Researchers at the University of Oxford have determined that the Moon once generated a magnetic field comparable to, and at times potentially stronger than, Earth’s present-day shield, but only during brief, intense volcanic episodes. The finding, drawn from a fresh analysis of Apollo-era rock samples, resolves a long-running dispute over how powerful the lunar dynamo actually was and why certain moon rocks carry such strong magnetic signatures while others barely register.
High-Titanium Lavas Carried the Strongest Fields
The central puzzle has persisted for decades: some Apollo basalts record magnetic intensities that rival Earth’s field today, while others from similar ages show almost nothing. A new study in Nature Geoscience by Oxford’s Department of Earth Sciences offers a clean explanation. By compiling published paleointensity and geochemistry data from multiple Apollo missions, the team found a statistically robust link between magnetic strength and titanium dioxide content. Rocks rich in TiO2, specifically those exceeding 6 weight percent, consistently recorded strong fields of at least about 40 microteslas. One high-titanium sample returned a paleointensity of 69 plus or minus 16 microteslas, well above the 25 to 65 microtesla range typical of Earth’s surface field.
That pattern points to a dynamo that did not hum along steadily for billions of years. Instead, the Moon’s core appears to have fired up its magnetic engine in short, powerful bursts timed to episodes of titanium-rich volcanism. The implication is that dense, titanium-rich magma sank toward the core, altering heat flow and destabilizing convection patterns enough to temporarily amplify the field. This reading challenges the older assumption that the Moon sustained an Earth-like field continuously during its first billion years, and it also explains why low-titanium Apollo basalts from similar epochs carry far weaker magnetic records despite forming under broadly similar surface conditions.
A Dynamo That Flickered Across Billions of Years
Separate measurements from Apollo 17 mare basalts 75035 and 75055, described in a Nature Astronomy study, reported a mean paleointensity of roughly 50 microteslas around 3.7 billion years ago, along with a field inclination estimated at 34 plus or minus 10 degrees using boulder layering and orientation constraints. That reading sits within the cluster of high-titanium results and strengthens the case that the Moon briefly matched, and may have exceeded, Earth’s magnetic protection during peak volcanic activity. Together, these samples show that the early lunar field could be both strong and geometrically organized, rather than a chaotic or purely local phenomenon.
The field did not vanish overnight. Work on Apollo 15 regolith breccia sample 15498 extended the lunar dynamo’s lifetime by at least one billion years beyond earlier estimates, showing a weak but measurable field of roughly 5 plus or minus 2 microteslas persisting from about 1 to 2.5 billion years ago. Earlier review literature had placed the dynamo’s active window from at least 4.25 to 3.56 billion years ago with peak intensities at present-day Earth-like levels, then a decline of roughly 90 percent after about 3.56 billion years ago. The emerging picture is a magnetic engine that surged in the Moon’s youth, faded through the middle of its history, and lingered as a whisper for another billion years or more before finally shutting down.
Chinese Samples and Contamination Tests Support a Late Decline
Apollo rocks are not the only evidence for a long-lived but weakening lunar field. Basalts returned by China’s Chang’e‑5 mission recorded weak paleointensities of roughly 2 to 4 microteslas at approximately 2 billion years ago, supporting a persistent but feeble mid-stage lunar magnetic field. Because these samples come from a different landing site and were collected by a different space agency with their own curation practices, they provide an independent line of evidence for late-time dynamo activity that does not depend on any single Apollo collection or handling protocol. The Chang’e‑5 results dovetail with the Apollo breccia data, tightening the timeline for when the dynamo finally faded below detectable levels.
That independence matters because contamination has long been a concern in paleomagnetic work. A methodological study in Geophysical Research Letters tested whether spacecraft return or laboratory handling could have imprinted false magnetic signatures on Apollo samples by exposing analog rocks to strong artificial fields and tracking how the imposed magnetization decayed under standard cleaning procedures. The experiments showed that contamination can be characterized and removed using established demagnetization techniques, and that robust primary signals survive these treatments. When combined with the Chang’e‑5 data, which were acquired decades later and through entirely separate logistics, the contamination tests strongly support the conclusion that both the strong early and weak late-stage readings reflect a real lunar field rather than an artifact of sample handling.
Shared Shields and Atmospheric Consequences
A steady, long-lived lunar magnetic field would have shielded the Moon’s surface from solar wind for billions of years, potentially preserving volatile compounds and influencing surface chemistry. A flickering one tells a different story: protection was intermittent, concentrated during volcanic surges, and largely gone by 2 billion years ago. NASA has confirmed that magnetized Apollo rocks demonstrate the Moon once had a magnetic field that persisted for more than a billion years and was at times comparable to modern Earth’s. In its early days, the Moon orbited much closer to Earth, and modeling work suggests the two bodies may have shared a combined magnetic cavity that deflected the solar wind and helped protect both atmospheres from erosion, at least during periods when the lunar dynamo was active.
As the dynamo weakened and the Moon migrated outward, that shared shield would have unraveled. Without its own strong magnetosphere, the lunar surface became increasingly exposed to the solar wind, accelerating space weathering and sputtering of surface volatiles. Recent analyses of isotopes and volatile elements in lunar materials, summarized in a broad review of Moon–Earth evolution, emphasize how changing magnetic conditions likely interacted with impacts, volcanism, and orbital dynamics to shape the present-day Moon. The Oxford-led work on titanium-rich lavas adds a crucial piece to this story by showing that the strongest protection coincided with episodic volcanic bursts, implying that the Moon’s habitability for transient atmospheres or frost deposits was modulated not just by distance from Earth, but by the timing of its own internal eruptions.
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*This article was researched with the help of AI, with human editors creating the final content.
