Moons' magnetism: from Io's and Ganymede's present magnetic signatures to the ancient lunar dynamo

Intrinsic magnetic fields, corresponding to virtual axial dipole moments of the order of 10(20) Am-2, have recently been evidenced for the Jovian satellites lo and Ganymede. By reviewing the rock magnetic and palaeomagnetic: properties together with the history of lo, the hypothesis of a moment due to induced or remanent magnetization of crustal rocks acquired within the ambient Jovian field is clearly eliminated. This demonstration is all the more valid for Ganymede, which experiences a much lower Jovian field. The demonstration of a present dynamo action for these Jovian moons, possibly sustained by Jupiter through tidal heating and background magnetic field, may be an actualistic model for the early lunar history. The hypothesis of a lunar dynamo, active at 3-4 Ga, seems to be strongly supported by this analogy.