Jupiter’s giant moon Ganymede is the only known moon to have its own magnetic field – and it could be warming in a process “not yet observed anywhere else”, new research suggests.
One of the four Galilean satellites that orbit JupiterGanymede is the largest moon in the world solar system. At nearly 3,300 miles (5,300 kilometers) in diameter, it is more than 1,000 miles (1,600 km) wider than Earth’s. moon and slightly larger than Mercury, our smallest planetary child. (Jupiter has more than 100 confirmed moons, with the four largest known as the Galilean moons.)
The intrinsic magnetic field of this remarkable satellite — discovered by NASA’s Galileo spacecraft in 1996 – is generated by a process called a dynamo, powered by the electrically conductive liquid iron churned in its core.
Yet the mechanism by which this process emerged is the subject of heated debate.
“Many formation studies suggest that Ganymede formed too cold to start with a metallic core,” study co-author Kevin Trinha Caltech planetary scientist, said in a statement. “Meanwhile, many modeling studies of the Ganymede dynamo assume that Ganymede formed its metallic core around the time the Moon itself formed, as did the Earth. These two things cannot be true simultaneously.”
Thus, in an article published on May 6 in the journal Scientific advancesresearchers propose a new inside-out mechanism that could have formed Ganymede’s mysterious metallic core and dynamo later rather than earlier, as blobs of molten iron sank into the moon’s massive core. This activity could continue today.
An illustration depicting Jupiter and its largest moon, Ganymede, featuring auroras discovered by the Hubble Space Telescope.
(Image credit: NASA/ESA)
Hot or cold start?
The “warming-driven dynamo” presented in the study is the opposite of traditional ideas of dynamo origins, which propose that they form early in large bodies like Earth and then gradually cool.
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For example, the formation of metallic cores in planetary bodies is believed to have occurred approximately 200 million years after the formation of the solar system. Yet moons may be too small to retain enough heat from birth to power this process.
But all hope is not lost, as a body formed from a “cold start” may still be capable of developing a magnetic field-generating core, the researchers’ new model suggests.
This model incorporates and simplifies characteristics of Ganymede, such as its composition and core temperature, by assuming that its core is composed of iron and iron sulfide, because these components have lower melting temperatures.
“Cold” and “hot” scenarios for the formation of a dynamo, such as within Ganymede, as modeled in this study.
(Image credit: (Trinh et al., Science Advances, 2026))
In this model, drops of molten metal sink into Ganymede’s bowels to power its core and stir up its magnetic field. They are warmed by two main mechanisms: radioactive heating and tidal heating.
First, as heavier radioactive isotopes decay into lighter elements, they release heat. Second, Jupiter’s gigantic gravitational influence squeezes and stretches Ganymede as the moon moves toward and away from its parent, as if “kneading” a planet-sized piece of rocky, icy dough. The resulting internal friction generates heat. This heat powers the dynamo that gives Ganymede its magnetic field.
Overall, this hypothesis does not rule out the possibility that Ganymede formed with a core producing a magnetic field.
Extraterrestrial implications?
But if “cold start” cores exist throughout the universe, this could present a previously unexplored process allowing magnetic fields to form and protect aging worlds, perhaps facilitating an intriguing involvement in the search for extraterrestrial life.
Magnetic fields are necessary to protect life from harmful solar and cosmic radiation, making them a prerequisite for most searches for habitable planets. But a little magnetism can go a long way: as seen on Earth, with a magnetic field significantly weaker than a refrigerator magneteven a modest magnetosphere can transform the appearance of planetary bodies.
“There could be young, rocky exoplanets or exoplanets with lower radioactive isotope abundances (i.e. slower heating) that would be favorable for a recent warming-driven dynamo,” Trinh said in an email to Live Science. “The challenge is that no one has yet detected an exoplanet dynamo.”
Trinh, KT, Petricca, F., Hemingway, DJ, & Vance, SD (2026). Fueling the Ganymede dynamo with extended core formation. Scientific advances, 12(19), eaed8021. https://doi.org/10.1126/sciadv.aed8021
