How does Ganymede sustain a core dynamo today?
In plain words
Jupiter's moon Ganymede is the only moon known to make its own magnetic field. Models of how such a small body cools suggest its core should have stopped stirring long ago.
Precise statement
Galileo detected an intrinsic dipole with surface equatorial strength near $7 \times 10^{-3}$ G. Identify the power source that keeps Ganymede's Fe-FeS core convecting after 4.5 Gyr: iron snow, FeS crystallization, compositional convection, or heat from past tidal heating during resonance passage. An answer is a thermal-chemical evolution model producing a present dynamo of the observed strength.
What would settle it
JUICE orbital measurements of field harmonics, induction response and gravity (Love numbers) constraining core size and state.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
JUICE is planned to arrive at Jupiter in 2031 and orbit Ganymede from 2034.
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