Ganymede is not only Jupiter’s largest moon, but also the largest in our solar system and one of the few that hosts a massive ice ocean. Adding to this planet-like moon’s uniqueness is the fact that among the hundreds of moons in our solar system, Ganymede is the only one that generates its own magnetic field. While the prevailing view was that Ganymede generates this magnetic field through convection in an already-formed core, there are still uncertainties surrounding this idea.
Now, a new model of Ganymede’s inner evolution suggests different processes may be at play. The new model, described in a study published in Science Advances, indicates that Ganymede’s core is actually still forming, and this formation is driving the magnetic field instead.
Ganymede’s mysterious magnetic dynamo
The magnetic fields of celestial bodies are caused by active magnetic dynamos—natural processes where the motion of electrically conducting fluid, like molten iron in a planet’s core or plasma in a star, generates and sustains a magnetic field. It’s been theorized that Ganymede’s magnetic field is generated by “iron snow” convection, where solidified iron flakes fall through the liquid core. This motion creates a magnetic field.
The accretion process that happens during a planetary body’s formation can generate the heat needed to form a core through the differentiation of metals. This process acts as a magnetic dynamo. Generally, if enough heating from accretion occurs, the process should finish within 1 to 200 million years after solar system formation (our solar system is now around 4.6 billion years old).
The magnetic field then dissipates. Because of this, many of the solar system’s moons, like Earth’s moon, have formed already-cooled cores, which no longer generate magnetic fields. Even Mars, which is a little larger than Ganymede, has already undergone the whole process and now lacks its own internally generated magnetic field.
On the other hand, some planetary bodies never get hot enough to form a core, and never generate their own magnetic field. The study authors explain that some icy moons likely accreted after the decay of radioactive aluminum that might help heat up their interior and are too small to differentiate a metal core using accretional heat alone, which can delay or prevent core formation altogether.
All of this raises questions about why Ganymede has an active magnetic dynamo to this day. Because of the substantial evidence indicating that Ganymede does indeed have a metal core and an active magnetic dynamo, the authors think that its heating must have been delayed.
A cold beginning for long-lasting formation
To explore how and why Ganymede still has an active magnetic dynamo, the team involved with the new study used one-dimensional thermal evolution models to simulate what would happen in Ganymede’s interior if it had a “cold start.” The team tested a range of parameters in the model, including Ganymede’s initial composition, water content, and past tidal heating, to see which scenarios could sustain a dynamo.
The new models show that Ganymede’s magnetic dynamo could be powered by ongoing, slow core formation rather than a long-cooled core with iron snow. According to the models, the gradual warming of Ganymede’s interior may still be causing iron to separate and sink, stirring the core and sustaining the magnetic field. They say that the heat sources in the model include radioactive isotopes, gravitational energy from core formation, and tidal heating. The model relies on the assumption that Ganymede has a Fe-FeS system, with a lower range of melting temperatures than that of other Fe alloys.
“Our models show that Ganymede’s observed dynamo is consistent with ongoing core formation, a process not yet observed elsewhere. If Ganymede has an Fe-FeS core with a sub-eutectic composition, then gradual mantle warming may expel dense Fe melt onto the growing protocore and stir liquid metal, sustaining a dynamo for billions of years,” the study authors write.
Implications for other moons
Understanding Ganymede’s core formation can help researchers learn more about evolution and magnetic field generation in other moons as well. For example, the team says that Europa may have experienced a warmer evolution than Ganymede, making core formation less likely to be ongoing in their models.
They also say Callisto had unfavorable conditions for a core dynamo: “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. Callisto is often assumed to be partially ice-rock differentiated, but nonhydrostatic effects could have inflated the inferred MOI, leaving open the possibility of a metallic core. Our models indicate that later accretion, enhanced 40K leaching, higher ice-to-rock ratio, and smaller size could each inhibit ongoing core formation in Callisto.”
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Publication details
Kevin T. Trinh et al, Powering Ganymede’s dynamo with protracted core formation, Science Advances (2026). DOI: 10.1126/sciadv.aed8021
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Ganymede’s unique magnetic field may be powered by ongoing core formation—not a cooling core (2026, May 9)
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