Remanent ferromagnetism and the interior structure of Ganymede

The Galileo spacecraft discovered that Jupiter's moon, Ganymede, has both an internal magnetic field and a strongly differentiated interior. This has prompted new models of Ganymede's interior, featuring an iron core and dynamo action to produce the magnetic field. However, extant dynamo action is not necessary to produce Ganymede's magnetic fields. The rocky material within Ganymede is believed to be rich in iron-bearing minerals: particularly magnetite, and its outer layer is capable of holding strong remanent magnetism. We investigate two sources of remanent ferromagnetism: Jupiter's magnetic field, especially in the past, when Ganymede was closer to Jupiter due to tidal evolution, and a paleomagnetic field from dynamo action which is no longer active. In this analysis we use a model of remanent magnetization similar to those which have been applied to the Moon, Mars, and Mercury, but we have generalized it to apply to strongly magnetic materials. We find that magnetization by the jovian background field can account for no more than 5.6% of Gallymede's internal field. However, a paleomagnetic field of roughly 10,000 nT or greater (at least 15 times the current field of Ganymede and a third that of the Earth's current field) is capable of producing sufficient remanent magnetization to account for the Galileo observations, given very favorable assumptions about the magnetic properties of the rock layer. For more probable values a paleomagnetic held equal to or greater than the Earth's current field is necessary.