BY:SpaceEyeNews.
The Moon’s ancient magnetic field may have left a remarkable record beneath its far side. Researchers have identified a buried volcanic formation that could preserve evidence of magnetism from roughly 4.2 billion years ago. Its unusual combination of density and magnetization offers a fresh way to investigate the Moon’s interior.
The discovery centers on the Dewar region, where spacecraft measurements reveal overlapping gravity and magnetic anomalies. Together, those signals strengthen the case for an early lunar dynamo. Yet they also sharpen a difficult question: how did such a small core generate a substantial magnetic field?
A buried volcanic complex beneath Dewar
An unusual match of signals
Dewar stands out because two different measurements point toward the same underground region. The rocks appear denser than the surrounding crust and carry unusually strong magnetization.
The team’s model indicates a formation approximately 60 kilometers wide, extending to around nine kilometers below the surface. Researchers interpret it as a buried volcanic complex, drawing on its density, surface chemistry, and the area’s arched terrain.
These clues suggest that magma rose from the interior and eventually solidified. The resulting rock retained magnetization that scientists can still detect from orbit.
Establishing its ancient origin
Deposits from ancient lunar impacts help constrain the formation’s age. Their geological relationship with the buried structure points to an origin around 4.2 billion years ago.
That timing matters. It places the formation early in lunar history, making Dewar valuable for investigating the Moon’s internal evolution.
However, scientists inferred the structure and its age through remote observations and geological interpretation. They have not directly sampled the buried complex.
China’s Chang’e-7 Is Ready to Explore the Lunar South Pole!
Mapping the Moon’s magnetic past from orbit
The researchers combined gravity measurements from NASA’s GRAIL mission with magnetic observations from Lunar Prospector and Kaguya. Each dataset supplied a different part of the picture.
Gravity variations helped constrain the distribution of underground density. Meanwhile, magnetic measurements revealed how strongly the underlying rocks retained magnetization.
Combining the datasets allowed the team to connect a magnetic anomaly with a plausible geological source. That connection matters because magnetic measurements alone cannot always establish which rocks produce a signal.
Researchers from ETH Zurich, DLR, and the Technical University of Berlin collaborated on the study. Science Advances published the findings online on September 23, 2026.
Their approach adds an independent line of evidence to a debate previously shaped heavily by returned lunar samples. Those samples have yielded conflicting interpretations of the Moon’s magnetic history.
Some analyses suggest a strong field persisted for a substantial period, while others offer little support for that interpretation. Dewar approaches the disagreement through the relationship between an entire rock formation and its magnetic signature. This gives researchers another way to assess the evidence. It also ties their explanation to a particular location, where future observations could examine the proposed volcanic origin more closely.
How strong was the Moon’s ancient magnetic field?
Reading the record in cooling rock
Cooling magma can preserve information about the magnetic field surrounding it. Magnetic minerals retain part of that record after the rock solidifies.
For Dewar, researchers used the inferred rock composition to estimate the field needed to explain its magnetization. Their calculations indicate an ancient strength exceeding 11 microtesla.
ETH Zurich’s public summary rounds that threshold to more than 10 microtesla. For comparison, it gives around 50 microtesla as a representative value for Earth’s present field.
The comparison highlights a substantial ancient lunar field. However, the estimate represents a minimum under the study’s assumptions, rather than an exact measurement of its strength.
Separating ancient and present magnetism
The estimate concerns conditions when the volcanic rocks cooled billions of years ago. It does not describe the magnetic field currently measured above Dewar.
Today’s anomaly represents magnetization remaining within the crust. Understanding that distinction helps connect the present observations with the ancient process researchers hope to reconstruct.
Why an internal lunar dynamo fits
Scientists have considered more than one explanation for magnetized lunar rocks. Alongside an internal dynamo, they have investigated temporary magnetic effects associated with large impacts.
The researchers argue that Dewar lies outside the regions considered plausible for the proposed impact explanation. They therefore favor a longer-lasting field generated within the Moon.
Slowly cooling magma also makes the persistence of the surrounding field relevant. A sustained source offers a coherent explanation for the magnetization preserved in the volcanic formation.
Still, the findings leave an important problem unresolved. Researchers must explain how the Moon’s relatively small core could produce a field of this strength.
ETH Zurich acknowledges that the early dynamo question remains open. Dewar strengthens the evidence while giving scientists a more specific geological record to explain.
Lunar swirls offer another clue
A bright lunar swirl sits above the Dewar magnetic anomaly. Its curved markings connect the buried formation with another persistent lunar puzzle.
One proposed explanation involves magnetic fields that run horizontally near the surface. These fields can deflect solar-wind particles, reducing the weathering that darkens lunar soil.
As a result, some areas may remain brighter than their surroundings. The study also identifies iron oxide enrichment as relevant to explaining the swirl.
This connection could help researchers understand how surface chemistry and local magnetism interact. However, possible solar-wind deflection does not establish comprehensive radiation protection for future astronauts.
What future exploration could reveal
Dewar could help guide the selection of targets for surface measurements and lunar sample investigations. Such work would provide opportunities to test the orbital interpretation.
The researchers also see potential for applying their combined approach elsewhere. Mars offers an interesting possibility, although available observations lack the detail needed for equivalent analysis.
For now, Dewar’s importance rests on the connection between underground geology and preserved magnetism. The Moon’s ancient magnetic field has gained a valuable new clue, but its source still demands explanation. Future evidence must show how well this volcanic record fits the wider history of the lunar interior.
Main sources:
- ETH Zurich: The far side of the Moon provides clues to a previous magnetic field.
- Yang, Mittelholz, Broquet, and Moorkamp, Science Advances: Magmatic origin of the Dewar magnetic anomaly: Implications for an early lunar dynamo.
- SciTechDaily: A Hidden Structure on the Moon’s Far Side Could Rewrite Its Magnetic History.