Why stop at iron? Why not nickle, chrome or any of the other ferromagnetics or rare earth elements?. Sure, there probably is a rudimentary core of magnetic material comprised of meteoric debris. The best theory we have at the moment though is that an electrically conducting sublayer of salt water is reponsible for a dynamo effect that is analogous to the fluid magma beneath the Earth's mantle. Determining the internal stucture of Saturn's moons and defining the changing magnetic field is a part of Cassini's mission. We really don't know for sure at the moment - investigation is part of what most folk recognise as S-C-I-E-N-C-E. Cassini isn't just there for a joyride to take pretty pictures however enthralling they are.

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By asking questions we sometimes get the wrong answers, from wrong answers we learn to ask the right questions.