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Models of how such a small body cools suggest its core should have stopped stirring long ago.","posed_since":"1996","precise":"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. 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