Jupiter’s moon Ganymede is larger than the planet Mercury, hides a massive ocean beneath its ice, and is the only moon in the solar system known to generate its own magnetic field. 

For decades, scientists thought that the magnetic field came from motions inside a fully formed metallic core deep below the frozen surface. However, a new study now suggests Ganymede’s core itself may still be forming billions of years after the solar system was born.

That idea could finally explain why Ganymede still has an active magnetic field even though most moons lost theirs billions of years ago.

The moon that may have cooled too slowly

Magnetic fields are usually powered by internal dynamos—movements of electrically conducting liquid metal inside a planet or moon. However, such dynamos normally weaken once a world cools and its core formation ends. 

For example, Earth’s moon no longer has one. Even Mars lost its internally generated magnetic field billions of years ago, despite being slightly larger than Ganymede.

The mystery becomes even harder to explain because planetary cores are generally thought to form quickly. Heat from accretion (violent buildup of material during planetary formation) normally separates heavy metals from lighter rock within roughly 1 to 200 million years.

“For decades, studies of planetary formation and dynamo action have progressed in parallel but with conflicting assumptions on Ganymede’s initial state,” the study authors said. 

To understand why Ganymede still behaves like a magnetically active world, researchers built computer models that recreated the moon’s thermal history from its earliest days to the present. 

Most earlier theories assumed Ganymede formed hot, allowing heavy metallic material to rapidly sink inward and create a core early in its history. The new work explored a different possibility. What if Ganymede began much colder?

The researchers tested multiple scenarios using one-dimensional thermal evolution models. They adjusted factors such as the moon’s water content, internal composition, and the amount of tidal heating generated by Jupiter’s gravitational pull. They also examined how radioactive elements inside the moon could slowly warm the interior over time.

Their simulations suggested that Ganymede may contain a mixture of iron and sulfur with relatively low melting temperatures. In such a system, the moon would not need extreme early heating to start separating metal from rock. Instead, warming could happen gradually over billions of years.

That slow warming changes everything

Rather than completing core formation early in the solar system’s history, dense metallic liquid may still be moving toward the center today. 

According to the study, iron-rich melt inside Ganymede could slowly separate from surrounding material and sink deeper into the moon’s interior, feeding a partially formed metallic center sometimes described as a protocore. 

“We propose that Ganymede’s dynamo may arise from a warming interior, with slow differentiation dribbling a steady supply of Fe melt onto a growing protocore,” the study authors said.

As this liquid metal moves downward, it stirs electrically conducting material inside the moon and creates the conditions needed for a magnetic dynamo. In simple terms, the moon may still be building the very engine that powers its magnetic field.

“Our models show that Ganymede’s observed dynamo is consistent with ongoing core formation, a process not yet observed elsewhere,” the study authors note. This idea differs sharply from earlier models based on ‘iron snow’ convection, where solid iron particles crystallize inside an already-existing liquid core and fall downward like metallic snowfall. 

The new study instead suggests the dynamo may come from continuous core growth itself.

The researchers argue that this slow, ongoing separation of metal could maintain magnetic activity for billions of years—far longer than expected for a moon-sized body.

The findings may also help explain why other nearby icy moons of Jupiter evolved differently despite sharing similar environments. Europa may have experienced stronger early heating that allowed its core to form much earlier. While Callisto may have stayed too cold for efficient core development. 

Small differences in timing, composition, and heating may have pushed these neighboring moons down completely different evolutionary paths. “Callisto likely evolved along the opposite, colder path. The classic conundrum in comparing Ganymede and Callisto lies in their similar sizes, bulk densities, and adjacent orbits,” the study authors said.

A hidden process that could exist across the solar system

The study could change how scientists think about the evolution of icy worlds. Instead of forming quickly and slowly cooling into inactivity, some planetary cores may develop over billions of years while continuing to power magnetic dynamos.

That matters because magnetic fields can shield worlds from charged particles and help stabilize subsurface oceans over long periods. Since Ganymede likely hides a vast ocean beneath its ice shell, understanding how its magnetic field survives could offer clues about potentially habitable environments elsewhere in the solar system.

The idea remains unconfirmed. However, the models rely heavily on assumptions about Ganymede’s internal chemistry, and scientists still cannot directly observe the moon’s deep interior. 

Future missions such as JUICE from the European Space Agency could help test the theory by studying Ganymede’s magnetic environment and internal structure during the 2030s. If the theory holds up, Ganymede may become the first known world whose magnetic field survives because its core never fully stopped forming.