The Moon's ancient magnetic field has been debated for fifty years. Apollo astronauts brought back rocks that were magnetized — evidence that the Moon once had a magnetic field generated by a convecting liquid iron core, like Earth's. Some rocks showed field strengths as strong as or stronger than Earth's. But the physics was difficult: the Moon's core is small, and generating a strong dynamo field in a small core requires implausible energy sources. Other evidence, including orbital measurements, suggested the field was mostly weak.
Oxford scientists resolved the contradiction by finding a correlation that had been hiding in the Apollo data. Published in Nature Geoscience, their analysis showed that the magnetization of lunar rocks correlates with their titanium content. High-titanium rocks are strongly magnetized. Low-titanium rocks are weakly magnetized. The field was not uniformly strong — it was briefly strong, during episodes when titanium-rich rocks melted at the core-mantle boundary and drove intense but short-lived convection bursts. These bursts lasted perhaps 5,000 years, possibly only decades. Between bursts, the field was weak.
The structural insight is about sampling bias creating false consensus. The Apollo landing sites — chosen for safety and accessibility, not geological representativeness — happened to be in mare regions rich in titanium-bearing basalts. The rocks that were most likely to be collected were precisely the rocks that recorded the rare, intense magnetic bursts. The sample was biased toward the anomaly, making the anomaly look like the norm. The fifty-year debate about whether the Moon's field was strong or weak was produced by the distribution of landing sites, not by the physics of the Moon.
This is a general failure mode of observational science: when the sampling mechanism correlates with the variable being measured, the sample misrepresents the population. The Apollo missions did not sample the Moon randomly. They sampled specific terrain, and that terrain encoded specific magnetic history. The strong-field evidence was real — those rocks genuinely recorded strong fields. But the frequency of strong-field rocks in the collection overstated the frequency of strong-field episodes in lunar history.
The resolution — brief intense bursts in a background of weakness — is more interesting than either side of the original debate. A persistently strong field would require exotic physics. A persistently weak field would leave the magnetized rocks unexplained. Intermittent bursts explain both the rocks and the physics. The answer was always available in the data. The bias prevented anyone from finding it.