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Originally Posted by filrabat
Unless there is a way for an earth-like body (even if a moon) to generate a strong magnetic field other than rotation, then that practically bars the moons closest in to the gas giant - those moons are likely to be rotation-locked (the same hemisphere always facing the planet).
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A rotation-locked moon still rotates, and a moon CLOSE to a gas giant planet rotates pretty fast. Io makes one revolution every 1.8 days -- fast enough to generate magnetic field if it were big enough.