Ganymede’s magnetic field may be powered by a core still forming, new study finds

Scientists have long known that Ganymede, Jupiter’s largest moon, is extraordinary: It is the only moon in the solar system known to generate its own magnetic field.

Now, a new study challenges the primary ideas used to explain that field, suggesting that Ganymede’s metallic core may still be forming today and that the very process of core formation could be driving its dynamo.

NASA’s Galileo spacecraft, which explored the Jupiter system in the 1990s, discovered Ganymede’s intrinsic magnetic field using an onboard magnetometer. This field comes from vigorous churning of liquid iron inside the moon’s core, a process known as a dynamo. 

While many planets across the solar system host active dynamos, including Mercury, Earth, Jupiter, Saturn, Uranus and Neptune, Ganymede stands alone among the 300-plus moons in our solar system as the only one known to host an ongoing dynamo. Understanding what powers it offers a rare window into the deep interior of a world whose surface is somewhat mysterious. 

The new research is led by Arizona State University’s School of Earth and Space Exploration alum Kevin Trinh, who earned his PhD in geological sciences in 2025. 

Trinh and his team zeroed in on a long-standing contradiction in the field. Most models studying Ganymede’s dynamo assume that the moon’s metallic core formed roughly 4.5 billion years ago, around the same time Ganymede itself formed. 

But a separate body of work on how the Galilean moons came to be tells a different story: Ganymede likely formed too cold to have separated out a metal core at birth.

The results have been recently published in Science Advances.

“For decades, studies have progressed in parallel with conflicting assumptions about how Ganymede formed and evolved,” lead author Trinh said. “Many formation studies suggest that Ganymede formed too cold to start with a metal core. Meanwhile, many modeling studies of Ganymede’s dynamo assume that Ganymede formed its metal core roughly when the moon itself formed, as Earth did. 

“Both of these things cannot be simultaneously true.”

To address these two lines of research, the team used computer models to simulate Ganymede’s interior evolution from a cold start, lacking an initial metal core. Their results show that metallic core formation could still be underway inside Ganymede today, and that this ongoing process is itself consistent with the observed dynamo.

The proposed mechanism is fundamentally different from conventional “cooling dynamos,” in which thermal or chemical convection stirs an already-formed core. Instead, the team describes a “warming-driven dynamo” in which liquid iron sinking toward the moon's center actively stirs a growing proto-core.

“Our study hypothesizes a warming-driven dynamo for Ganymede, where the downward migration of liquid iron could stir the growing protocore. This idea contrasts with conventional cooling dynamos, which invoke thermal or chemical convection to mix the core,” Trinh said. 

The study does not rule out the possibility that Ganymede’s dynamo is driven by conventional thermal cooling. Rather, the researchers introduce an alternative explanation — one that is coherent with the moon’s cold-formation history and consistent with the magnetic field we detect today.

“Our results do not rule out a cooling-driven dynamo at Ganymede. However, we introduce a new dynamo mechanism that is aligned with the idea that Ganymede started out cold and without a metal core. More work is needed to identify the most likely mechanism to explain Ganymede’s dynamo today,” Trinh said. 

The study’s data have broader significance for understanding the diversity of planetary interiors across the solar system. Ganymede’s dynamo is one of the few observed constraints scientists have to probe deep interior processes on this moon, making the question of its origin especially significant. Missions, such as ESA’s Juice (Jupiter Icy Moons Explorer), en route to the Jovian system, may provide new data to help explain these new mechanisms.