Researchers studying a region of unusually strong magnetism emanating from a mysterious subsurface feature on the Moon’s far side, which always faces away from Earth, have determined that our planet’s largest natural satellite likely had an ancient magnetic field similar to Earth’s more than 4.2 billion years ago.
The team, which included researchers from ETH Zurich, the DLR Institute of Space Research, and the Technical University of Berlin, said their new models of the mysterious subsurface feature support the so-called ‘dynamo theory,’ suggesting the Moon supported a now inactive rotating iron core capable of generating a magnetic field similar to Earth’s that is absent today.
Far Side of The Moon’s Mysterious Subsurface Feature Offers Hints at Past Magnetic Field
“Today, there is an ongoing heated debate as to whether the Moon also operated a dynamo in the past,” explained Xi Yang, a PhD student in the Department of Earth and Planetary Sciences at ETH Zurich and a co-author of a study detailing the team’s findings.
Co-author and Geophysicist Anna Mittelholz, who is a lecturer in the same department, said some researchers support the existence of an internally generated lunar magnetic field, while others “find no evidence” for it.
The researchers also note that rock samples collected during the 1970s Apollo missions have provided “contradictory” evidence. For example, meteorite impacts could explain some of those results.
Curious whether the solution could be hidden in satellite gravity data, the research team examined data from the Dewar crater on the far side of the Moon. They also incorporated topography and composition studies.
A Potential Window into the Moon’s Internal Structure
“The Dewar region is a genuine stroke of luck: one of the strongest magnetic field anomalies on the far side of the Moon and a distinct gravity anomaly coincide spatially there,” Mittelholz explained.
Yang said these qualities, combined with the area’s density of magnetic rocks, make Dewar’s crater “a potential window into the Moon’s internal structure.”
Although orbital magnetic measurements are insufficient, the researchers noted that the evidence for an ancient lunar dynamo could be found in gravity measurements made by NASA’s ‘GRAIL’ probes. Mittelholz said these readings provided the team with “insight into the density and thus into the material beneath the surface.”
“Where the magnetic field and gravity signals coincide, it is possible to combine the two and attribute the anomaly to a specific geological structure,” the researcher explained. “This is precisely the opportunity that Dewar offered.”
Models Reveal Massive Volcanic Feature Supporting Dynamo Hypothesis
After collecting the available gravity and magnetic field data, the ETH-led team created what they described as an “accurate model” of the lunar subsurface. This included a focus on the Dewar region on the Moon’s far side. The researchers also included orbital measurements from the Lunar Prospector and Kaguya missions.
After processing the data, the team’s models produced some compelling results. This included theoretical confirmation of the mysterious subsurface feature containing high levels of magnetism.
The structure appears to be a 60-kilometer-wide rocky body lying just beneath the area around Dewar’s crater. Gravity and magnetic data models suggest the mysterious subsurface feature is roughly nine kilometers deep. Finally, the models showed that the rocky section is much denser than the surrounding lunar crust and “strongly” magnetized.
The researchers said combining these results with lunar surface geochemistry and arched topography revealed that the feature is solidified magma that rose from the subsurface, consistent with an ancient molten core capable of producing a magnetic field.
“(It’s) a buried volcanic complex,” they explained.
Mittelholz said knowing the area’s composition, and hence, likely iron content, allowed the team to estimate the minimum strength such an ancient lunar magnetic field must have possessed “as the magma cooled slowly.”
“For me, that is a very important finding,” the researcher explained.
The study authors noted that the structure’s age can also be determined from impact materials spread across the surface. Their analysis dates its creation to roughly 4.2 billion years ago, providing chronological support for an ancient lunar dynamo.
“We have found that the magnetic field on the Moon at that time was very likely stronger than 10 microtesla,” Yang explained, noting that Earth’s present-day magnetic field strength stands at around 50 microtesla.
From ‘Was there a dynamo?’ to ‘How did it work?
Although previous efforts have suggested that lunar magnetism could result from violent meteor impacts, the team said the Dewar region “lies outside the areas that are considered possible candidates.”
“We can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the core,” Yang said.
Although the new research supports the presence of an ancient lunar dynamo, the team noted that “it remains unclear, however, how the small lunar core could have generated such a strong magnetic field.” As a result, they conceded that the existence of a lunar dynamo has not been fully explained.
“But we are examining the question from an entirely new perspective,” Mittelholz said, adding that the question has now shifted from ‘Was there a dynamo?’ to ‘How did it work?’
The study “Magmatic origin of the Dewar magnetic anomaly: Implications for an early lunar dynamo” was published in Science Advances.
Christopher Plain has spent the last six years as Associate News Editor and Head Science Reporter at The Debrief. Follow and connect with him on X, learn about his novels at plainfiction.com, or email him at christopher@thedebrief.org.