Solar System’s largest moon is unique with a still forming core for billions of years

Amazing stuff!

"In 1996, NASA’s Galileo spacecraft made a shocking discovery at Ganymede, Jupiter’s—and the Solar System’s—largest moon. Unlike every other known moon, it hosted an internally powered magnetic field called a dynamo. Now, scientists think the reason might make the moon doubly unique: its core may still be forming. ...

Typically, the elements within a planet or moon stratify over time, with heavier elements like iron sinking down to form a dense core. When that core churns, it generates electrical currents that sustain a magnetic field.

Scientists only know of such dynamos on the gas giants, Earth, and Mercury. But when researchers modeled the process on Ganymede to match the magnetic field measurements from Galileo, they found that the core might not simply be churning, but actively growing, with new iron still dripping down from rocks in the moon’s mantle. If true, that would render Ganymede the only known body still to be forming its center. ..."

From the abstract:

"Ganymede is the only known moon with an active dynamo today.

Previous studies interpret Ganymede’s dynamo as arising from convection in a metal core that formed billions of years ago.

However, Ganymede likely accreted too cold to form with a metal core, which confounds interpretations of Ganymede’s magnetic field as a constraint on the moon’s broader history.

Here, we reevaluate the thermal evolution of Ganymede’s rock-metal interior from a cold start. 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."

ScienceAdviser

Core of Solar System’s largest moon may still be forming "Ganymede discovery could force rethink of how worlds power their magnetic fields"

Powering Ganymede’s dynamo with protracted core formation (open access)

Fig. 1. Possible thermal evolution of Ganymede’s interior based on assumed initial temperatures.

Deep inside Jupiter’s largest moon, Ganymede, molten iron may be sinking, adding to a still-growing core.