Archived orbital data from the Dewar region on the Moon’s far side reveals a buried volcanic complex formed 4.2 billion years ago, providing strong evidence that the early Moon sustained an internal geodynamo and generated its own planetary magnetic field.
For decades, planetary scientists have argued over whether the young Moon ever possessed a true internal magnetic field. Now, a deep dive into archival orbital measurements has brought fresh momentum to the side favoring an ancient lunar geodynamo. Researchers examining a massive, buried geological structure on the lunar far side have concluded that the feature is solidified magma dating back roughly 4.2 billion years, carrying a magnetization signature too strong to be explained by meteorite impacts alone.
Archival Probe Data Exposes the Dewar Region Anomaly
The discovery centers on the Dewar region, a heavily rugged area on the Moon’s far side that remains permanently hidden from Earth. By combining gravity data gathered by NASA’s GRAIL mission with orbital magnetic measurements from Japan’s Kaguya mission and NASA’s Lunar Prospector—which was originally launched in 1998—scientists constructed a unified subsurface model.
The analysis revealed a dense, strongly magnetized rock body stretching approximately 60 kilometers—about 37 miles—in width and extending roughly 9 kilometers (5.6 miles) beneath the surface. According to the research team, this subterranean mass is a buried volcanic complex formed when magma rose from deep within the interior and solidified before breaking through the surface.
Calculating Ancient Field Strength and Ruling Out Impacts
Because the rock body contains a measurable concentration of iron, the research team could calculate the minimum magnetic field required to magnetize the material as it slowly cooled over evolutionary timescales. Study authors Xi Yang and Anna Mittelholz, a PhD student and planetary scientist respectively at ETH Zurich, determined that the ancient lunar field likely exceeded 10 microteslas. For comparison, Earth’s modern magnetic field stands at roughly 50 microteslas.
“We have found that the magnetic field on the Moon at that time was very likely stronger than 10 microtesla. On Earth today, the magnetic field strength stands at around 50 microtesla.”
Xi Yang, PhD student in the Department of Earth and Planetary Sciences at ETH Zurich
Researchers also evaluated whether a massive asteroid or meteorite impact could have generated the localized magnetization through shockwaves. However, the Dewar region lies well outside the geographic zones considered plausible candidates for impact-induced magnetization. That spatial distribution left the team confident in an internal origin. We can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the core,
Yang noted, as cited by ETH Zürich.
Contradictory Apollo Rock Samples and a Shift in Scientific Debate
The debate over lunar magnetism stretches back to the 1970s, when samples returned by the Apollo missions first indicated that ancient lunar rocks had recorded a magnetic field. While those initial findings convinced generations of astronomers that an early dynamo existed, subsequent analyses of different Apollo samples over the past decade produced conflicting results, with some testing showing zero magnetic signature for overlapping periods.
By relying on orbital gravity and magnetic data instead of physical samples, the international team bypassed those terrestrial disruptions.
Even so, the new findings do not completely reconcile the conflicting interpretations derived from Apollo samples. Instead, researchers suggest the overarching scientific inquiry has matured. As Mittelholz observed in a university statement, the primary question has shifted from simply asking whether a dynamo existed to investigating How did it work?
given that the Moon’s relatively small core would have faced severe thermodynamic constraints to generate such a strong field.
Connection to Lunar Swirls and Future Exploration Goals
The Dewar location also hosts a prominent lunar swirl—bright, sinuous surface markings that routinely coincide with localized magnetic anomalies. Scientists theorize that horizontal magnetic field lines at these sites partially deflect charged solar wind particles, shielding the underlying soil from space weathering that typically darkens lunar regolith.