a more random selection of lunar rocks, it would have been highly unlikely to find samples that recorded these rare magnetic surges.
Co-author Associate Professor Jon Wade (Department of Earth Sciences, University of Oxford) offered a comparison: “If we were aliens exploring the Earth, and had landed here just six times, we would probably have a similar sampling bias especially if we were selecting a flat surface to land on. It was only by chance that the Apollo missions focussed so much on the Mare region of the Moon — if they landed somewhere else, we would likely have concluded that the Moon only ever had a weak magnetic field and missed this important part of early lunar history entirely.”
Co-author Dr. Simon Stephenson (Department of Earth Sciences, University of Oxford) added: “We are now able to predict which types of samples will preserve which magnetic field strengths on the Moon. The upcoming Artemis missions offer us an opportunity to test this hypothesis and delve further into the history of the lunar magnetic field.”
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